god damn shunt is killing me
Discussion
blitzracing said:
I had a quick look at the timing figures for the RV8 pre emissions, and they vary between TDC and 8', so it cant be simply having a late burn to reduce pollutants that the timing is set at these modest advance values.
But do they run full manifold vacuum or ported vacuum to the vac advance? I strongly suspect pre emissions RV8s ran a full manifold vacuum signal to the vac advance. In which case if you read the full instructions will almost certainly say something like... "set your ignition timing at 8 degrees at an idle speed of 850rpm with the vac advance disconnected!". Of course when you reconnect the vac advance it will add at least 4 degrees to give you 8 + 4 = 12 degrees at a lowly 850rpm 
Some people mistakenly disconnect their vac advance because they see race cars running like this and they think its a performance mod, this is of course total bollix. If you want your TVR to return it’s best fuel economy connecting your vacuum advance capsule and making sure it's working correctly is essential. With vac advance not only are you likely to gain 2-3 mpg at cruise but if you use a full vacuum signal the engine will idle far smoother and have less tendency to foul it's plugs, because it will idle on a leaner mixture properly so there is less overly wet charge passing through the system.
This brings us back to the subject of ported vacuum versus full manifold vacuum, take a look at your TVRs ported vacuum point and you'll see its located just above the throttle butterfly, this means at idle there is fek all vacuum signal and so no vacuum advance. This also means at idle the timing will be way retarded over what's ideal, it is only when the throttle is opened slightly that the butterfly goes past the vacuum port, at this point the port is now connected to manifold vacuum and so you suddenly get vac advance.
This tipping back and forth between zero vacuum and manifold vacuum as the throttle butterfly passes back and forth over the ported vacuum point on the throttle body is obviously going to rapidly add and subtract at least 4 degrees of timing, such violent swings in timing are definitely bad news for drivability. I'd put money on the throttle butterfly passing back and forth over the ported vacuum point when you are modulating your slow road speed which is typically when you're driving through town between 1,650-1850rpm. It's these violent on/off shifts in timing that initiate the shunt, this combined with the lean 14.7:1 AFR imposed by the lurching narrow band lambda 14CUX just makes the situation even worse.
Manifold vacuum (unlike ported vacuum) is a nice consistent and reliable solid signal, if you are looking for the best idle it’s manifold vacuum all the way. The ported vacuum strategy used on our cars was just a means of cleaning up emissions at idle, firstly it makes heat to light the pre-cats earlier... but also by forcing the engine demand more air (and consequently more fuel) at idle the hydrocarbon parts per million could be reduced, to get the idle speed down ignition timing was retarded.
Ported vacuum reduces emissions but that's the only good thing about it, in all other respects it's a terrible idea, ported vacuum gives a rougher idle and poor low speed drivability, now make the engine burn a lean idle 14.7:1 AFR and the situation gets even worse. Switch to the way more consistent signal full manifold vacuum delivers and your idle will become smoother and low speed drivability will be greatly improved, the only downside being inevitably your idle emissions will go up.
Edited by ChimpOnGas on Saturday 27th January 08:43
On the left of the image we can see ported vacuum, the example is showing the engine at idle, with the throttle butterfly closed at idle there is little or no vacuum so no timing is added by the vac advance capsule, your Chimaera is idling at 10 degrees.
Now... this is what happens with ported vacuum when you move off idle, as the throttle butterfly is opened it passes the ported vacuum port and suddenly the vac advance capsule sees full manifold vacuum.... so it instantly adds 4-6 degrees of timing (probably more like 10 degrees).
Now imagine your throttle butterfly passing back and forth over the ported vacuum port as you modulate your road speed driving through town, as much as 10 degrees of timing will be instantly added and then instantly removed.
Your ignition timing is now swinging violently from 10 degrees to 20 degrees and back to 10 again, these relatively large and rapid changes in ignition timing are very bad for drivability and smooth engine operation!
Switch your vac advance from the standard ported vacuum to a full manifold vacuum signal and not only are all those nasty violent swings in timing removed, but in theory you'll also be idling at a far more optimal 16-18 degrees.... not the retarded ported vacuum 10 degrees Land Rover used to lower idle emissions
The results of this simple, quick and free to implement change should be a smoother idle and improved drivability when the throttle butterfly is being moved from closed to almost closed as you modulate your slow road speed through town
Now... this is what happens with ported vacuum when you move off idle, as the throttle butterfly is opened it passes the ported vacuum port and suddenly the vac advance capsule sees full manifold vacuum.... so it instantly adds 4-6 degrees of timing (probably more like 10 degrees).
Now imagine your throttle butterfly passing back and forth over the ported vacuum port as you modulate your road speed driving through town, as much as 10 degrees of timing will be instantly added and then instantly removed.
Your ignition timing is now swinging violently from 10 degrees to 20 degrees and back to 10 again, these relatively large and rapid changes in ignition timing are very bad for drivability and smooth engine operation!
