god damn shunt is killing me
Discussion
Four key points:
1. This is not a mod, its simply going back to the 18 degrees of idle most V8s ran in the 60's
2. Going full vacuum is mostly about improving the way the engine idles by giving the engine the timing it wants
3. Not only will 18 degrees give you a much more stable idle, you will reduce all that shocking heat in the exhaust manifolds which offers many benefits especially given the TVR manifold design
4. On the shunting point its not so much about adding timing in the shunting zone its about how many degrees of advance the ignition system must add to get you to the right number
On point four, one of the first things you learn early on when playing with ignition tables is big jumps in timing are bad news, you need to get from wherever you are on the table to your other timing numbers on the table (up or down) as smoothly as possible, from idle to 1,750rpm it stands to reason the easiest way to achieve this is to narrow the gap, and easiest way to do that is start from a higher idle timing number like 18 degrees, which is handy as 18-20 degrees is exactly what old Rover likes to idle at.
Lets say for arguments sake the target optimal timing figure at 1,750rpm at a smallish throttle opening produces an engine load figure of 44kPa and your optimal target timing figure at that point is 25 degrees, if you're idling at 10 degrees the distributor has to add 15 degrees from 1,000rpm to 1,750rpm which is a lot of timing to add in just 750rpm. On the other hand if you start at 18 degrees you only need to add 7 degrees to get to your magic 25, the 750rpm window is the same, believe me... with low engine speed ignition timing a jump of 7 degrees is a lot less than 15.
Of course it doesn't actually work like that, what happens with a full vacuum strategy is you start at 18 degrees idle then lose all vacuum as you first crack open the throttle and pull away. However, with full vacuum applied to the vac advance unit things are much more consistent than this sounds, even at low load & low engine speeds where shunting resides when you lift of the throttle to regulate your slow speed progress in urban situations some of your timing will be coming from centrifugal advance and some will come from the vac advance unit, in effect the two work together and in on/off throttle speed regulating situations such as urban driving the engine is way smoother. Those who suffer shunting subconsciously know a fixed throttle angle helps maintain smooth slow speed progress, they avoid on/off shifts in throttle position at all costs because that's the fasted and most effective way to initiate the shunt.
In busy urban driving conditions that lets face it we all must suffer on this little crowded island of ours, no sooner have you gathered momentum and found yourself moving at 25mph than you find yourself needing to slow to regulate progress in line with other road users who will be slowing to turn, stopping at junctions or slowing for a cyclist and then accelerating after they've passed ect ect ect. So if you study it you'll find urban driving is all about delicately modulating speed using the throttle and in doing so you are constantly opening and closing the throttle butterfly by tiny amounts and so creating more and less vacuum. With ported vacuum you might get a degree or two on top of centrifugal advance at certain throttle angles best, but this is soon lost with the smallest twitch of your big toe, the ported vacuum signal is so weak, inconsistent and on/off at this point it'll probably be fluttering the vac unit which is not good for smooth progress as timing will be added and taken away in a very irregular and inconsistent way.
I've heard many tales of people reducing shunting by disconnected their vac advance unit, all they are doing is removing that flutter the weak on/off ported vacuum signal gives around 1,750rpm right when the driver finds he needs to keep changing the throttle angle to regulate his road speed through town. The truth is I've run my Chimaera at the shunting point at many different timing settings from 18 degrees to 32 degrees and the car drives perfectly at these figures... and anything in between, what makes the car shunt is when I try to shift the timing number up in big jumps from idle to the 1,750rpm figure.
Big jumps in timing are always going to give drivability issues, if you start at 18 degrees idle then run 22 degrees at 1,750rpm and find the car is smoother than when you ran 29 degrees at 1,750rpm... you haven't proved 22 degrees is what the engine wants, you've just narrowed the gap. 18-22 degrees is a small gap of just 4 degrees which allows you to create a nice even and smooth transition, 18-29 is a jump of 11 degrees and from 1,000rpm to 1,750rpm is just a 750rpm window to get there, so the steps must be more dramatic and so the risk of a shunt becomes greater.
Like I say, you haven't discovered less timing (22 degrees) in the shunting zone is better than 29 degrees, all you've done is reduced the big 11 degree jump in timing you had when you ran 29 degrees at 1,750rpm. Slowing the rate of attack by simply reducing the gap between idle timing and optimum timing at 1,750rpm (25 degrees) is the key to a smooth TVR, switching to full vacuum and idling at 18 degrees closes the gap because even though you lose vacuum when your first pull away.... rpms are already on the way up so centrifugal force takes over, now come off the throttle and the vac advance unit connected to full vacuum steps back in to cushion the fall..... unlike ported vacuum which tends to see you falling in a low number timing hole.
Try some of this on/off throttle low speed urban driving running ported vacuum, then try the same route using same on/off throttle driving style with full vacuum applied to the vac advance unit instead.... and see which setup you prefer
. Applying full vacuum to the vac advance unit is just part of a package of things that can help, however, people should apply caution and common sense when experimenting with this strategy because at the end of the day the Lucas distributor and vac unit were designed to work with ported vacuum.
A better solution would come in in the form of an MSD distributor with an adjustable vac advance unit and customisable advance curves, but by the time you've bought one.... for a few quid more you could have fitted a Canems, MegaJolt or Omex 200 mappable ignition only system which will always be infinitely better than any distributor, even a tunable MSD one
Finally you can improve shunting by adding fuel if you have that facility, this works because richer mixtures are easier to ignite and (up to a point) burn better than lean mixtures. Because rich mixtures are easier to ignite you can run less timing, if you run a leaner mixture it will need to be lit earlier to ensure complete combustion simply because it's both harder to strike and takes longer to burn completely. If you cover the shunt in fuel you can indeed run a lower timing number, indeed it'll probably demand it, but it's not the best strategy in my opinion.
The better approach is to find the best balance between AFR and timing, which at 1,750 on petrol on my 4.0 litre running a Stealth cam but no head work is 14.3:1 @ 25 degrees. People may see this as lean but there is absolutely no shunting on my car like this, if I run 22 degrees @ 14.3:1 combustion is fractionally incomplete before the exhaust valve starts to open so micro misfires exist, and the engine is not its smoothest, if I leave timing at 22 degrees I must richen the AFR to 13.5:1 or richer to solve the problem.
So my question is.... is 14.3:1 too lean or is 22 degrees too little timing? I'll let you decide that one. Personally I like to burn less petrol to get to the same place because petrol is costly and adding timing is free, using less fuel to achieve the most efficient burn while completely eliminating shunting is always my objective.
Finally if you're still on the 14CUX the system will be constantly shooting for 14.7:1 at the dreaded shunt point (1,750rpm light throttle 43kPa), now that is lean and with the 14CUX its very hard to add fuel as it is with an aftermarket ECU so most people just live with it. The narrow band lambda sensors only see stioch so the AFRs on a 14CUX TVR will be up and down like a w
e's draws as the and slow responding ECU constantly tries to hit 14.7:1. So given it's not easy to add fuel, and given lean mixtures are harder to strike and burn slower that richer ones surely it's beneficial and a lot easier just to add some timing to compensate.
