god damn shunt is killing me
Discussion
Likewise, if anything, you were defensive of them.
But as over on the ACT thread, it's nice when people actually get to talk and sort things out the old fashioned way, mano a mano. Washing (dirty or otherwise) shouldn't be done in public. I blame Fakebook, that's where it all started.
But as over on the ACT thread, it's nice when people actually get to talk and sort things out the old fashioned way, mano a mano. Washing (dirty or otherwise) shouldn't be done in public. I blame Fakebook, that's where it all started.
, and fair play to UKDJ to offer 
Haha. They will sort it out for me and I’ll let you know what went wrong!
Thanks all for help I just didn’t want these guys to go any more miles for me as they have already done a ton of work, it’s quite a distance away and I’m not the best customer for making money on!
Thanks Take care all and speak soon 😎
Thanks all for help I just didn’t want these guys to go any more miles for me as they have already done a ton of work, it’s quite a distance away and I’m not the best customer for making money on!
Thanks Take care all and speak soon 😎
The pre load on these diffs is set quite low, some say that pre load becomes even less over time.
What oil is changed the least and often never changed at all on these cars!
There is a good possibility many Tvr suffer from slack diffs.
The shunting is in my view caused by poor fuelling at lower revs which the diff masks until it gets loose, shunting then becomes obvious.
My new Ecu does a fine job at lower revs, but there was always a bit of snatch when changing gear or if I went too low as in 800-1000 revs the snatch could become clunky.
New diff for me anyway,
Much smoother gear-changes ( cold gearstick)
No snatch when using the clutch and it’s much easier to use.
Less clunkiness when on off throttle abruptly
No shunt at any revs unless I’m clumbsy with the throttle.
It was the final piece in what becomes an expensive but in my case very effective way to re create the ( new car ) feel.
Shunting ruins your enjoyment and the car.
I’d changed gearbox oil but it’s like a new box, takes a hot day and many miles of driving before gearstick is anything but Luke warm.
It used to get hot !
The best way to describe the combination of a new Ecu and diff if you have a mild ish cam is
New car feel.
How you’d expect it to be if you were collecting it new.
Surely the cars didn’t shunt from new so it’s age related in both Ecu management and the diffs case.
The diffs can have had an easy life or a very bad one depending on how the cars been used so it’s a bit of a lottery to know.
From a safety position,
Acceleration is more consistent and far less likely to spin up one wheel and spin you out !
What oil is changed the least and often never changed at all on these cars!
There is a good possibility many Tvr suffer from slack diffs.
The shunting is in my view caused by poor fuelling at lower revs which the diff masks until it gets loose, shunting then becomes obvious.
My new Ecu does a fine job at lower revs, but there was always a bit of snatch when changing gear or if I went too low as in 800-1000 revs the snatch could become clunky.
New diff for me anyway,
Much smoother gear-changes ( cold gearstick)
No snatch when using the clutch and it’s much easier to use.
Less clunkiness when on off throttle abruptly
No shunt at any revs unless I’m clumbsy with the throttle.
It was the final piece in what becomes an expensive but in my case very effective way to re create the ( new car ) feel.
Shunting ruins your enjoyment and the car.
I’d changed gearbox oil but it’s like a new box, takes a hot day and many miles of driving before gearstick is anything but Luke warm.
It used to get hot !
The best way to describe the combination of a new Ecu and diff if you have a mild ish cam is
New car feel.
How you’d expect it to be if you were collecting it new.
Surely the cars didn’t shunt from new so it’s age related in both Ecu management and the diffs case.
The diffs can have had an easy life or a very bad one depending on how the cars been used so it’s a bit of a lottery to know.
From a safety position,
Acceleration is more consistent and far less likely to spin up one wheel and spin you out !
When you have an aftermarket ECU and get into the mapping side as I have, it becomes very easy to dial in and dial out shunting, this helps prove what it really is and why it exits on 14CUX equipped cars. Contrary to common thinking I can absolutely guarantee it has nothing to do with the inlet manifold design, the problem has everything to do with air and ignition, I have proved this by eliminating all shunting with mapping my fuel & ignition tables and eliminating all induction leaks... all this without needing to change the standard inlet manifold & plenum arrangement.