Switch your vac advance from the standard ported vacuum to a full manifold vacuum signal and not only are all those nasty violent swings in timing removed, but in theory you'll also be idling at a far more optimal 16-18 degrees.... not the retarded ported vacuum 10 degrees Land Rover used to lower idle emissions
The results of this simple, quick and free to implement change should be a smoother idle and improved drivability when the throttle butterfly is being moved from closed to almost closed as you modulate your slow road speed through town
Let's face it Dave, search 'shunting' in the chimaera forum and you will find so many 'cures' that you wonder how tvr got this car sold out of their dealerships in the first place, let alone it being their best selling car.
Granted, perhaps they didn't do it from new but it is such a common thing with these cars now that it seems most people find that the only proper way of sorting it once and for all is to rip it all out and replace everything with a nice modern ecu and loom.
I was so close to pushing the button on doing just that, as a recent thread I started discussed, and fitting a diy MS system but part of me felt that there is more to this clockwork engine management system and that I should give it a little more time, and look into it further.
Up to now I successfully 'cured' the shunting on my HC with changing the chip to a TVR mapped green tune version and this did the trick, which I understand ignores the lambda readings and relies on a manually set AFM to regulate data the ECU needs to set the cars fuelling.
The downside is that my car seems to drink more fuel but it really becomes much smoother and easier to drive.
I'm going to do another full health check on my ignition and fuelling systems but I have a feeling that all is not well in the vacuum department and perhaps my advance is not working properly, hence the bad fuel consumption, even when driving like my mum.
I'm going to try taking a line out of the plenum base but would like to know what the optimum timing degrees should be at idle and at 2.8k as a few figures have been knocked about in this thread and I need further clarification.
Granted, perhaps they didn't do it from new but it is such a common thing with these cars now that it seems most people find that the only proper way of sorting it once and for all is to rip it all out and replace everything with a nice modern ecu and loom.
I was so close to pushing the button on doing just that, as a recent thread I started discussed, and fitting a diy MS system but part of me felt that there is more to this clockwork engine management system and that I should give it a little more time, and look into it further.
Up to now I successfully 'cured' the shunting on my HC with changing the chip to a TVR mapped green tune version and this did the trick, which I understand ignores the lambda readings and relies on a manually set AFM to regulate data the ECU needs to set the cars fuelling.
The downside is that my car seems to drink more fuel but it really becomes much smoother and easier to drive.
I'm going to do another full health check on my ignition and fuelling systems but I have a feeling that all is not well in the vacuum department and perhaps my advance is not working properly, hence the bad fuel consumption, even when driving like my mum.
I'm going to try taking a line out of the plenum base but would like to know what the optimum timing degrees should be at idle and at 2.8k as a few figures have been knocked about in this thread and I need further clarification.
jazzdude said:
I'm going to try taking a line out of the plenum base but would like to know what the optimum timing degrees should be at idle and at 2.8k as a few figures have been knocked about in this thread and I need further clarification.
The real answer to that is there's no definitive answer, because there are so many variables:- Preferred idle speed
- How much valve overlap you're running (cam profile)
- Cylinder head efficiency
- Compression ratio
- Engine temp
- And most importantly your AFR
While it can be very effective, the 'More Fuel' strategy is of course a rather crude approach, at this point its important to understand how it actually works. Richer mixtures burn more readily, this means you can ignite the fuel later in the cycle than you could with a lean mixture to ensure complete combustion.
Make no mistake, air fuel ratio and timing are unavoidably linked, change one and you'll need to change the other and vise versa to achieve the desired result. The 'More Fuel' strategy will work well on a TVR that's still idling at 10 degrees because 10 degrees is not ideal for the lean 14.7:1 the 14CUX imposes, of course if you'd just left the fueling element alone and worked on optimizing the ignition timing to deal with the 14.7:1 the same results could be achieved using a lot less fuel.
When tuning an engine our objectives should always be driven by efficiency, smooth engine operation from efficient combustion, this is very a different approach to leaving your retarded ignition timing where it is and then throwing fuel at it to match. Take the opposite approach of adjusting your ignition timing to match the lean 14.7:1 the 14CUX imposes to achieve complete combustion and you are now burning less fuel while still delivering the drivability improvements you seek.
The truth is the best approach is apply a mix of the two strategies, IE add a little bit of fuel while the same time adding some timing too, working like this if you have a good ear and an AFR gauge eventually you'll always find the efficiency sweet spot, but definitive proof can only come from studying where peak torque is found and for that you need a dyno. This tuning process is very straightforward to achieve with an after market ECU, start slightly rich with slightly over advanced timing then gradually lean the fueling out as you equally gradually retard the timing a degree at a time, eventually the two closing elements will meet at the sweet spot of best lean torque while delivering optimum smooth engine operation and best drivability.