LPG as a fuel behaves in many ways just like a lean petrol mixture, LPG is harder to strike and burns slower than petrol so this is why we must add timing to compensate. If I run insufficient timing at low rpm and high load conditions I get a pronounced pop in my exhaust and a stumble in power deliver, this is a misfire. What's happening is I have lit my LPG too late, combustion is incomplete and continues as the the exhaust valve starts to open forcing combustion pressures into the exhaust... hense the POP! At low rpm my spark voltage is at its weakest and at high load my cylinder pressures are at their highest, so if I'm going to get a misfire it'll be when I dump the throttle open climbing a hill around 1,500-2,000rpm. At this point to eliminate the fault I could throw all the fuel I can at it and it'll have no effect whatsoever, what I need to do is simply light the charge of LPG and air earlier by advancing my timing by a few degrees.
My point is timing and AFR are unavoidably linked and must work hand in hand, if you cant easily change the super lean 14.7:1 AFR your 14CUX runs at in the shunting zone all you need to do is light the inappropriately lean charge of air and petrol a little earlier, this is why for those still running the largely un-tunable and lean running 14CUX and distributor system may benefit from experimented with applying a full vacuum signal to their vac advance unit and so deliver the additional advance the lean mixture demands in the shunting zone.
Trust me the 14.7:1 the 14CUX forces the engine to run at 99% of the time is lean, read up on the behaviors of lean mixtures and study the condition known as 'Lean Misfire', you'll find running more timing is always the answer especially if you can't make thinks a bit a richer.
1. This is not a mod, its simply going back to the 18 degrees of idle most V8s ran in the 60's
2. Going full vacuum is mostly about improving the way the engine idles by giving the engine the timing it wants
3. Not only will 18 degrees give you a much more stable idle, you will reduce all that shocking heat in the exhaust manifolds which offers many benefits especially given the TVR manifold design
4. On the shunting point its not so much about adding timing in the shunting zone its about how many degrees of advance the ignition system must add to get you to the right number
On point four, one of the first things you learn early on when playing with ignition tables is big jumps in timing are bad news, you need to get from wherever you are on the table to your other timing numbers on the table (up or down) as smoothly as possible, from idle to 1,750rpm it stands to reason the easiest way to achieve this is to narrow the gap, and easiest way to do that is start from a higher idle timing number like 18 degrees, which is handy as 18-20 degrees is exactly what old Rover likes to idle at.
Lets say for arguments sake the target optimal timing figure at 1,750rpm at a smallish throttle opening produces an engine load figure of 44kPa and your optimal target timing figure at that point is 25 degrees, if you're idling at 10 degrees the distributor has to add 15 degrees from 1,000rpm to 1,750rpm which is a lot of timing to add in just 750rpm. On the other hand if you start at 18 degrees you only need to add 7 degrees to get to your magic 25, the 750rpm window is the same, believe me... with low engine speed ignition timing a jump of 7 degrees is a lot less than 15.
Of course it doesn't actually work like that, what happens with a full vacuum strategy is you start at 18 degrees idle then lose all vacuum as you first crack open the throttle and pull away. However, with full vacuum applied to the vac advance unit things are much more consistent than this sounds, even at low load & low engine speeds where shunting resides when you lift of the throttle to regulate your slow speed progress in urban situations some of your timing will be coming from centrifugal advance and some will come from the vac advance unit, in effect the two work together and in on/off throttle speed regulating situations such as urban driving the engine is way smoother. Those who suffer shunting subconsciously know a fixed throttle angle helps maintain smooth slow speed progress, they avoid on/off shifts in throttle position at all costs because that's the fasted and most effective way to initiate the shunt.
In busy urban driving conditions that lets face it we all must suffer on this little crowded island of ours, no sooner have you gathered momentum and found yourself moving at 25mph than you find yourself needing to slow to regulate progress in line with other road users who will be slowing to turn, stopping at junctions or slowing for a cyclist and then accelerating after they've passed ect ect ect. So if you study it you'll find urban driving is all about delicately modulating speed using the throttle and in doing so you are constantly opening and closing the throttle butterfly by tiny amounts and so creating more and less vacuum. With ported vacuum you might get a degree or two on top of centrifugal advance at certain throttle angles best, but this is soon lost with the smallest twitch of your big toe, the ported vacuum signal is so weak, inconsistent and on/off at this point it'll probably be fluttering the vac unit which is not good for smooth progress as timing will be added and taken away in a very irregular and inconsistent way.
I've heard many tales of people reducing shunting by disconnected their vac advance unit, all they are doing is removing that flutter the weak on/off ported vacuum signal gives around 1,750rpm right when the driver finds he needs to keep changing the throttle angle to regulate his road speed through town. The truth is I've run my Chimaera at the shunting point at many different timing settings from 18 degrees to 32 degrees and the car drives perfectly at these figures... and anything in between, what makes the car shunt is when I try to shift the timing number up in big jumps from idle to the 1,750rpm figure.
Big jumps in timing are always going to give drivability issues, if you start at 18 degrees idle then run 22 degrees at 1,750rpm and find the car is smoother than when you ran 29 degrees at 1,750rpm... you haven't proved 22 degrees is what the engine wants, you've just narrowed the gap. 18-22 degrees is a small gap of just 4 degrees which allows you to create a nice even and smooth transition, 18-29 is a jump of 11 degrees and from 1,000rpm to 1,750rpm is just a 750rpm window to get there, so the steps must be more dramatic and so the risk of a shunt becomes greater.
Like I say, you haven't discovered less timing (22 degrees) in the shunting zone is better than 29 degrees, all you've done is reduced the big 11 degree jump in timing you had when you ran 29 degrees at 1,750rpm. Slowing the rate of attack by simply reducing the gap between idle timing and optimum timing at 1,750rpm (25 degrees) is the key to a smooth TVR, switching to full vacuum and idling at 18 degrees closes the gap because even though you lose vacuum when your first pull away.... rpms are already on the way up so centrifugal force takes over, now come off the throttle and the vac advance unit connected to full vacuum steps back in to cushion the fall..... unlike ported vacuum which tends to see you falling in a low number timing hole.
Try some of this on/off throttle low speed urban driving running ported vacuum, then try the same route using same on/off throttle driving style with full vacuum applied to the vac advance unit instead.... and see which setup you prefer
. Applying full vacuum to the vac advance unit is just part of a package of things that can help, however, people should apply caution and common sense when experimenting with this strategy because at the end of the day the Lucas distributor and vac unit were designed to work with ported vacuum.A better solution would come in in the form of an MSD distributor with an adjustable vac advance unit and customisable advance curves, but by the time you've bought one.... for a few quid more you could have fitted a Canems, MegaJolt or Omex 200 mappable ignition only system which will always be infinitely better than any distributor, even a tunable MSD one
Finally you can improve shunting by adding fuel if you have that facility, this works because richer mixtures are easier to ignite and (up to a point) burn better than lean mixtures. Because rich mixtures are easier to ignite you can run less timing, if you run a leaner mixture it will need to be lit earlier to ensure complete combustion simply because it's both harder to strike and takes longer to burn completely. If you cover the shunt in fuel you can indeed run a lower timing number, indeed it'll probably demand it, but it's not the best strategy in my opinion.