The truth is what TVR got from Land Rover in the 1990's was a fuel and ignition system that were very much 80's tech, it was already very out of date when TVR got hold of it, things got a lot better for the old Rover V8 in the mid 90's when Land Rover gave the engine an ECU managed distributorless ignition (DIS) and fully sequential fuel injection system, sadly TVR never got this setup but I'm sure Land Rover were very happy to offload their stock out of date 14CUX ECU's and prehistoric distributors on the boys in Blackpool.
So what is shunting and more importantly what really causes it? Well, you can break it down into four subsections:
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.
The Solutions to Shunting
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.
Start by servicing your ignition system, some starting points are:
Finally, consider switching from the standard ported vacuum signal to the vac advance unit and move to a full vacuum signal, this will give you an additional 8 degrees of timing at idle on top of your 10 degrees starting point and in the shunting zone another 4 degrees of timing which will give you a smoother idle, cooler running and generally better low speed drivability.
None of the above involves spending £3k on a new engine management system, it's just understanding the system you have, understanding it's issues, and correcting the faults so it works like it did when your TVR was new.
I hope all this helps,and I wish you good luck with it.
Dave.
The truth is what TVR got from Land Rover in the 1990's was a fuel and ignition system that were very much 80's tech, it was already very out of date when TVR got hold of it, things got a lot better for the old Rover V8 in the mid 90's when Land Rover gave the engine an ECU managed distributorless ignition (DIS) and fully sequential fuel injection system, sadly TVR never got this setup but I'm sure Land Rover were very happy to offload their stock out of date 14CUX ECU's and prehistoric distributors on the boys in Blackpool.
So what is shunting and more importantly what really causes it? Well, you can break it down into four subsections:
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.
The Solutions to Shunting
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.
Start by servicing your ignition system, some starting points are:
- Run the engine at night and look for sparks indicating leaking HT leads ect
- Delete the failure prone plug extenders and replace with socks
- Visually inspect HT leads for splits and burning
- Replace the inappropriate shrouded electrode plugs specified by TVR for a new set extended electrode number 6 NGK plugs
- Check coil output
- Ect Ect Ect
Finally, consider switching from the standard ported vacuum signal to the vac advance unit and move to a full vacuum signal, this will give you an additional 8 degrees of timing at idle on top of your 10 degrees starting point and in the shunting zone another 4 degrees of timing which will give you a smoother idle, cooler running and generally better low speed drivability.
None of the above involves spending £3k on a new engine management system, it's just understanding the system you have, understanding it's issues, and correcting the faults so it works like it did when your TVR was new.
I hope all this helps,and I wish you good luck with it.
Dave.
SILICONEKID 345HP 12.03 said:
Take it to Jools and then go to the pub , on return pay him and drive away with the problem solved ..
No need to blow your mind ??
Admittedly a far better option for most Daz, but some like to fix their own car, it's actually not a complex vehicle so no need to blow your mind.... or Jools for that matter No need to blow your mind ??
See above, in essence Range Rover chose ported vacuum for emission reasons.
Ported vacuum is taken from a point in line with the throttle butterfly, as such and strictly speaking it is neither on the full vacuum side of the throttle butterfly or the atmosphere side either.
Basically using ported vacuum as the signal for the vac advance module means at idle you get zero effect, while people will tell you to disconnect the vac advance pipe on a Chimaera when setting your ignition timing to 10-12 degrees the truth is it makes no difference at all to leave it connected.
The advice to disconnect the vac advance when setting your ignition timing comes from the days when a full vacuum signal was used, on the Range Rovers, Chimaera's and Griffs where the ported vacuum signal was used.... you set your idle timing to 10-12 degrees and that's right where it'll stay, all be it with the normal jumps up and down by 3-4 degrees you see from a distributor under your timing light.
You need the engine pumping way harder than it can at idle and pumping hard against an almost closed throttle butterfly before you'll generate enough vacuum to start the vac advance unit pulling on the distributor base plate, so the truth is on our cars the vac advance only comes into play at small throttle openings over 1,600 rpm IE when your cruising.