Like this my relatively standard 4.0HC with no head work and running a V8 Developments Stealth cam on petrol idles beautiful at 1000rpm at 14.3:1 AFR but to do so it also needs 18 degrees of timing. Idle timing is one thing, of course what you need to do is repeat the above tuning process over a myriad rpm & load sights. All the time you're doing this your objective remains the same, you are looking to achieve the most complete and efficient combustion to produce best torque with the least fuel while still enjoying smooth engine operation and best drivability.
What I've been trying to say in this post is..... if you can't easily change the AFR because you're running the 14CUX which in most situations runs the engine too lean for the ancient and inefficient RV8 combustion chamber design limitations, you can at least optimize your ignition timing so you are burning the 14CUX's lean 14.7:1 AFR more efficiently, and of course making changes to your ignition timing is free.
You've smoothed your shunt by smothering it in fuel, but you've done nothing with your ignition timing, if you take the more complete approach of tuning your ignition timing as well as your fueling you will find you'll need less fuel to achieve the same result. Clearly this is a more intelligent way to tune your engine because tuning your air fuel ratio as well as when you choose to bun it in the cycle is a strategy built around the principle of efficiency.
^^^^^^ l like all that very much 
The more you understand the engines needs and can effect it the more efficient you can make it.
This timing stategy and then leaning it off as much as is safe seems to be exactly what I have. Bit rich when cold but cat is being considered in the mapping, warm it’s very efficient indeed.
This use of both advance and retard to keep it in the sweet spot seems to keep the engine running cooler, I’m mostly at 2000 revs ish so very little load either way on engine, these accurate firing points seem to keep the engine in balance and smooth
The fuelling seems trimmed to an inch of its life, very impressive stuff these modern Ecu
The more you understand the engines needs and can effect it the more efficient you can make it.
This timing stategy and then leaning it off as much as is safe seems to be exactly what I have. Bit rich when cold but cat is being considered in the mapping, warm it’s very efficient indeed.
This use of both advance and retard to keep it in the sweet spot seems to keep the engine running cooler, I’m mostly at 2000 revs ish so very little load either way on engine, these accurate firing points seem to keep the engine in balance and smooth
The fuelling seems trimmed to an inch of its life, very impressive stuff these modern Ecu
Classic Chim said:
^^^^^^ l like all that very much
The more you understand the engines needs and can effect it the more efficient you can make it.
This timing stategy and then leaning it off as much as is safe seems to be exactly what I have. Bit rich when cold but cat is being considered in the mapping, warm it’s very efficient indeed.
This use of both advance and retard to keep it in the sweet spot seems to keep the engine running cooler, I’m mostly at 2000 revs ish so very little load either way on engine, these accurate firing points seem to keep the engine in balance and smooth
The fuelling seems trimmed to an inch of its life, very impressive stuff these modern Ecu
Alun, in simple terms what you're describing above is a well tuned engine, which in most cases a 14CUX and distributor equipped TVR wont be because emissions targets and system limitations got in the way.The more you understand the engines needs and can effect it the more efficient you can make it.
This timing stategy and then leaning it off as much as is safe seems to be exactly what I have. Bit rich when cold but cat is being considered in the mapping, warm it’s very efficient indeed.
This use of both advance and retard to keep it in the sweet spot seems to keep the engine running cooler, I’m mostly at 2000 revs ish so very little load either way on engine, these accurate firing points seem to keep the engine in balance and smooth
The fuelling seems trimmed to an inch of its life, very impressive stuff these modern Ecu
What a lot of people dont realize is there isn't an engine in the world that doesn't suffer misfires to some degree or another, no internal combustion engine will actually burn 100% of it's air fuel charge 100% on the time. A 14CUX and distributor equipped TVR is potentially much worse than the same TVR with a well mapped after market ECU running wasted spark because you can optimize both AFRs and ignition timing which work hand in hand....you also have a much more powerful spark which no question helps a lot too.
Actually I have the upmost respect for the engineers who used the 14CUX and distributor setup to get the ancient and highly inefficient Rover V8 to idle and run reasonably well at low engine speeds/light loads at the inappropriately lean 14.7:1 AFR imposed on them by emissions. The problem is to achieve this everything was right on the edge of what it was capable of, so if anything is slightly off our cars in standard trim tend to run like ship. Of course the problem is compounded because the wiring on a TVR isn't the best and earths are often an issue, combine this with the forward facing exhaust manifolds on Chims and Griffs that tend to cook ignition components like HT leads and those dreadful plug extenders and the situation is even worse.
But that's not to say you cant make a TVR with the 14CUX and distributor run well, it's just everything has to be absolutely in tip top condition, which nine times out of ten based on most of the Chims and Griffs I've seen and listened to... they aren't!
Admittedly this is on LPG, but the above shows you can indeed run right in the shunting zone at stoich.