The better approach is to find the best balance between AFR and timing, which at 1,750 on petrol on my 4.0 litre running a Stealth cam but no head work is 14.3:1 @ 25 degrees. People may see this as lean but there is absolutely no shunting on my car like this, if I run 22 degrees @ 14.3:1 combustion is fractionally incomplete before the exhaust valve starts to open so micro misfires exist, and the engine is not its smoothest, if I leave timing at 22 degrees I must richen the AFR to 13.5:1 or richer to solve the problem.
So my question is.... is 14.3:1 too lean or is 22 degrees too little timing? I'll let you decide that one. Personally I like to burn less petrol to get to the same place because petrol is costly and adding timing is free, using less fuel to achieve the most efficient burn while completely eliminating shunting is always my objective.
Finally if you're still on the 14CUX the system will be constantly shooting for 14.7:1 at the dreaded shunt point (1,750rpm light throttle 43kPa), now that is lean and with the 14CUX its very hard to add fuel as it is with an aftermarket ECU so most people just live with it. The narrow band lambda sensors only see stioch so the AFRs on a 14CUX TVR will be up and down like a w
e's draws as the and slow responding ECU constantly tries to hit 14.7:1. So given it's not easy to add fuel, and given lean mixtures are harder to strike and burn slower that richer ones surely it's beneficial and a lot easier just to add some timing to compensate.LPG as a fuel behaves in many ways just like a lean petrol mixture, LPG is harder to strike and burns slower than petrol so this is why we must add timing to compensate. If I run insufficient timing at low rpm and high load conditions I get a pronounced pop in my exhaust and a stumble in power deliver, this is a misfire. What's happening is I have lit my LPG too late, combustion is incomplete and continues as the the exhaust valve starts to open forcing combustion pressures into the exhaust... hense the POP! At low rpm my spark voltage is at its weakest and at high load my cylinder pressures are at their highest, so if I'm going to get a misfire it'll be when I dump the throttle open climbing a hill around 1,500-2,000rpm. At this point to eliminate the fault I could throw all the fuel I can at it and it'll have no effect whatsoever, what I need to do is simply light the charge of LPG and air earlier by advancing my timing by a few degrees.
My point is timing and AFR are unavoidably linked and must work hand in hand, if you cant easily change the super lean 14.7:1 AFR your 14CUX runs at in the shunting zone all you need to do is light the inappropriately lean charge of air and petrol a little earlier, this is why for those still running the largely un-tunable and lean running 14CUX and distributor system may benefit from experimented with applying a full vacuum signal to their vac advance unit and so deliver the additional advance the lean mixture demands in the shunting zone.
Trust me the 14.7:1 the 14CUX forces the engine to run at 99% of the time is lean, read up on the behaviors of lean mixtures and study the condition known as 'Lean Misfire', you'll find running more timing is always the answer especially if you can't make thinks a bit a richer.
Sardonicus said:
If you add advance in the lurch/shunt zone (below 2k RPM ish) then I got bad news for you 
.... it will get even worse IMO part of the masking this problem with an after-market ECU for example is to richen this area a little and pull advance out not add more 
Simon, with respect I beg to differ, but it may be we're just talking a cross proposes, so to explain my argument we must first try to put ourselves in the position of someone still running the 14CUX, for these guys adding fuel isn't the easy option it is for us mate. .... it will get even worse IMO part of the masking this problem with an after-market ECU for example is to richen this area a little and pull advance out not add more As an experiment set your MegaSquirt to run at the nasty lean 14.7:1 a 14CUX Chim is forced to run at 99% of the time including the shunting zone, run it like this with 22 degrees of timing in the shunting zone and then again at 25/6 degrees, do not touch your fuel table, keep your AFR nailed at 14.6:1 and tell me what timing number works best.
Keep in mind the 14CUX while shooting for it's already excessively lean 14.7:1 target will actually be overshooting 14.7:1 regularly, the lurching rich lean behavior of narrow band lambda sensors combined with a slow to respond ECU makes for significant periods where the true AFR will actually be hitting 15.5:1 or leaner, especially at lightish load in the lower RPMs ie the shunting zone.
Obviously we all know that lean mixtures are harder to strike and burn more slowly than rich mixtures, this is partly why we must advance our ignition timing when we're in light load leaner cruising conditions, engine speed is another reason as all fuels burn at a fixed speed but lean mixtures definitely demand more timing too. If you're running leaner mixtures you absolutely must initiate combustion earlier to achieve an efficient and complete burn, with a 14CUX momentarily lurching AFRs up to a potential crazy lean 15.5:1 it's no wonder these cars shunt, now if you throw in a vacuum leak (even a tiny one) the situation will be leaner and so worse still.
Of course as you quite rightly point our the other way to solve the situation is to add fuel at which point it may well be advisable to retard not advance your timing, easy for us but of for the 14CUX users. This is what I mean about speaking at cross purposes Simon, what came first for you.... adding fuel or pulling timing? I think you get what I'm saying mate. The thing we must always keep in mind is laying fuel over the shunt is not really a very practical option for those still on the 14CUX, they are stuck with the overly lean situation so the owners of standard Chimaeras should firstly make sure their ignition system is in tip top condition, then look the eliminate every single tiny vacuum leak that exists.
At this point we need to accept both the idle air control valve system (stepper motor) and the crude un-valved crankcase ventilation system used on our cars are really both just vacuum leaks themselves. While these are intentional vacuum leaks added by the designer, make no mistake they are vacuum leaks all the same. All vacuum leaks (rouge or intentional) are a bad thing for smooth internal combustion engine operation, this was one the first lessons drummed into me when I completed my formal training on the subject all those years ago, and further beaten into be by the clever old boys who I worked with in restoration back in the early 90's.
So even with a fit but still rather weak original distributor and canister coil ignition system working to its best, and all vacuum leaks completely eliminated, the 14CUX crowd must accept while they could really still do with throwing some fuel at the shunt the system they have simply wont let them, honestly at this point they would would be better off with a carb.
So, what to do with that overly lean situation Land Rover inflicted on the old Rover V8 to try and make the ancient engine a little cleaner? Well, as we now know leaner mixtures are harder to strike and burn slower, so the next best strategy has to be to add some timing to light that lean fire a little earlier. And the easiest way to do this at idle is to switch from the feeble ported vacuum signal to full vacuum just as all Yank V8s ran in the 1960's before all the emissions nonsense gave birth to the terrible retarded timing strategy that is ported vacuum.
However, if you do try this strategy remember the Lucas distributor a vac advance unit fitted to our cars was always designed to work with ported vacuum, while my theory is sound it still needs someone to carefully test it to see if it works with the standard dizzy and vac advance unit. Please keep in mind detonation is a very bad thing so I hold no responsibility for engine damage the change may or may not deliver, saying that I've done enough testing with my Canems equipped Chimaera to prove this essentially low compression RV8 engine is very resistant to detonation. For example, my Chimaera will happily pull away under load on petrol with 28 degrees of timing with zero detonation issues, and as the standard distributor is all done advancing at 28 degrees people should take reassurance from my tests that the experiment is not really that risky at all.
