MX5 Turbo - Hesitation on Boost
Discussion
Hi,
Just wondering if anyone can provide any advice on the following. Recently I've been noticing some hesitation when the car comes onto boost and this is apparent right through the rev range. Just this week I took the car into a local specialist and they first had a look at the plugs and advised they were fouled up slightly and thought the car was possibly running rich. In doing so the spark itself wasn't therefore adequate which in turn would cause the hesitation. Having spoke to them today they have advised that the Greddy e-manage Blue the car is setup with doesn't support the Innovate Wideband O2 sensor and since disconnecting it the problem has almost fully gone bar at the very top of the rpm range.
I'm wondering can the car run on the map it has constantly without a wideband o2 sensor and given the sensor has now been disconnected, will the car continue to run rich and foul up spark plugs? The map the car has seems good in the sense of the torque curve seems healthy and the power output is good. I'm thinking the Wideband was possibly added later on or that the Greddy e-manage blue does support it and it's crapped out?
Just wondering if anyone can provide any advice on the following. Recently I've been noticing some hesitation when the car comes onto boost and this is apparent right through the rev range. Just this week I took the car into a local specialist and they first had a look at the plugs and advised they were fouled up slightly and thought the car was possibly running rich. In doing so the spark itself wasn't therefore adequate which in turn would cause the hesitation. Having spoke to them today they have advised that the Greddy e-manage Blue the car is setup with doesn't support the Innovate Wideband O2 sensor and since disconnecting it the problem has almost fully gone bar at the very top of the rpm range.
I'm wondering can the car run on the map it has constantly without a wideband o2 sensor and given the sensor has now been disconnected, will the car continue to run rich and foul up spark plugs? The map the car has seems good in the sense of the torque curve seems healthy and the power output is good. I'm thinking the Wideband was possibly added later on or that the Greddy e-manage blue does support it and it's crapped out?
I suggest you find a tuner who is competent.
Because that explanation makes no sense.
" they have advised that the Greddy e-manage Blue the car is setup with doesn't support the Innovate Wideband O2 sensor and since disconnecting it the problem has almost fully gone bar at the very top of the rpm range."
Can you explain this better ?
You seem to be suggesting a new problem, so clearly whatever way it was set up worked fine before, but doesnt now. Or have you added the Innovate somehow prior to the problem ?
Disconnected how ? Is it even wired into the Greddy and is the Greddy actually using it ? Have they tested the wideband to see if it is working ?
It sounds like this shop has little ability and havent actually tested anything ? and are just making wild guesses/assumptions ?
Because that explanation makes no sense.
" they have advised that the Greddy e-manage Blue the car is setup with doesn't support the Innovate Wideband O2 sensor and since disconnecting it the problem has almost fully gone bar at the very top of the rpm range."
Can you explain this better ?
You seem to be suggesting a new problem, so clearly whatever way it was set up worked fine before, but doesnt now. Or have you added the Innovate somehow prior to the problem ?
Disconnected how ? Is it even wired into the Greddy and is the Greddy actually using it ? Have they tested the wideband to see if it is working ?
It sounds like this shop has little ability and havent actually tested anything ? and are just making wild guesses/assumptions ?
stevieturbo said:
I suggest you find a tuner who is competent.
Because that explanation makes no sense.
" they have advised that the Greddy e-manage Blue the car is setup with doesn't support the Innovate Wideband O2 sensor and since disconnecting it the problem has almost fully gone bar at the very top of the rpm range."
Can you explain this better ?
You seem to be suggesting a new problem, so clearly whatever way it was set up worked fine before, but doesnt now. Or have you added the Innovate somehow prior to the problem ?
Disconnected how ? Is it even wired into the Greddy and is the Greddy actually using it ? Have they tested the wideband to see if it is working ?
It sounds like this shop has little ability and havent actually tested anything ? and are just making wild guesses/assumptions ?