- RPM 1,716
- Map 52 kPa
- 7% throttle opening
- AFR 14.7:1 (Lambda 1 - So a true 15.5:1 running LPG)
- Ignition timing 33 degrees
- RPM 1,716
- Map 52 kPa
- 7% throttle opening
- AFR 14.3:1
- Ignition timing 24 degrees
This in a 4.0HC running the standard plenum & inlet manifold with the addition of a V8 Developments Stealth cam and a bespoke lightweight 18lb steel flywheel from TTV Racing, two modifications that in theory should negatively impact drivability.
Hopefully this proves you absolutely do not not need to put up with shunting on these cars
carsy said:
5th gear is one thing but can you drive in 2nd gear at a steady 1500rpm and still be shunt free.
Give that man a cigar small throttle opening or trailing throttle approaching roundabout or slow moving traffic is where its at 1500/1800 RPM is the sweet spot 
well not exactly sweet but .... 
getting on the gas at anytime in this zone just cleans/smooths things right up Sardonicus said:
Give that man a cigar small throttle opening or trailing throttle approaching roundabout or slow moving traffic is where its at 1500/1800 RPM is the sweet spot well not exactly sweet but .... getting on the gas at anytime in this zone just cleans/smooths things right up
Here's another little trick that worked wonders on my Chimaera: small throttle opening or trailing throttle approaching roundabout or slow moving traffic is where its at 1500/1800 RPM is the sweet spot well not exactly sweet but .... getting on the gas at anytime in this zone just cleans/smooths things right up 1. Screwing the base idle screw right home so it does nothing
2. Jack the throttle butterfly rest position to give the desired idle speed
The base idle screw is an air bleed circuit much the same as used on many carburetors, when I was taught how to tune carburetors the two rules that were always drummed into me by the old boys I worked with, were....
- All vacuum leaks are bad
- An idle air bleed circuit is just a managed vacuum leak (as is a crankcase ventilation system), so less air bleed, or better still no air bleed at all... should always be your goal!
To set your throttle butterfly rest position remove the plenum and invert it, study the area under throttle body and you'll find a little grub screw, remove this and re-insert it from above so when the plenum is back in place you can easily adjust your throttle butterfly rest position. Now wind the base idle screw right home so there is no longer an air bleed, then bring your idle speed back up using the grub screw. With my Stealth cam and lightened flywheel I run my idle at 1050rpm which after much experimentation has consistently proved to be the optimum setting.
At 1050rpm with the throttle butterfly in the standard rest position and the base idle screw used to set this idle speed my engine will pull an average of 54 kPA, like this it also tends to bounce between 50 & 60 kPa. Now, by winding the base idle screw all the way home and adjusting the throttle butterfly rest position to achieve the exact same 1050rpm idle that figure falls to 40 kPa. Not only does the vacuum/load drop by a full 14 kPa but it sits rock solid at 40 kPa with hardly a flicker or fluctuation, this strategy delivers a significantly smoother and more stable idle that's audibly calmer and more refined at the exact same 1050rpm, all this with the engine running lightened flywheel and a V8 Developments Stealth cam which they themselves describe as "a fast road camshaft".
I have back to back tested both ways to achieve my chosen idle speed within 10 minutes of each other as the changes are easy to implement and reverse, these back to back tests and the kPa figures do not lie. The best way to describe the idle effects of swapping the standard base idle air bleed managed idle to a throttle butterfly rest position managed idle... is that it's just like running a milder cam profile profile with less overlap.
People should also take a close look at the passive un-valved crankcase ventilation circuit used on our cars, follow the hose connected directly to full vacuum at the plenum and ask yourself what effects this open circuit is likely to have on your idle? Using a hose clamp to temporarily remove the effect this circuit has on idle quality and drivability reveals it's influence. Clearly some way of managing crankcase gasses remains an essential requirement for any internal combustion engine, so I am not for one minute suggested you should permanently remove this circuit.
What I'm saying is in it's standard un-valved configuration the crankcase ventilation system is an extremely crude setup as it essentially connects full manifold vacuum not only to the crankcase (fine), but also connects it directly to atmosphere via the throttle body where the designers used venturi vacuum to ensure the engine would still consume blowby gasses during large throttle openings, ie when manifold vacuum is lost. Make no mistake, connecting full manifold vacuum directly to atmosphere is very bad for idle quality because what it really is, is just another vacuum leak.
At idle for what is a relatively low engine speed, vacuum is high... so that open to atmosphere un-valved circuit that directly connects your full manifold vacuum to the throttle body inevitably has a significant impact on idle speed and quality. You can test this by using a hose clamp on the part of the circuit that connects full manifold vacuum to atmosphere at the throttle body, at idle you'll find the engine is pulling air from this point which contributes to the lean ragged idle you so often observe on our cars (hunting).
Worse still if you connect a vacuum gauge at this point to see what's really going on with the standard setup at idle you'll see the figures fluctuate quite a bit, now go for a slow speed drive using small throttle openings and it's even worse, indeed you'll see big swings and even evidence of reverse pulses under certain situations. Obviously you can't just seal off the crankcase ventilation circuit but does it really need to be connected to the throttle body at all?... well yes it does, because as already stated you still need some method of removing crankcase gasses when you're are accelerating hard and manifold vacuum is lost.