Anyone experimenting with moving from a ported to a full vacuum signal applied to their vac advance unit should first establish if their vac advance is actually working in the first place (failures are common), they should then use a timing light at idle (no load) to see how many degrees at idle the new strategy truly adds. Finally if the engine is going to pink, (and I doubt it will) it'll be just off idle as you pull away under load, so turn up your ears and abort if hear the onset of that well know sound.
Who will be the first to try it?
ChimpOnGas said:
Sardonicus said:
If you add advance in the lurch/shunt zone (below 2k RPM ish) then I got bad news for you .... it will get even worse IMO part of the masking this problem with an after-market ECU for example is to richen this area a little and pull advance out not add more
Simon, with respect I beg to differ, but it may be we're just talking a cross proposes, so to explain my argument we must first try to put ourselves in the position of someone still running the 14CUX, for these guys adding fuel isn't the easy option it is for us mate. .... it will get even worse IMO part of the masking this problem with an after-market ECU for example is to richen this area a little and pull advance out not add more As an experiment set your MegaSquirt to run at the nasty lean 14.7:1 a 14CUX Chim is forced to run at 99% of the time including the shunting zone, run it like this with 22 degrees of timing in the shunting zone and then again at 25/6 degrees, do not touch your fuel table, keep your AFR nailed at 14.6:1 and tell me what timing number works best.
t , I experimented over a long period and settled on low 20's BTDC and the mentioned AFR's above gives a good compromise but lets face it 2 completely different RV8's IMO your is running efficiently around 2k RPM mine is well off cam and it spits its dummy inc burping spent exhaust gases back into the plenum only for the cycle to start all over again 
that I have no doubt I am happy with the result considering it was never going to run sublime for obvious reasons 
this is why I plan to go full seq at some point although not a priority at the moment TVR's fettling is way off my radar at the moment with this tuning lark whats good for the goose is not always .................. well you know the rest 
EDITED just to say on the plenty of advance front at idle spot on I run around 20 IIRC less makes the cam lope more with the same RPM (no purpose) and more gives no more benefit right up to 25 BTDC then things go down hill from there not betterEdited by Sardonicus on Thursday 18th January 15:21
This is what I mean Simon, your engine is a very different animal to mine and the majority of RV8s in Chims, respect to you for building a screamer but big cams demand different strategies, unlike the 14CUX crowd you're also not forced to run at an excessively lean 14.7:1 AFR in the shunting zone, there are clearly big differences between your situation and the majority of Chimaera owners.
What I'm trying to do here is talk to the majority, and the majority run a fairly stock RV8 compared to yours and most are still on the 14CUX which means they'll be burdened with an inappropriately lean 14.7:1 AFR in the shunting zone, it's also very hard for these folk to add the extra fuel needed to correct the situation.
My point is while the people in the 14CUX camp may not be able to easily add fuel, they can burn the lean condition imposed on them more effectively by advancing their timing. Obviously doing so by simply turning the distributor to give the desired 18 degrees at idle is a bad idea because by the time the engine is spinning at 2,800rpm they will be running 18 degrees at idle plus 18 degrees of mechanical advance from the distributor. At 2,800rpm and above under full load this will deliver 36 degrees of timing which is a couple of degrees more than many people may feel comfortable with.
However if they set their idle timing at 12 degrees, then applied a full vacuum signal to their vac advance unit it will add a further 6 degrees to deliver 12 + 6 = the ideal 18 degrees at idle, ported vacuum at idle is simply too weak to do this which is the very reason it was used. Land Rover wanted the engine to idle at the retarded timing figure of 10 degrees because it helped them get idle emissions down on what was/is a very dirty inefficient engine design by modern standards. Obviously under full load even with the full vacuum strategy the vac advance unit will be adding nothing, so the peak timing number under these conditions will top out at a nice safe 30 degrees. Following the acceleration event and as the driver starts to close the throttle to cruise on the motorway around 80mph & 3,000rpm the vac unit will come into play once more, again there will be 30 degrees of mechanical advance but this time also 12 degrees of vacuum imitated advance, 30 + 12 = 42 degrees to deliver improved cruising fuel economy. To be fair ported vacuum gives you this 42 degrees ar cruise too, so switching to full vacuum is really all about a better idle, less heat in the exhaust ports at idle... and potentially better drivability in the shunting zone too.
This is why I'm suggested it might be worth owners of standard Chimaeras with the lean running 14CUX and distributor experimenting with switching from ported vacuum to full vacuum applied to their vac advance, its hard to add fuel with the 14CUX but you can use vacuum to give you the safe 18 degrees idle the engine wants without any risk of pushing total timing beyond safe limits under hard acceleration.
The only possible downside to the idea I can see is emissions will go up at idle, but I'd hazard a guess most TVR owners will prioritize a smooth idle, lower exhaust manifold heat and a generally smoother driving car in the shunting zone over a small increase in tailpipe emissions?
I know I do
What I'm trying to do here is talk to the majority, and the majority run a fairly stock RV8 compared to yours and most are still on the 14CUX which means they'll be burdened with an inappropriately lean 14.7:1 AFR in the shunting zone, it's also very hard for these folk to add the extra fuel needed to correct the situation.
My point is while the people in the 14CUX camp may not be able to easily add fuel, they can burn the lean condition imposed on them more effectively by advancing their timing. Obviously doing so by simply turning the distributor to give the desired 18 degrees at idle is a bad idea because by the time the engine is spinning at 2,800rpm they will be running 18 degrees at idle plus 18 degrees of mechanical advance from the distributor. At 2,800rpm and above under full load this will deliver 36 degrees of timing which is a couple of degrees more than many people may feel comfortable with.
However if they set their idle timing at 12 degrees, then applied a full vacuum signal to their vac advance unit it will add a further 6 degrees to deliver 12 + 6 = the ideal 18 degrees at idle, ported vacuum at idle is simply too weak to do this which is the very reason it was used. Land Rover wanted the engine to idle at the retarded timing figure of 10 degrees because it helped them get idle emissions down on what was/is a very dirty inefficient engine design by modern standards. Obviously under full load even with the full vacuum strategy the vac advance unit will be adding nothing, so the peak timing number under these conditions will top out at a nice safe 30 degrees. Following the acceleration event and as the driver starts to close the throttle to cruise on the motorway around 80mph & 3,000rpm the vac unit will come into play once more, again there will be 30 degrees of mechanical advance but this time also 12 degrees of vacuum imitated advance, 30 + 12 = 42 degrees to deliver improved cruising fuel economy. To be fair ported vacuum gives you this 42 degrees ar cruise too, so switching to full vacuum is really all about a better idle, less heat in the exhaust ports at idle... and potentially better drivability in the shunting zone too.