They have advised it doesn't support it and in not supporting it the wideband sensor has become faulty. This I don't quite understand? I'm lead to believe they can become faulty with overfueling issues amongst lots of other things but not that - perhaps he explained it poorly. It didn't work fine before as there was hesitation on boost and this is the reason they have given.Because that explanation makes no sense.
" they have advised that the Greddy e-manage Blue the car is setup with doesn't support the Innovate Wideband O2 sensor and since disconnecting it the problem has almost fully gone bar at the very top of the rpm range."
Can you explain this better ?
You seem to be suggesting a new problem, so clearly whatever way it was set up worked fine before, but doesnt now. Or have you added the Innovate somehow prior to the problem ?
Disconnected how ? Is it even wired into the Greddy and is the Greddy actually using it ? Have they tested the wideband to see if it is working ?
It sounds like this shop has little ability and havent actually tested anything ? and are just making wild guesses/assumptions ?
They also stated the map is setup for drifting hence why towards the top of the rpm range it overfuels.
As stevie says, find a new specalist.
The LC-1 has two linear (or non-linear, they're programmable) 0-5v outputs - these can also be configured to output any voltage range within that 0-5v, so you can emulate a stock narrowband by programming one of the outputs to 0-1v.
This is something the eManage Blue most definitely supports, not necessarily through it's O2 sensor input, but you can definitely utilise it. A lot of people use the TPS input.
To me it sounds like you have some form of vac leak. the eMB retains the stock ECU as it's a piggyback. This means you're still running a mass airflow system (unlike most aftermarket ECU's which use Speed Density), long and short is if there are any vac leaks the ECU won't know about it and will struggle to fuel the car properly.
I suggest you sign up and post on www.mx5nutz.com/forum There are a lot of modified mx5 owners and specialists who post on there and will be able to help further.
The LC-1 has two linear (or non-linear, they're programmable) 0-5v outputs - these can also be configured to output any voltage range within that 0-5v, so you can emulate a stock narrowband by programming one of the outputs to 0-1v.
This is something the eManage Blue most definitely supports, not necessarily through it's O2 sensor input, but you can definitely utilise it. A lot of people use the TPS input.
To me it sounds like you have some form of vac leak. the eMB retains the stock ECU as it's a piggyback. This means you're still running a mass airflow system (unlike most aftermarket ECU's which use Speed Density), long and short is if there are any vac leaks the ECU won't know about it and will struggle to fuel the car properly.
I suggest you sign up and post on www.mx5nutz.com/forum There are a lot of modified mx5 owners and specialists who post on there and will be able to help further.
Thanks for the advice. I spoke to another of the guys who seemed far more knowledgeable today. He advised the wideband sensor was installed incorrectly due to the way it was wired up. Healso advis its faulty. He stated the car now pulls smoothly and the only issue is the car running slightly rich which will lessen the economy (not fussed) and shorten the life of the spark plugs so these are being replaced due to being so cheap so I can get an idea on how quickly they foul up.
He thinks the o2 sensor was added at a later date to the turbo install and that it won't make any difference due to the car having a map that was done with an aux o2 wideband.
He thinks the o2 sensor was added at a later date to the turbo install and that it won't make any difference due to the car having a map that was done with an aux o2 wideband.
Buzzkill said:
Thanks for the advice. I spoke to another of the guys who seemed far more knowledgeable today. He advised the wideband sensor was installed incorrectly due to the way it was wired up. Healso advis its faulty. He stated the car now pulls smoothly and the only issue is the car running slightly rich which will lessen the economy (not fussed) and shorten the life of the spark plugs so these are being replaced due to being so cheap so I can get an idea on how quickly they foul up.
He thinks the o2 sensor was added at a later date to the turbo install and that it won't make any difference due to the car having a map that was done with an aux o2 wideband.
Firstly, don't guess/think, you should be checking facts.He thinks the o2 sensor was added at a later date to the turbo install and that it won't make any difference due to the car having a map that was done with an aux o2 wideband.