To resolve the issues the standard un-valved setup imposes and to reduce the excessively strong vacuum it sees at idle I have used two PCV valves, these are effectively shuttle valves that mean at idle the engine is only drawing crankcase gasses and zero air from atmosphere. Indeed the only point that venturi vacuum comes into effect is when venturi vacuum is high enough in throttle body to pull the PCV valve off it's seat and open that circuit. The result is crankcase gasses are managed correctly at both high manifold vacuum and no manifold vacuum WOT conditions, at idle because manifold vacuum is no longer directly connected to atmosphere idle quality becomes much smoother, more stable and easier to adjust, there are also no lean spikes that can cause the engine to hunt and micro-misfire with the standard arrangement.
Combine this valve managed crankcase ventilation arrangement with zero air bleed from the base idle screw and idle the speed now set exclusively by throttle butterfly rest position, and dramatic improvements in idle quality can be achieved. In addition to achieving a significantly more stable idle, on/off throttle response is greatly enhanced as is low speed drivability. For those running a distributor you can now switch your vac advance capsule from the fluctuating ported vacuum signal to the much more stable and reliable signal you receive from full manifold vacuum. Like this your idle timing should now move from the retarded 10 degrees to something much closer to the optimum idle timing figure of 18 degrees which helps burn the lean 14.7:1 idle AFR much more effectively, the results being a further enhancement of idle quality and on/off throttle position slow speed drivability/throttle response in urban driving conditions.
In summary, if you deploy all three of the above strategies their combined effect is to transform the way a Chim or Griff idles and drives at low speed. All the above is the result of some two years tuning my TVR, by doggedly perusing drivability and idle perfection using sound theory and a close study of the standard setup, I've been able to create a car that's infinitely nicer to drive. The critical point being... while my Canems system certainly helps, I honestly don't believe you need an expensive aftermarket ECU to deliver the same results.
The common lesson learned is, in my experience, many of the issues with the standard setup and so the areas that offer most scope for improvement are the elements forced on the designer by emissions targets. Retarded idle timing, our crank case ventilation arrangement, restrictive catalytic converters and lean idle AFRs only exist as function of the designer desperately trying to make an ancient dirty engine with very inefficient combustion chamber design hit challenging emission targets the engine was never intended to meet. Remove all the emissions driven systems and strategies and for sure the engine will become more polluting, but it'll also drive much nicer too.
If you're running an old engine like the Rover V8 in simple terms the dilemma will often boil down to.... "Dolphins or Drivability"
The choice is yours
Dave you can control how the air bleed reaches the inlet manifold but you cant alter the vacuum signal InHG/ KPA doing so 
no matter how this bleed is introduced or method be it idle valve, cranked butterfly, PCV valve etc either metered or unmetered 
if you introduce enough air to allow a 800RPM idle via a bleed screw,butterfly,idle valve the vacuum will remain the same ao long as the target value of 800RPM is constant across any method , methinks I have misinterpreted your post 
no matter how this bleed is introduced or method be it idle valve, cranked butterfly, PCV valve etc either metered or unmetered if you introduce enough air to allow a 800RPM idle via a bleed screw,butterfly,idle valve the vacuum will remain the same ao long as the target value of 800RPM is constant across any method , methinks I have misinterpreted your post Edited by Sardonicus on Friday 2nd February 11:08
While manifold pressure is a measure of the depression between the throttle and the cylinders, and the more the throttle is opened the closer that manifold pressure returns to atmospheric pressure, air speed over the throttle butterfly should also be considered. The influence an air bleed from the base idle screw (or anywhere else) has on engine speed is only part of the story. Study the location of the base idle circuit on the throttle body and it's relationship to the throttle butterfly and it's not hard to see why there might be better ways to achieve a chosen stable idle speed... its not dissimilar to why I'm suggesting ported vacuum may not be the most stable signal to influence your engine speed governed ignition timing curve.
In addition to this you can of course reduce or increase manifold absolute pressure very effectively using ignition timing, as you increase timing MAP will fall (up to a point) and increase as you pull timing, as I'm sure people will be quick to point out this is to some degree a function of the changes in idle speed driven by the changes in timing.... but that's not all that's going on. Furthermore, idle stability at the same engine speed will vary with differing air fuel ratios, but an excessively lean 15.5:1 1050rpm idle is audibly a very different thing to a richer 13.5:1 also at 1050rpm. Now forget your ears for a minute and look at the difference in MAP between the lean and richer examples, both engines are at 1050rpm and both have the same resting throttle position and or base idle setting, but the MAP will be far from the same and fluctuations in MAP will be far greater in our lean example.