This is why I'm suggested it might be worth owners of standard Chimaeras with the lean running 14CUX and distributor experimenting with switching from ported vacuum to full vacuum applied to their vac advance, its hard to add fuel with the 14CUX but you can use vacuum to give you the safe 18 degrees idle the engine wants without any risk of pushing total timing beyond safe limits under hard acceleration.
The only possible downside to the idea I can see is emissions will go up at idle, but I'd hazard a guess most TVR owners will prioritize a smooth idle, lower exhaust manifold heat and a generally smoother driving car in the shunting zone over a small increase in tailpipe emissions?
I know I do
ChimpOnGas said:
This is what I mean Simon, your engine is a very different animal to mine and the majority of RV8s in Chims, respect to you for building a screamer but big cams demand different strategies, unlike the 14CUX crowd you're also not forced to run at an excessively lean 14.7:1 AFR in the shunting zone, there are clearly big differences between your situation and the majority of Chimaera owners.
What I'm trying to do here is talk to the majority, and the majority run a fairly stock RV8 compared to yours and most are still on the 14CUX which means they'll be burdened with an inappropriately lean 14.7:1 AFR in the shunting zone, it's also very hard for these folk to add the extra fuel needed to correct the situation.
My point is while the people in the 14CUX camp may not be able to easily add fuel, they can burn the lean condition imposed on them more effectively by advancing their timing. Obviously doing so by simply turning the distributor to give the desired 18 degrees at idle is a bad idea because by the time the engine is spinning at 2,800rpm they will be running 18 degrees at idle plus 18 degrees of mechanical advance from the distributor. At 2,800rpm and above under full load this will deliver 36 degrees of timing which is a couple of degrees more than many people may feel comfortable with.
However if they set their idle timing at 12 degrees, then applied a full vacuum signal to their vac advance unit it will add a further 6 degrees to deliver 12 + 6 = the ideal 18 degrees at idle, ported vacuum at idle is simply too weak to do this which is the very reason it was used. Land Rover wanted the engine to idle at the retarded timing figure of 10 degrees because it helped them get idle emissions down on what was/is a very dirty inefficient engine design by modern standards. Obviously under full load even with the full vacuum strategy the vac advance unit will be adding nothing, so the peak timing number under these conditions will top out at a nice safe 30 degrees. Following the acceleration event and as the driver starts to close the throttle to cruise on the motorway around 80mph & 3,000rpm the vac unit will come into play once more, again there will be 30 degrees of mechanical advance but this time also 12 degrees of vacuum imitated advance, 30 + 12 = 42 degrees to deliver improved cruising fuel economy. To be fair ported vacuum gives you this 42 degrees ar cruise too, so switching to full vacuum is really all about a better idle, less heat in the exhaust ports at idle... and potentially better drivability in the shunting zone too.
This is why I'm suggested it might be worth owners of standard Chimaeras with the lean running 14CUX and distributor experimenting with switching from ported vacuum to full vacuum applied to their vac advance, its hard to add fuel with the 14CUX but you can use vacuum to give you the safe 18 degrees idle the engine wants without any risk of pushing total timing beyond safe limits under hard acceleration.
The only possible downside to the idea I can see is emissions will go up at idle, but I'd hazard a guess most TVR owners will prioritize a smooth idle, lower exhaust manifold heat and a generally smoother driving car in the shunting zone over a small increase in tailpipe emissions? I know I do
Even pre build a richer mix and less timing still worked best thats my point What I'm trying to do here is talk to the majority, and the majority run a fairly stock RV8 compared to yours and most are still on the 14CUX which means they'll be burdened with an inappropriately lean 14.7:1 AFR in the shunting zone, it's also very hard for these folk to add the extra fuel needed to correct the situation.
My point is while the people in the 14CUX camp may not be able to easily add fuel, they can burn the lean condition imposed on them more effectively by advancing their timing. Obviously doing so by simply turning the distributor to give the desired 18 degrees at idle is a bad idea because by the time the engine is spinning at 2,800rpm they will be running 18 degrees at idle plus 18 degrees of mechanical advance from the distributor. At 2,800rpm and above under full load this will deliver 36 degrees of timing which is a couple of degrees more than many people may feel comfortable with.
However if they set their idle timing at 12 degrees, then applied a full vacuum signal to their vac advance unit it will add a further 6 degrees to deliver 12 + 6 = the ideal 18 degrees at idle, ported vacuum at idle is simply too weak to do this which is the very reason it was used. Land Rover wanted the engine to idle at the retarded timing figure of 10 degrees because it helped them get idle emissions down on what was/is a very dirty inefficient engine design by modern standards. Obviously under full load even with the full vacuum strategy the vac advance unit will be adding nothing, so the peak timing number under these conditions will top out at a nice safe 30 degrees. Following the acceleration event and as the driver starts to close the throttle to cruise on the motorway around 80mph & 3,000rpm the vac unit will come into play once more, again there will be 30 degrees of mechanical advance but this time also 12 degrees of vacuum imitated advance, 30 + 12 = 42 degrees to deliver improved cruising fuel economy. To be fair ported vacuum gives you this 42 degrees ar cruise too, so switching to full vacuum is really all about a better idle, less heat in the exhaust ports at idle... and potentially better drivability in the shunting zone too.
This is why I'm suggested it might be worth owners of standard Chimaeras with the lean running 14CUX and distributor experimenting with switching from ported vacuum to full vacuum applied to their vac advance, its hard to add fuel with the 14CUX but you can use vacuum to give you the safe 18 degrees idle the engine wants without any risk of pushing total timing beyond safe limits under hard acceleration.
The only possible downside to the idea I can see is emissions will go up at idle, but I'd hazard a guess most TVR owners will prioritize a smooth idle, lower exhaust manifold heat and a generally smoother driving car in the shunting zone over a small increase in tailpipe emissions? I know I do
Edited by ChimpOnGas on Thursday 18th January 18:33
it was a stock 4.0 HC still had the 500 spec cam pre-op , obviously I am not comparing what I have now to your average Joe that would not be a good comparison Dave anyway 
Sardonicus said:
ven pre build a richer mix and less timing still worked best thats my point it was a stock 4.0 HC still had the 500 spec cam pre-op , obviously I am not comparing what I have now to your average Joe that would not be a good comparison Dave anyway
Mate, clearly going richer is the answer, but that's not my point...... what I'm saying is 14CUXers will find adding fuel very tricky indeed. it was a stock 4.0 HC still had the 500 spec cam pre-op , obviously I am not comparing what I have now to your average Joe that would not be a good comparison Dave anyway Leaner petrol mixtures are just like LPG, they are harder to strike and burn slower than rich petrol mixtures.... so it you're a 14CUXer and stuck with 14.7:1 advancing the timing to light the fire earlier is the way forward. I must do the same when burning LPG or suffer the pop in my exhaust when accelerating at lower RPMs up a hill, this is a lean misfire or if you look at it another way.... insufficient timing!
Throw a bunch of fuel at the shunt as you and I can (I did it too mate), and the situation changes dramatically, so lets forget that for a minute, if you can't add fuel because you're locked out of the ECU switching to a full vacuum signal lets you light that lean 14CUX fire earlier and right when you need to.