The LC-1 is an auxiliary wideband. I doubt it's faulty either, probably just badly wired. It'll have been installed with one output simulating a narrowband to keep the stock ECU happy, the other will have been used to either view on a gauge, or plumbed into the eMB.
The eMB can't/doesn't run any form of closed loop fuelling though. Ergo it can't act on any information the wideband it providing, it's purely for manually tuning. By that I mean you log a run and look at the widband output and where it's lean add fuel, where it's rich you remove fuel.
By removing the LC-1 entirely, you've forced the stock ECU into running open loop all the time, this will cause your car to run excessively rich, you'll waste fuel, you'll rob yourself of power, and you risk washing the bores and causing excessive wear and premature engine failure.
What you've done is fixed the symptoms, not the cause.
Re-fit the wideband, install it EXACTLY as per the instructions, they're very sensitive to poor grounds, you should tap into the ECU grounds, don't tap into the radio grounds or anything daft like that. Give the stock ECU a simulated narrowband signal, (or re-fit an OEM narrowband sensor and then weld a second bung for the wideband).
Then log full throttle runs and ensure the AFR's are correct, if they're not tune the car until they are.
You may also need to invest in an O2 clamp, this 'clamps' the O2 signal as you enter boost but are still in closed loop mode, it stops the stock ECU fighting the eMB. The Stock ECU will try to pull fuel when the eMB will be adding it - the stock ECU has no idea extra air is being forced into the engine.
My 2000 NB 1.8 runs like a dream on a Microsquirt with a GT2860 turbo and a Techedge wideband. I have had nothing but bad experiences with the Innovate widebands. Have tried several of them over the years all have been shortlived, inacccurate and unreliable plus their tech support sucks. You really don't want it running excessively rich, it will bore wash and dilute the oil, as well as missing and fouling plugs.
I'm currently using a techedge 2j2 and have also installed a PLX to compare the readings. Both are nice units and they agree to within 0.1 AFR with each other. The microsquirt V3 install was very easy and the transformation over a piggyback system was night and day. May be worth considering doing it if time and budgets allow.
Just for the benefit of others considering Turbocharging, I used a BEGI cast manifold and matching downpipe, RX8 injectors, GT2860RS 0.86 AR pukka garret turbo. Standard fuel pump, fuel rail and regulator. Intercooler kit form universal intercoolers which needed a bit of work to fit but works nicely. The crank wheel was replaced with a Jenvey 36-1 and uses the standard MX5 pickup. No cam sensor needed. Idle control is with the standard MX5 idle control valve. Manley forged rods and Wiseco 8.3:1 forged pistons, which are very quiet even clock cold. Cams are standard hydraulic with a MK1 head assembly.
It runs fantastic, makes over 300HP at 20PSI and has barely a hint of lag.
I'm currently using a techedge 2j2 and have also installed a PLX to compare the readings. Both are nice units and they agree to within 0.1 AFR with each other. The microsquirt V3 install was very easy and the transformation over a piggyback system was night and day. May be worth considering doing it if time and budgets allow.
Just for the benefit of others considering Turbocharging, I used a BEGI cast manifold and matching downpipe, RX8 injectors, GT2860RS 0.86 AR pukka garret turbo. Standard fuel pump, fuel rail and regulator. Intercooler kit form universal intercoolers which needed a bit of work to fit but works nicely. The crank wheel was replaced with a Jenvey 36-1 and uses the standard MX5 pickup. No cam sensor needed. Idle control is with the standard MX5 idle control valve. Manley forged rods and Wiseco 8.3:1 forged pistons, which are very quiet even clock cold. Cams are standard hydraulic with a MK1 head assembly.
It runs fantastic, makes over 300HP at 20PSI and has barely a hint of lag.