Air speed over the throttle butterfly, the stability and quality of the air you introduce (blow-by is not air), ignition timing, valve events and air fuel ratio can all influence MAP at an exact same engine speed, which seems counter-intuitive until you remember an engine is just a big vacuum pump and even small changes in any of these listed elements can influence it's efficiency and so the level of depression it creates within the inlet manifold (MAP). Your big old vacuum pump is still turning at 1050rpm... but it's efficiency can definitely be altered in many different ways, that in turn will have an impact on manifold absolute pressure and idle quality.
As we all know valve overlap, ignition timing and air fuel ratios are all very much critical influencers of idle quality, but lets park these elements for a minute. Where you get your air from, the quality of that air, where and how many places you allow that air into the engine... and finally the speed of that air... all become considerations that most definitely do matter.
In addition to this you can of course reduce or increase manifold absolute pressure very effectively using ignition timing, as you increase timing MAP will fall (up to a point) and increase as you pull timing, as I'm sure people will be quick to point out this is to some degree a function of the changes in idle speed driven by the changes in timing.... but that's not all that's going on. Furthermore, idle stability at the same engine speed will vary with differing air fuel ratios, but an excessively lean 15.5:1 1050rpm idle is audibly a very different thing to a richer 13.5:1 also at 1050rpm. Now forget your ears for a minute and look at the difference in MAP between the lean and richer examples, both engines are at 1050rpm and both have the same resting throttle position and or base idle setting, but the MAP will be far from the same and fluctuations in MAP will be far greater in our lean example.
Air speed over the throttle butterfly, the stability and quality of the air you introduce (blow-by is not air), ignition timing, valve events and air fuel ratio can all influence MAP at an exact same engine speed, which seems counter-intuitive until you remember an engine is just a big vacuum pump and even small changes in any of these listed elements can influence it's efficiency and so the level of depression it creates within the inlet manifold (MAP). Your big old vacuum pump is still turning at 1050rpm... but it's efficiency can definitely be altered in many different ways, that in turn will have an impact on manifold absolute pressure and idle quality.
As we all know valve overlap, ignition timing and air fuel ratios are all very much critical influencers of idle quality, but lets park these elements for a minute. Where you get your air from, the quality of that air, where and how many places you allow that air into the engine... and finally the speed of that air... all become considerations that most definitely do matter.
Well aware of swinging timing to influence the vacuum signal along with all the cam duration,engine displacement CR etc etc but cranking open butterflies or air bleeds be it via an ICV or whatever does not change the vacuum rate Dave, direction of airflow through the plenum maybe and it may be this which may have an affect to the dreaded zone (thats a maybe) you may have worded it differently but you and I know what I mean
and it may be this which may have an affect to the dreaded zone (thats a maybe) you may have worded it differently but you and I know what I mean Edited by Sardonicus on Friday 2nd February 15:03
jazzdude said:
'To resolve the issues the standard un-valved setup imposes and to reduce the excessively strong vacuum it sees at idle I have used two PCV valves'
Can you elaborate on this a little and perhaps post what these are and how you fitted them, Dave?
Before you start adding PCV valves my advice would be to use a hose clamp at various points on the system, this will help you establish if you are indeed suffering from an excessive vacuum leak from the breather hoses and will force you to think about how the system works.Can you elaborate on this a little and perhaps post what these are and how you fitted them, Dave?
Keep in mind oil mist and acidic/caustic crankcase vapors are very effective at attacking and relaxing rubber hoses, take a look and you'll also see these hoses often relied solely on a push fit, in a number of places where the hoses are susceptible to becoming baggy no jubilee clips were used.
Clamping off the main source of vacuum at the plenum, listening to how this effects idle speed, and how it may also be influencing drivability (go for a short test drive) should be your starting point. Please do not run this clamp test for very long, combined with a visual inspection of your hoses running the clamp for 5 minutes is safe and will tell you everything you need to know. All internal combustion engines make crankcase gasses (blow by) and pressure, it is therefore essential to manage this correctly or you'll end up with oil leaks and other issues, so never run your clamp test for longer than a few minutes on a fully warned engine.
Chimp On Gas - I've gotten myself confused so need a little clarity.
My Chimaera now has pcv, the fuel fume purge is removed and this is where i'm taking my vacuum signal from. T piece into the fuel regulator line gives a very weak vacuum which has no effect on the distributor.
Adding jubilee clips on all hoses improved the idle, as did the fuel purge system removal, and the addition of a pcv valve.
I had a quick go at timing but have gotten confused. Ive read all your posts (printed off...not stalky at all...) but need clarity on the idle timing. Vacuum connected or disconnected and blocked?
I left it for now as 16 deg idle with it connected and at 3500 rpm maxes out at about 32 degree.
Do i set it at 18 to 20 vac removed. Check the figure 3500 rpm (about 34 deg from memory) then connect vac and set base idle? Idle advance should increase but max advance should remain constant?
Ive timed cars before but they were with centrifugal distributors with no option for vac.
My Chimaera now has pcv, the fuel fume purge is removed and this is where i'm taking my vacuum signal from. T piece into the fuel regulator line gives a very weak vacuum which has no effect on the distributor.