The idea is clearly for 14CUXers.... not us!
I’m just glad I’ve got the best drive in/ out after market Ecu available to the Tvr family for driving in then driving out once and no tweaks needed 
It’s called MBE
Dave doesn’t seem to have heard of it
I have no idea if your right about this but it all sounds plausible to my unsophisticated mind.
I can see what your trying to say, fair play Dave
It’s called MBE
Dave doesn’t seem to have heard of it
I have no idea if your right about this but it all sounds plausible to my unsophisticated mind.
I can see what your trying to say, fair play Dave
Edited by Classic Chim on Thursday 18th January 20:48
just a thought why is it that lots of chims dont shunt whilst still running the 14cux my old maroon one had over 100.000 miles on the clock never shunted plus i am working at present on a 4.0ltrhc completely standard not a hint of shunt so is it possible there is an explanation for shunting other than the original design because not all do it.
john
john
Engineer1949 said:
just a thought why is it that lots of chims dont shunt whilst still running the 14cux my old maroon one had over 100.000 miles on the clock never shunted plus i am working at present on a 4.0ltrhc completely standard not a hint of shunt so is it possible there is an explanation for shunting other than the original design because not all do it.
john
It's a good point John, and one I was careful to make earlier in this post.john
ChimpOnGas said:
Firstly it's important to remind ourselves the 14CUX/Distributor system worked fine from new, while there are special considerations in the TVR application that include additional heat, the extra stress this puts on ignition components, the higher lift cams used and the generally poor wiring standards at Blackpool, even new Chims and Griffs ran nicely. Shunting is therefore a fault not a trait that should be accepted as the norm..
My money is on a combination of things.... ChimpOnGas said:
1. Induction leaks
On any internal combustion engine all induction/vacuum leaks are bad, they are especially bad news when the injection system is measuring the mass of air entering the engine using an air flow meter (AFM), if the engine receives air that has bypassed the AFM the ECU has no knowledge of its prescience until it's too late. It will be detected in the form of a high oxygen content in the exhaust gasses so the ECU will try to add fuel to correct the situation, sadly the 14CUX uses narrow band lambda sensors which without getting too technical are slow and imprecise. The processing power of the ECU is also comically slow even by 90's engine management standards, so the ECU is getting poor quality knowledge of the induction leak from the oxygen content in exhaust which is obviously out of data as it is post combustion... and then, implements it's correction strategy very slowly. This is where you get the cycle of rich/lean shunting, you feel this as an on/off power delivery at low/mid load between 1700-1850rpm.
2. Ignition timing & Exhaust Heat
The Range Rover ignition system was designed to run the engine's exhaust ports hot to reduce emissions and light off the cats, the ten degrees of timing at idle mid 80's Range Rovers and mid 90's Chims/Griffs run at is not really what an old push rod V8 with poor combustion chamber design wants. This retarded timing strategy was developed and used by the Yanks in the 70's as a sticking plaster attempt to extend the life of ancient V8 power plants and Land Rover followed suit in the 80's, go back to the 60's and all Yank V8s ran full vacuum to the vac advance unit and 15-18 degrees of timing at idle. All the heat from this retarded timing strategy is bad enough on a Range Rover but on a TVR Chim/Griff with it's forward facing exhaust manifolds it's very hard on HT leads indeed, one cooked HT lead can easily be enough to promote a misfire and stimulate the dreaded shunt.
3. The idle air control valve (stepper motor)
Often seen as shunting criminal number one the stepper motor in truth is really just part of the problem, and may not be the biggest criminal at all. The stepper motor simply introduces air to manage idle, but as we all know on an internal combustion engine all vacuum leaks are bad, even on my Bosch idle valve equipped Canems system it's always best to manage idle with scatter spark ignition timing management and limit the effects of the idle air control valve. Obviously the stepper motor on a standard Chimaera is a dumb device, it's behaviour is managed by the 14CUX ECU which is only as good as the data it receives from various sensors. This is true for fuel metering too, but is especially critical when you consider the idle strategy needs the ECU to see a road speed signal that in the case of Griffs & Chims comes from a box of electronics made by Dorris on a Friday afternoon in a shed in Blackpool, it is not reliable!
4. The crankcase ventilation system
Take a close look at the crankcase ventilation system on Range Rovers and TVRs during the 14CUX period and you'll see it is un-valved, it is whats known as a passive system, this idea didn't last long as all manufacturers realised fitting a PCV valve and so creating a positive crank ventilation system was far superior. The un-valved system we are burdened with is really just another unregulated vacuum leak, crank case gasses are nasty but still contain oxygen, these are burnt by the engine but do not pass through the AFM so their quantity and influence are only detected post combustion by the narrow band lambda sensors (see point 3) and rememberon an internal combustion engine all vacuum leaks are bad.
It's also worth reminding ourselves the quality of wiring on these cars is not great, and issues associated with poor earths are common, if the car is suffering from a weak spark and at the same time is running lean (which lets face it is what the 14CUX does) then you're going to suffer a lean misfire under certain conditions. You can definitely run the engine at 14.7:1 on petrol in the shunting zone and get away with it (JUST!), I've done it, but even with wasted spark, twin MSD coil packs, MSD Super Conductor HT leads, no plug extenders, Iridium number 6 plugs and all vacuum leaks rigorously eliminated the engine is still right on the edge of shunting at such lean mixtures.On my Canems system I have set a lean limit in the shunting zone of 14.0:1, it'll will briefly hit 14.3:1 in this area of the map but typically the engine is held at 14.0:1 or fractionally richer, given the tunability an after market ECU gives us no body in their right mind would run the old Rover V8 at the 14CUX 14.7:1 lean AFR... 14.7:1 is definitely right of the very edge of the leanest AFR the engine will cope with at 1,750rpm at light load before it starts to protest.On any internal combustion engine all induction/vacuum leaks are bad, they are especially bad news when the injection system is measuring the mass of air entering the engine using an air flow meter (AFM), if the engine receives air that has bypassed the AFM the ECU has no knowledge of its prescience until it's too late. It will be detected in the form of a high oxygen content in the exhaust gasses so the ECU will try to add fuel to correct the situation, sadly the 14CUX uses narrow band lambda sensors which without getting too technical are slow and imprecise. The processing power of the ECU is also comically slow even by 90's engine management standards, so the ECU is getting poor quality knowledge of the induction leak from the oxygen content in exhaust which is obviously out of data as it is post combustion... and then, implements it's correction strategy very slowly. This is where you get the cycle of rich/lean shunting, you feel this as an on/off power delivery at low/mid load between 1700-1850rpm.
2. Ignition timing & Exhaust Heat
The Range Rover ignition system was designed to run the engine's exhaust ports hot to reduce emissions and light off the cats, the ten degrees of timing at idle mid 80's Range Rovers and mid 90's Chims/Griffs run at is not really what an old push rod V8 with poor combustion chamber design wants. This retarded timing strategy was developed and used by the Yanks in the 70's as a sticking plaster attempt to extend the life of ancient V8 power plants and Land Rover followed suit in the 80's, go back to the 60's and all Yank V8s ran full vacuum to the vac advance unit and 15-18 degrees of timing at idle. All the heat from this retarded timing strategy is bad enough on a Range Rover but on a TVR Chim/Griff with it's forward facing exhaust manifolds it's very hard on HT leads indeed, one cooked HT lead can easily be enough to promote a misfire and stimulate the dreaded shunt.