Sounds like an interesting car. I respectfully disagree on Innovate products however. Once installed properly they're a fantastic unit that respond extremely quickly and allow you to re-calibrate if sensor drift occours - This will happen btw, but with your techedge, there's nothing you can do about it, and you've have no idea it's happened.
I do understand that some people have issues with Innovate products, IME it's always been down to the install, I'm not saying it's the same in your case by the way, no company has a 100% track record!
Can I ask why you did you not use the stock cam and crank signals though? The uS has the code to run them perfectly fine. It would also mean you could have retained sequential injection which is a boon when using larger injectors and want to idle at stoich and improve MPG when pootling about.
I'm also extremely curious as to why you swapped the much better flowing MK2 head to the inferior MK1 head.
I do understand that some people have issues with Innovate products, IME it's always been down to the install, I'm not saying it's the same in your case by the way, no company has a 100% track record!
Can I ask why you did you not use the stock cam and crank signals though? The uS has the code to run them perfectly fine. It would also mean you could have retained sequential injection which is a boon when using larger injectors and want to idle at stoich and improve MPG when pootling about.
I'm also extremely curious as to why you swapped the much better flowing MK2 head to the inferior MK1 head.
OP, I've got a very similar problem to you at present. My setup is 1.6, greddy turbo, intercooler, 320 injectors and and emanage blue. I have the LC1 (wired correctly), so as was written earlier, 1 feed goes to the original ecu (narrow band) and the other will go to the piggy back.
At presen, mine will hold a correct AFR (around 14.7) until either it is just coming on boost or you rev it to 4950 revs, at which point the AFR will rapidly descend to 9.5.
If you apply full throttle from lower revs, it will initially drop the AFR to 9/10, then hold 13.5 (as it should) until the revs climb, then it will lean off to 14.7. When it was behaving properly, it would hold 13.5 approx all the way through.
So far, I have done a leak test on the boost pipework, replaced injector seals, checked every vac line I can find, isolated tubes to boost gague, fuel pressure reg, map sensor. Disconnected EMB so it runs stock ecu, still does it.
I am concerned about my fuel pressure reg, as I can hear a faint hissing through the pipework when it is connected. Surely that's wrong. As the vac line to the reg only affects the engine at idle, I blanked that off - still the same.
A few weeks ago, I had the opposite happeining, in that it would go lean at idle (20 AFR), this turned out to be a squished injector seal, replaced, all good! 5 days later, I have the problem we're discussing now. I'm getting dangerously close to taking it for a diagnostic.
Also OP, I wonder if you should asks the mods to move this topic to the MX5 forum.
If you have had any developments please let us know!
At presen, mine will hold a correct AFR (around 14.7) until either it is just coming on boost or you rev it to 4950 revs, at which point the AFR will rapidly descend to 9.5.
If you apply full throttle from lower revs, it will initially drop the AFR to 9/10, then hold 13.5 (as it should) until the revs climb, then it will lean off to 14.7. When it was behaving properly, it would hold 13.5 approx all the way through.
So far, I have done a leak test on the boost pipework, replaced injector seals, checked every vac line I can find, isolated tubes to boost gague, fuel pressure reg, map sensor. Disconnected EMB so it runs stock ecu, still does it.
I am concerned about my fuel pressure reg, as I can hear a faint hissing through the pipework when it is connected. Surely that's wrong. As the vac line to the reg only affects the engine at idle, I blanked that off - still the same.
A few weeks ago, I had the opposite happeining, in that it would go lean at idle (20 AFR), this turned out to be a squished injector seal, replaced, all good! 5 days later, I have the problem we're discussing now. I'm getting dangerously close to taking it for a diagnostic.
Also OP, I wonder if you should asks the mods to move this topic to the MX5 forum.
If you have had any developments please let us know!
we will agree to differ on Innovate.