Adding jubilee clips on all hoses improved the idle, as did the fuel purge system removal, and the addition of a pcv valve.
I had a quick go at timing but have gotten confused. Ive read all your posts (printed off...not stalky at all...) but need clarity on the idle timing. Vacuum connected or disconnected and blocked?
I left it for now as 16 deg idle with it connected and at 3500 rpm maxes out at about 32 degree.
Do i set it at 18 to 20 vac removed. Check the figure 3500 rpm (about 34 deg from memory) then connect vac and set base idle? Idle advance should increase but max advance should remain constant?
Ive timed cars before but they were with centrifugal distributors with no option for vac.
The crankcase breather system is a separate thing altogether, it's a vacuum leak thing and shouldnt be confused with the ported to full manifold vacuum and so ignition timing strategy also being discussed.
MOVING ON....
The timing idea is an experiment you may wish to consider if you are still running a distributor, it's merely a theory on my part but it is based on what I've found to work well as I have the clear advantage of fully a mapped ignition ignition system. The theory, and I must stress again it is just a theory, is some if not all the benefits of adopting the advanced timing strategy I run, could also potentially be enjoyed by those still running a distributor... and simply by switching the signal applied the vac advance unit from ported vacuum to full vacuum.
At no point am I suggesting these changes are a guaranteed way to make your engine run better, all I'm sharing is the ignition timing strategy that definitely made mine run smoother, use less fuel and ultimately make more power, some of which I've translated to something that may or may not help people still running a distributor. I've also tried to evidence my theory by providing information on why car makers in the 1970's started switching from the long established full vacuum signal applied to the vac advance unit to ported vacuum.
What I can tell you is:
1. First measure your timing with everything as standard making sure you are idling at the correct 10 degrees specified by Land Rover
2. Now remove the vac advance hose from the ported vacuum point, seal the ported vacuum nipple on the throttle body, and now put the hose to full vacuum (you could 'T' the fuel pressure regulator hose for this)
3. Your idle speed should go up because in theory the vac advance unit is now adding XX?? number of degrees of advance
4. Screw your base idle screw back in to return your idle to where it was before
5. Now using a dial back timing light record your degrees of advance at idle (16-20 degrees???) and any audible changes in idle quality
6. Increase engine speed to 3,250rpm, and using your dial back timing light record your total timing number
7. Assuming your total timing number is a safe 36 degrees or less go for a gentle drive
8. Drive the car at light load between 1,650rpm and 1,850rpm to see if the additional timing provided by the stronger and more consistent full vacuum signal improves drivability and throttle response
9. Carefully take the car to 4,000rpm at load, safe in the knowledge you should never exceed 36 degrees of total mechanical timing
10. Under such conditions there should be insufficient vacuum for the vac advance unit to have any effect so the engine should only ever get that total mechanical timing number observed in point 6
WARNING! - Because with a distributor you can't actually monitor the real time true timing figure when you're driving as you can with an aftermarket ECU, you absolutely must keep a very careful ear open for the onset of an detonation when you accelerate to 4,000rpm!
11. Back off the throttle but hold the engine between 2,800rpm and 3,000rpm on the smallest throttle input you can to maintain engine speed in that bracket, you are now light load cruising and adding the maximum amount of advance the vac advance unit is capable of providing
12. Under these light load cruise conditions detonation is very unlikely but as with the acceleration test, you once again... absolutely must keep a very careful ear open for the onset of an detonation!
When you've completed the above tests, report back here on your findings, IE...
NB: My engine burning petrol will easily and safely tolerate as much as 22 degrees if idle timing, 38 degrees of total timing and 48 degrees at light load cruise timing with no issues whatsoever. I give you these figures merely to reassure you the above distributor based tests should, in theory, be perfectly safe.
To be clear... 22/38/48 are most definitely not the optimal timing figures, far from it
For the record, with the benefit of an aftermarket engine management system and some rolling road work to establish peak torque under different load conditions... here is what I run on petrol:
Idle: 18°
Total timing: 36°
Light load cruise: 46°
I look forward to hearing the results of simply switching your vac advance unit from the standard ported vacuum to a full manifold vacuum signal. It's possible you'll see some real benefits, it's also possible you'll notice no improvements at all, but because there's also a very slim chance you'll find the point of detonation so I want to be 100% clear with you now by saying I hold no responsibility for that as all this is just theory until someone takes the initiative and tests it.
DISCLAIMER!
Please remember Land Rover designed and tested the vac advance unit to work with ported vacuum, not full vacuum, so clearly how it'll behave and how much timing it adds under full vacuum needs careful evaluation before you consider putting your engine under any load whatsoever! I hold no responsibility for any issues arising from your engine reaching the point of detonation during the above theory based tests.
Regards, Dave
MOVING ON....