By using ported vacuum Land Rover was able to get the benefits of 8-12 degrees of additional ignition advance at light load higher road speed cruise conditions but still keep the car idling at 10-12 degrees. The thing is this engine does not like to idle at 10-12 degrees, so why did they use ported vacuum...
. Well if you run retarded timing at idle two things happen, firstly you can reduce certain pollutants in the exhaust gasses and secondly you generate a lot of heat in the exhaust ports which aids early lighting of the cats. Internal combustion engines are most polluting at cold start and during the warm up phase so the earlier you can get the cats working the better, and cats need heat to start working.
Ported vacuum was developed by the Yanks in the 70's when they were still using engines designed in the 1950's but ever sticker smog laws were forcing designers to come up with all kind of nasty sticking plaster emission reducing strategies like smog pumps and retarded idle timing. As always the British motor industry were even slower to develop new and more efficient power plants, so ten years later in the 1980's Land Rover started to adopt the rubbish sticking plaster strategies developed a decade before in order that they might squeeze a few more years out of the old Rover V8.
Make no mistake this engine does not want to idle at 10-12 degrees, any more than a small block Chevy wants to idle at 10-12 degrees, in the 1960'sand before all this emissions bollax all Yank V8s ran the 18-20 degrees at idle they wanted for a nice smooth idle. This is no different for our Rover V8 because essentially in all the important aspect the engine is like any other Yank V8, free yourself from worrying about emissions and fit an aftermarket engine management system and see what your Rover V8 idles like when you take it for 10-12 degrees to the 18-20 degrees it really wants.
Obviously the first thing that happens is your idle speed immediately goes up by 5-800rpm, screw it back down to 1,00rpm using your base idle screw and what your left with is an engine that idles so much smoother and cooler than you ever thought possible. Obviously if you are still running a distributor you'll need a different strategy to achieve the same thing, fortunately you can easily achieve an 18 degree idle by simply following how V8s were set up in the 60's, simply switch from the stupid ported vacuum and put the vac advance pipe to full vacuum and hey presto you're idle at 18 degrees.
And before people claim this is dangerous consider what happens as soon as you pull away, you open the throttle so that vacuum is lost instantly and the distributor will immediately go back to it's pre-set centrifugal advance curve that's governed by engine speed, bob weight mass and advance spring load. The other thing that will happen when you switch to full vacuum is the vac advance unit will add 3-4 degrees around 1,700-1,850 rpm right where the engine needs it, 3-4 degrees on top of the centrifugal advance doesn't sound like a lot but it's actually quite a bit of timing to add at such low engine speeds and is certainly plenty enough to notably smooth out the dreaded shunt.
Further more it's my belief that this still using ported vacuum may be getting vac advance flutter at 1,700-1,850 rpm, it may be there's an on/off effect at this engine speed/throttle opening that means the a couple of degrees of timing is being rapidly added and removed, obviously this is a nasty situation for any engine and smooth running, this flutter may well be a contributing to the shunt. Get it on a solid full vacuum signal and the system will add timing in a much smoother way for sure!
The best bit about switching from ported to full vacuum is it's free and easy for anyone with a Chim or Griff to implement, if like most of us you're not bothered about emissions it's well worth doing, and for the MoT emissions test you can always revert back the dreadful ported system and the poor idle the 10-12 degrees of timing inflicts.
Ported vacuum is taken from a point in line with the throttle butterfly, as such and strictly speaking it is neither on the full vacuum side of the throttle butterfly or the atmosphere side either.
Basically using ported vacuum as the signal for the vac advance module means at idle you get zero effect, while people will tell you to disconnect the vac advance pipe on a Chimaera when setting your ignition timing to 10-12 degrees the truth is it makes no difference at all to leave it connected.
The advice to disconnect the vac advance when setting your ignition timing comes from the days when a full vacuum signal was used, on the Range Rovers, Chimaera's and Griffs where the ported vacuum signal was used.... you set your idle timing to 10-12 degrees and that's right where it'll stay, all be it with the normal jumps up and down by 3-4 degrees you see from a distributor under your timing light.