My Mk2 head was cracked between the valves as the first build was on the original 99,000 mile MK2 engine with standard compression ratio. It was well quick but didn't last long! With regard to the triggers, the 4 tooth standard trigger can be miles out over 1 engine cyle out due to crank acceleration on a light engine. You get a lot of timing jitter with it. Given that the Jenvey setup is only £50 I went for it. I take your point on the sequential, it was a right pig to get the idle clean with the big injectors, especially as the idle fuel pressure is so low that they dribble more than squirt. I'm still not 100% happy with it as I'm running 14:1 at idle when it should be happy at stoich. The general MPG is very good though. Without being lead footed I'm hovering around 35MPG with a nice lean cruise of 15.5:1 but rapidly dropping to 13:1 at moderate acceleration when just going into boost and 11.8:1 at full tilt. The 6 speed box was a disappointment though, it's not nearly as slick as the five speed one was and no significant overdrive in top compared to the five.
With hindsight I would go for the full megasquirt V3 with the extra inputs and outputs. Handy for things like alternator control and all the weird and wonderful valves like the one that applies atmospheric pressure to the fuel pressure reg when your on idle to up the fuel pressure.
fatjon said:
we will agree to differ on Innovate.
My Mk2 head was cracked between the valves as the first build was on the original 99,000 mile MK2 engine with standard compression ratio. It was well quick but didn't last long! With regard to the triggers, the 4 tooth standard trigger can be miles out over 1 engine cyle out due to crank acceleration on a light engine. You get a lot of timing jitter with it. Given that the Jenvey setup is only £50 I went for it. I take your point on the sequential, it was a right pig to get the idle clean with the big injectors, especially as the idle fuel pressure is so low that they dribble more than squirt. I'm still not 100% happy with it as I'm running 14:1 at idle when it should be happy at stoich.
If you still have the MK1 CAS, you can cut the inner disc to give only 1 puse per cam revolution, feed this into your MegaSquirt and you have enough information to run sequential.My Mk2 head was cracked between the valves as the first build was on the original 99,000 mile MK2 engine with standard compression ratio. It was well quick but didn't last long! With regard to the triggers, the 4 tooth standard trigger can be miles out over 1 engine cyle out due to crank acceleration on a light engine. You get a lot of timing jitter with it. Given that the Jenvey setup is only £50 I went for it. I take your point on the sequential, it was a right pig to get the idle clean with the big injectors, especially as the idle fuel pressure is so low that they dribble more than squirt. I'm still not 100% happy with it as I'm running 14:1 at idle when it should be happy at stoich.
It's pretty simple to add an additional 2 injector drivers and then you're golden
fatjon said:
With hindsight I would go for the full megasquirt V3 with the extra inputs and outputs. Handy for things like alternator control and all the weird and wonderful valves like the one that applies atmospheric pressure to the fuel pressure reg when your on idle to up the fuel pressure.
MSIII is absolutely superb, (I've been through every iteration and am on MS3 now). Alternator control isn't quite there yet, but there are simple circuits on-line which replicate the stock MX5's alternator control. They easilly fit in the prototype area of a V3 mainboard.I particularly like the native on board knock conditioner and RTC. Makes tuning ignition a lot easier with a nice safety net when really pushing the engine, and the RTC means finding the SDcard datalogs I want is much easier!
fatjon said:
especially as the idle fuel pressure is so low that they dribble more than squirt.
mmmm? really? why is that the case? If you have your FPR referenced to intake manifold pressure, Injector Delta Pressure should remain constant (yes, rail pressure falls, but IDP remains constant.) Actually, IDP generally increases at idle as the pressure regulator is at maximum relief opening (hardly any fuel is going into the engine) so the spring rate of the regulator piston results in a larger pintle load and a higher rail pressure.
If your FRP is referenced to atmosphere, the IDP goes up as MAP falls.