3. The idle air control valve (stepper motor)
Often seen as shunting criminal number one the stepper motor in truth is really just part of the problem, and may not be the biggest criminal at all. The stepper motor simply introduces air to manage idle, but as we all know on an internal combustion engine all vacuum leaks are bad, even on my Bosch idle valve equipped Canems system it's always best to manage idle with scatter spark ignition timing management and limit the effects of the idle air control valve. Obviously the stepper motor on a standard Chimaera is a dumb device, it's behaviour is managed by the 14CUX ECU which is only as good as the data it receives from various sensors. This is true for fuel metering too, but is especially critical when you consider the idle strategy needs the ECU to see a road speed signal that in the case of Griffs & Chims comes from a box of electronics made by Dorris on a Friday afternoon in a shed in Blackpool, it is not reliable!
4. The crankcase ventilation system
Take a close look at the crankcase ventilation system on Range Rovers and TVRs during the 14CUX period and you'll see it is un-valved, it is whats known as a passive system, this idea didn't last long as all manufacturers realised fitting a PCV valve and so creating a positive crank ventilation system was far superior. The un-valved system we are burdened with is really just another unregulated vacuum leak, crank case gasses are nasty but still contain oxygen, these are burnt by the engine but do not pass through the AFM so their quantity and influence are only detected post combustion by the narrow band lambda sensors (see point 3) and rememberon an internal combustion engine all vacuum leaks are bad.
Even if you're running the original single coil and distributor ignition system and its earthed correctly, and all the other ignition components are in 100% perfect order... then you'll still get away with 14.7:1. However, if anything on the ignition side is even slightly marginal you're going to have a weaker than ideal spark and you'll struggle to effectively burn the lean mixtures the 14CUX imposes on our cars, especially with the ported vacuum retarded idle timing strategy inflicted on the engine. Now throw in a stepper motor that's allowing a small bleed of air into the engine when it shouldn't be and you could easily find yourself trying to drive in the shunting zone at AFRs as lean as 16.0:1. At this point its worth pointing out if I tune my Canems system so I'm running 15:1 or leaner at 1,750rpm, the car will shunt, no question!
Shunting is a lean misfire, or more accurately... a lean condition of 14.7:1 the 14CUX forces the engine to run at combined with insufficient ignition timing to burn that lean mixture completely. The problem is exacerbated by the rich/lean lurching behavior of the narrow band lambda sensors and slow responding ECU working with them. If the ECU detects a lean condition it will try to correct the situation by throwing in a big dollop of fuel, at which point the ECU will detect a rich condition and pull a big slice of fuel out. The poor engine is kangarooing down the road going Lean - Rich - Lean - Rich - Lean - Rich ect ect until you accelerate out of the cycling shunt condition.
If you're still on the 14CUX sadly you can't just lay fuel over the shunting zone as anyone with an Emerald, Megasquirt, Canems or MBE easily can, without professional help you're stuck with 14.7:1 so you better make sure you've eliminated every single vacuum leak and your ignition system is in tip top condition/correctly earthed or you're going to suffer the shunt. Range Rovers and Discos shunt from time to time too, and for the same reasons, its just a two ton 4x4 has a lot more inertia than 1060kg TVR so a big heavy Range Rover or Disco does a far better job of masking the dreaded shunt than the way lighter Chimaera can ever hope to.
My final point is if you're going to manage idle using an air bleed (idle air control valve), best you make sure your wiring and earths are in perfect order, idle valves be it a Lucas stepper motor or a Bosch rotary type idle valve (as used with the Canems system) seem to be very sensitive to EMI and poor earthing, I guess because they are very low amp devices. In my opinion all vacuum leaks are bad anyway, and an idle air control valve is just a managed vacuum leak, idle air control systems need to be working 100% perfectly or you're asking for trouble. You can see why the designers of the MBE system decided to completely do away with an idle air control valve in favor of managing idle speed using ignition timing only, if the wiring or earthing is anything other than perfect then idle air control valves can literally be more trouble than they're worth.
I still like my idle air control valve because it does a great job of increasing idle speed during the cold start warmup phase, and on the Canems system is also allows me to run an anti-stall strategy, both are excellent features that give OEM levels of driver convenience I would never be without. But in all other situations controlling idle speed with ignition timing is a way better idea than trying to do it with what is essentially a managed vacuum leak.
Edited by ChimpOnGas on Friday 19th January 09:06
Dave a rotary idle valve 2 or 3 wire should not be prone to EMI 
all grounding is driven by the ECU direct only the ign controlled live could possibly be susceptible to continuity problems and that lays with the installer anyway I have yet to see an install inc OE fitment for an idle control valve (rotary or pintle stepper) needed screened or twisted paired cables for example to cope with any interference agreed though re the lurching being down to a multitude of factors
all grounding is driven by the ECU direct only the ign controlled live could possibly be susceptible to continuity problems and that lays with the installer anyway I have yet to see an install inc OE fitment for an idle control valve (rotary or pintle stepper) needed screened or twisted paired cables for example to cope with any interference agreed though re the lurching being down to a multitude of factors Edited by Sardonicus on Friday 19th January 13:13
N7GTX said:
Sardonicus said:
agreed though re the lurching being down to a multitude of factors
Also, with several exhaust manifold gaskets blown, there is an air leak upsetting the balance? Curiously, the shunting is worst at 1800 rpm. 
I should have included this one on my original list
Exhaust leaks are bad news and will provide a false oxygen content signal to a lambda sensor, in closed loop an exhaust leak is as bad, if not worse, than a vacuum leak on the induction side.
Thanks for sharing this very valid point
ChimpOnGas - I sent you an e-mail regarding your fantastic theories, but in the meantime I have a question the answer to which may help us 14CUX'ers.
You say the vacuum pot is configured to ported vacuum, incidentally I plan on recording the results of moving the pipe from ported to full vacuum at idle and post the results to help support your ideas. Anyway, seeing as the RV8 has been around for a long time, is there not a distributor available that is configured for full vacuum and the ideal advance curve for the engine? I know Mallory do adjustable ones, but I'm thinking a cheap, bolt on, swap.
I've had a look but with all the variations over the years its rather overwhelming.
Did the SD1 run EFi? That was pre-emissions so presumably full vacuum? Same as pre-90's range rovers? Or were they all carb before emissions came in?
Stu
You say the vacuum pot is configured to ported vacuum, incidentally I plan on recording the results of moving the pipe from ported to full vacuum at idle and post the results to help support your ideas. Anyway, seeing as the RV8 has been around for a long time, is there not a distributor available that is configured for full vacuum and the ideal advance curve for the engine? I know Mallory do adjustable ones, but I'm thinking a cheap, bolt on, swap.
I've had a look but with all the variations over the years its rather overwhelming.