You need the engine pumping way harder than it can at idle and pumping hard against an almost closed throttle butterfly before you'll generate enough vacuum to start the vac advance unit pulling on the distributor base plate, so the truth is on our cars the vac advance only comes into play at small throttle openings over 1,600 rpm IE when your cruising.
By using ported vacuum Land Rover was able to get the benefits of 8-12 degrees of additional ignition advance at light load higher road speed cruise conditions but still keep the car idling at 10-12 degrees. The thing is this engine does not like to idle at 10-12 degrees, so why did they use ported vacuum...
. Well if you run retarded timing at idle two things happen, firstly you can reduce certain pollutants in the exhaust gasses and secondly you generate a lot of heat in the exhaust ports which aids early lighting of the cats. Internal combustion engines are most polluting at cold start and during the warm up phase so the earlier you can get the cats working the better, and cats need heat to start working.Ported vacuum was developed by the Yanks in the 70's when they were still using engines designed in the 1950's but ever sticker smog laws were forcing designers to come up with all kind of nasty sticking plaster emission reducing strategies like smog pumps and retarded idle timing. As always the British motor industry were even slower to develop new and more efficient power plants, so ten years later in the 1980's Land Rover started to adopt the rubbish sticking plaster strategies developed a decade before in order that they might squeeze a few more years out of the old Rover V8.
Make no mistake this engine does not want to idle at 10-12 degrees, any more than a small block Chevy wants to idle at 10-12 degrees, in the 1960'sand before all this emissions bollax all Yank V8s ran the 18-20 degrees at idle they wanted for a nice smooth idle. This is no different for our Rover V8 because essentially in all the important aspect the engine is like any other Yank V8, free yourself from worrying about emissions and fit an aftermarket engine management system and see what your Rover V8 idles like when you take it for 10-12 degrees to the 18-20 degrees it really wants.
Obviously the first thing that happens is your idle speed immediately goes up by 5-800rpm, screw it back down to 1,00rpm using your base idle screw and what your left with is an engine that idles so much smoother and cooler than you ever thought possible. Obviously if you are still running a distributor you'll need a different strategy to achieve the same thing, fortunately you can easily achieve an 18 degree idle by simply following how V8s were set up in the 60's, simply switch from the stupid ported vacuum and put the vac advance pipe to full vacuum and hey presto you're idle at 18 degrees.
And before people claim this is dangerous consider what happens as soon as you pull away, you open the throttle so that vacuum is lost instantly and the distributor will immediately go back to it's pre-set centrifugal advance curve that's governed by engine speed, bob weight mass and advance spring load. The other thing that will happen when you switch to full vacuum is the vac advance unit will add 3-4 degrees around 1,700-1,850 rpm right where the engine needs it, 3-4 degrees on top of the centrifugal advance doesn't sound like a lot but it's actually quite a bit of timing to add at such low engine speeds and is certainly plenty enough to notably smooth out the dreaded shunt.
Further more it's my belief that this still using ported vacuum may be getting vac advance flutter at 1,700-1,850 rpm, it may be there's an on/off effect at this engine speed/throttle opening that means the a couple of degrees of timing is being rapidly added and removed, obviously this is a nasty situation for any engine and smooth running, this flutter may well be a contributing to the shunt. Get it on a solid full vacuum signal and the system will add timing in a much smoother way for sure!
The best bit about switching from ported to full vacuum is it's free and easy for anyone with a Chim or Griff to implement, if like most of us you're not bothered about emissions it's well worth doing, and for the MoT emissions test you can always revert back the dreadful ported system and the poor idle the 10-12 degrees of timing inflicts.
ChimpOnGas said:
The best bit about switching from ported to full vacuum is it's free and easy for anyone with a Chim or Griff to implement, if like most of us you're not bothered about emissions it's well worth doing, and for the MoT emissions test you can always revert back the dreadful ported system and the poor idle the 10-12 degrees of timing inflicts.
Where's the pick-up point for full vacuum? There should be at least one or more blanks on the driver's side of the plenum base. If you fit one of these plugs - called a ram pipe plug - it is very easy and neat to connect a vacuum pipe. Use a suitable washer.
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