The timing idea is an experiment you may wish to consider if you are still running a distributor, it's merely a theory on my part but it is based on what I've found to work well as I have the clear advantage of fully a mapped ignition ignition system. The theory, and I must stress again it is just a theory, is some if not all the benefits of adopting the advanced timing strategy I run, could also potentially be enjoyed by those still running a distributor... and simply by switching the signal applied the vac advance unit from ported vacuum to full vacuum.
At no point am I suggesting these changes are a guaranteed way to make your engine run better, all I'm sharing is the ignition timing strategy that definitely made mine run smoother, use less fuel and ultimately make more power, some of which I've translated to something that may or may not help people still running a distributor. I've also tried to evidence my theory by providing information on why car makers in the 1970's started switching from the long established full vacuum signal applied to the vac advance unit to ported vacuum.
What I can tell you is:
- The standard ported vacuum setup has you idling a 10 degrees
- Ported vacuum provides insufficient vacuum at the vac advance unit to add timing
- Given the choice (which I have) I would never idle my engine at 10 degrees but it would lower my emissions
- Typically others running an aftermarket ECU will choose to idle higher than the standard distributor/ported vacuum 10 degrees too
- My car idles on petrol much much better at 18 degrees but it is more polluting like this
- Ported vacuum can deliver big on/off swings in vacuum as the throttle blade passes over the port because as you open the throttle beyond roughly 4% it's switching from virtually zero vacuum to full manifold vacuum
- Rapidly fluctuating timing numbers like this between the closed and 5% throttle open positions... can't be good for drivability
1. First measure your timing with everything as standard making sure you are idling at the correct 10 degrees specified by Land Rover
2. Now remove the vac advance hose from the ported vacuum point, seal the ported vacuum nipple on the throttle body, and now put the hose to full vacuum (you could 'T' the fuel pressure regulator hose for this)
3. Your idle speed should go up because in theory the vac advance unit is now adding XX?? number of degrees of advance
4. Screw your base idle screw back in to return your idle to where it was before
5. Now using a dial back timing light record your degrees of advance at idle (16-20 degrees???) and any audible changes in idle quality
6. Increase engine speed to 3,250rpm, and using your dial back timing light record your total timing number
7. Assuming your total timing number is a safe 36 degrees or less go for a gentle drive
8. Drive the car at light load between 1,650rpm and 1,850rpm to see if the additional timing provided by the stronger and more consistent full vacuum signal improves drivability and throttle response
9. Carefully take the car to 4,000rpm at load, safe in the knowledge you should never exceed 36 degrees of total mechanical timing
10. Under such conditions there should be insufficient vacuum for the vac advance unit to have any effect so the engine should only ever get that total mechanical timing number observed in point 6
WARNING! - Because with a distributor you can't actually monitor the real time true timing figure when you're driving as you can with an aftermarket ECU, you absolutely must keep a very careful ear open for the onset of an detonation when you accelerate to 4,000rpm!
11. Back off the throttle but hold the engine between 2,800rpm and 3,000rpm on the smallest throttle input you can to maintain engine speed in that bracket, you are now light load cruising and adding the maximum amount of advance the vac advance unit is capable of providing
12. Under these light load cruise conditions detonation is very unlikely but as with the acceleration test, you once again... absolutely must keep a very careful ear open for the onset of an detonation!
When you've completed the above tests, report back here on your findings, IE...
- After you've corrected your idle speed using the base idle screw, how did your idle timing number change when you switched from ported vacuum to full vacuum?
- Did you notice an audible improvement in idle quality and stability when running your new advanced timing strategy?
- What was your total mechanical timing figure when you manually raised engine speed to 3,250rpm?
- Did you suffer any detonation during the acceleration test?
- Did you suffer any detonation during the light load cruise test?
NB: My engine burning petrol will easily and safely tolerate as much as 22 degrees if idle timing, 38 degrees of total timing and 48 degrees at light load cruise timing with no issues whatsoever. I give you these figures merely to reassure you the above distributor based tests should, in theory, be perfectly safe.
To be clear... 22/38/48 are most definitely not the optimal timing figures, far from it
For the record, with the benefit of an aftermarket engine management system and some rolling road work to establish peak torque under different load conditions... here is what I run on petrol:
Idle: 18°
Total timing: 36°
Light load cruise: 46°
I look forward to hearing the results of simply switching your vac advance unit from the standard ported vacuum to a full manifold vacuum signal. It's possible you'll see some real benefits, it's also possible you'll notice no improvements at all, but because there's also a very slim chance you'll find the point of detonation so I want to be 100% clear with you now by saying I hold no responsibility for that as all this is just theory until someone takes the initiative and tests it.
DISCLAIMER!
Please remember Land Rover designed and tested the vac advance unit to work with ported vacuum, not full vacuum, so clearly how it'll behave and how much timing it adds under full vacuum needs careful evaluation before you consider putting your engine under any load whatsoever! I hold no responsibility for any issues arising from your engine reaching the point of detonation during the above theory based tests.
Regards, Dave
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