Did the SD1 run EFi? That was pre-emissions so presumably full vacuum? Same as pre-90's range rovers? Or were they all carb before emissions came in?
Stu
The 123 ignition system is a fully programmable plug in replacement distributor, so you do as you wish with the timing against RPM and vacuum, but its quite pricey, but very well made. Old school messing around with springs and bob weights to alter the advance curve is now obsolete, but still do able if you want to spend the time. Mallory was mechanical adjustments last time I looked for the RV8.
I have also tried to get the Aldon Amethyst system to work on the RV8 and locking the bob weights so effectively the dizzy becomes electronically mappable, but it suffered false triggering due to the way the ignition amp works. I think it could be made to work with an optical trigger, but I dont know any of the shelf units you can get, and something home made is not ideal as its such a critical component to get you home. A mechanical dizzy has a lot going for it in terms of reliability !
I have also tried to get the Aldon Amethyst system to work on the RV8 and locking the bob weights so effectively the dizzy becomes electronically mappable, but it suffered false triggering due to the way the ignition amp works. I think it could be made to work with an optical trigger, but I dont know any of the shelf units you can get, and something home made is not ideal as its such a critical component to get you home. A mechanical dizzy has a lot going for it in terms of reliability !
Edited by blitzracing on Wednesday 24th January 12:52
StuVT said:
Did the SD1 run EFi? That was pre-emissions so presumably full vacuum? Same as pre-90's range rovers? Or were they all carb before emissions came in?
Stu
My 92 range rover has what appears to be an identical distributer to my 92 Griff.Stu
Not checked part numbers or how they are set up, but they do look the same externally.
BIG DUNC said:
My 92 range rover has what appears to be an identical distributer to my 92 Griff.
Not checked part numbers or how they are set up, but they do look the same externally.
They are the same, and the advance curve is the same Range Rover modest too.Not checked part numbers or how they are set up, but they do look the same externally.
StuVT said:
ChimpOnGas - I sent you an e-mail regarding your fantastic theories, but in the meantime I have a question the answer to which may help us 14CUX'ers.
With respect and for the benefit of all its better to discuss this subject on an open forum rather than by PM.All I would add is it's easy to measure vacuum at the inlet manifold just as it's easy to experiment with applying the same amount of vacuum to the vac advance capsule, to do both all you need is:
1: A basic vacuum tester....
Such vacuum testers are not expensive and should be part of any home mechanics tool kit.
2: A timing light (preferably a dial back one)
Again, an essential part of any automotive took kit.
Use the vacuum gauge to record the amount of vacuum your engine produces from idle to WOT at peak RPM, be especially sure to accurately record the figures you see at idle (high vacuum low engine speed) and cruise (higher vacuum at higher engine speeds - greater vacuum), remember your engine is just a big pump trying to suck in as much air as it can.
When the throttle is closed or mostly closed your engine is still trying to inhale as much air as it can but because the door is closed the vacuum generated on the engine side of the throttle butterfly (in the plenum & inlet manifold) increases massively, it's this depression you're are looking to record. Of course at idle to door is almost shut but the engine speed is low, at cruise however the door is also mostly shut but engine speed is high so while in both examples the throttle positions will be similar obviously you can expect to record a higher vacuum figure at 2,800rpm cruise than you'll will when idling at 950rpm.
If you've carefully recorded your vacuum figures you can now artificially replicate them with your vacuum tester, connect the vacuum tester to the vac capsule and apply the same amount of vacuum your engine produced under different load conditions, do so with your timing light pointed at the crankshaft pulley and you can record the number of degrees of ignition advance your vacuum advance capture is applying under those different engine operating conditions. This is where a dial back timing light is much more convenient and accurate than a standard timing light because all you need to do is adjust it to put the mark back to TDC then the true no of degrees will be displayed on the screen.
Forgetting cruise conditions for a minute if your engine is largely standard you can easily run old Rover with as much as 36 degrees of advance at WOT perfectly safely, the standard setup runs 10 degrees at idle but because the distributor only gives roughly 18 degrees of mechanical advance on top of this and it's all done giving by 2,800rpm anyway you get 10 + 18 = 28 degrees of total mechanical timing which is mild to say the least, its certainly safe but also presents lots of scope for improvement as proved by anyone running an after market ECU.
Ignoring the vacuum advance capsule and what it does lets say we disconnected it and turn the distributor to give 18 degrees of advance at idle, like this the distributor can still only give you a further 18 degrees of mechanical advance. But this time as you've started at 18 you're now running a total timing figure of 36 degrees (18 + 18 = 36), which is much more like what typical hot rodders have run is SBC or SBFs at since the dawn of time to achieve best performance while still keeping things safely outside of the dreaded detonation zone. None of these guys have ever given two ships to emissions by the way and neither should they, and I'd hazard a guess most TVR owners are of a similar mind set.
Now, lets say you're not convinced and you're still not comfortable with running 36 degrees of total timing at WOT but you still want the better idle and the economy advantage of a bit of vacuum generated additional timing during cruising conditions, well just set your idle timing to 12 degrees with the vac advance disconnected, then connect it to full vacuum rather than the dreadful emission strategy ported vacuum our cars came with. Like this at idle you should get the 12 + 4 (ish) from the vac capsule so the car will idle smoother at a more suitable 16 degrees, it will however be more polluting.
With the vac capsule connected to full vacuum it'll still do nothing at WOT because with the throttle fully open there is little or no vacuum in the plenum, you'll get a super safe 12 + 18 = 30 degrees when you boot it... but now at cruise at 2,800rpm and above you'll enjoy 30 + 12 (ish) from the full vacuum connected vac advance capsule, so 42 degrees of economy improving timing right when you need it. Actually give or take a couple of degrees I've just described the standard setup because ported vacuum still gives you a pretty good suck on the vac capsule when cruising, what it can't do is provide much at idle like a solid and steady full vacuum signal will.
Keep in mind on petrol in the past I've run my Canems equipped Chimaera with as much as 36 degrees of total timing and 48 degrees at light load cruise with no issues whatsoever, of course more timing does not always make more power or fuel economy, this is why you really need a dyno to prove what timing numbers produces the best torque under different driving conditions and engine loads. But my point is even if you set your idle timing at 18 degrees (vac capsule disconnected) at WOT you'll still only see a safe 18 + 18 = 36... and when the vac capsule is re-connected and put to full vacuum you'll get and equally safe 18 + 18 + 12 (ish) from the vac capsule so 48 degrees at light load cruise.
The tests you conduct using your vacuum tester and dial back timing light are clearly very important, use this data to decide how you want to run your engine and reassure yourself your new mechanical and vacuum timing strategies remain detonation safe. I'll end by just saying it seems very likely to me given the emissions focused original setup and what I've learned from tuning my engine on the Canems system, with some moderate, careful and measured changes to ignition timing there's some power, economy and throttle response gains hidden within the original distributor setup too.
Just dont expect anything other than worse emissions if you do decide to run more timing at idle
Just found this....
http://www.superchevy.com/how-to/46178/
Well worth a read, it seems to support what I'm saying.
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