The issue with idle is usually that the engine friction is low, so BMEP is low (effectively, no flywheel work is being consumed, BMEP = FMEP if rpm is constant) so the injected fuel mass per firing event is low, and with high flow injectors this requires only a short energised duration, which can often be in the non linear region of the injector opening (pintle does not have time to fully open)
For "Normal" injectors, being driven directly off the vehicle battery / alternator (at ~14.7v) generally, below approximately 2ms duration they will start becoming very non-linear. (they won't even open below ~1ms, and between 1ms and 2ms, the fuel deliver mass will not be very consistent)
Also, if you are running sequential injection, have a play around with moving the injection timing around, you may find that injecting on an already open inlet valve can help idle stability (depending on things like your throttling system and cam overlap etc)
Finally, make sure you are running a lot of spark reserve, i.e. an ignition angle retarded away from MBT. This reduces torque, and allows you to open the throttle more, increase airflow (for a given torque and hence a given idle speed) and that means you need more fuel to get back to your target lambda. This results in longer injection durations and helps move the injector towards a more stable operating region. As a handy benefit, it also gives you a nice big torque reserve, that can be accessed quickly with cycle-by-cycle changes in ignition angle.
Also, if you are running sequential injection, have a play around with moving the injection timing around, you may find that injecting on an already open inlet valve can help idle stability (depending on things like your throttling system and cam overlap etc)
Finally, make sure you are running a lot of spark reserve, i.e. an ignition angle retarded away from MBT. This reduces torque, and allows you to open the throttle more, increase airflow (for a given torque and hence a given idle speed) and that means you need more fuel to get back to your target lambda. This results in longer injection durations and helps move the injector towards a more stable operating region. As a handy benefit, it also gives you a nice big torque reserve, that can be accessed quickly with cycle-by-cycle changes in ignition angle.
Edited by anonymous-user on Wednesday 5th June 16:49
Thanks for the excellent tip. I will get plugged in at the weekend and pull a bit of timing and up the duty cycle on the idle valve a little. I can see the logic of the argument. I think I may be limited by cooling system if I go too far with it though. It heats up pretty quick if it gets too much retard. That said, I have room for a second fan on the radiator and I can live with that solution for a cleaner idle. The symptoms certainly fit your description, in particular the very non linear behaviour of the injectors at idle with variations in electrical load. The ECU has a compensation figure for a change in voltage but as you say, the variation is so non linear that it is all but useless as a method of controlling the amount injected at idle. My typical idle PW at 900 RPM and 14:1 AFR is 1.5ms at it maxes out on full boost at around 80% duty cycle so I don't have a lot of scope for a change of injector size even tough it's a rather short idle pulse width.
Had a similar but unrelated problem on my Cerbera. It had an intake harmonic at very low RPM with fuel standoff when running at very small throttle angles at low speed. Made it shunt and was a very unpleasant drive. Same cure though, I pulled some timing and that means you naturally open the throttle a little more which solved the problem. That tip came from Dave Walker at Emerald, now there's a man that knows his stuff. Just didn't want to double the value of my MX5 by adding one of his ecu's.
Had a similar but unrelated problem on my Cerbera. It had an intake harmonic at very low RPM with fuel standoff when running at very small throttle angles at low speed. Made it shunt and was a very unpleasant drive. Same cure though, I pulled some timing and that means you naturally open the throttle a little more which solved the problem. That tip came from Dave Walker at Emerald, now there's a man that knows his stuff. Just didn't want to double the value of my MX5 by adding one of his ecu's.
Max_Torque said:
For "Normal" injectors, being driven directly off the vehicle battery / alternator (at ~14.7v) generally, below approximately 2ms duration they will start becoming very non-linear. (they won't even open below ~1ms, and between 1ms and 2ms, the fuel deliver mass will not be very consistent)
Also, if you are running sequential injection, have a play around with moving the injection timing around, you may find that injecting on an already open inlet valve can help idle stability (depending on things like your throttling system and cam overlap etc)
Finally, make sure you are running a lot of spark reserve, i.e. an ignition angle retarded away from MBT. This reduces torque, and allows you to open the throttle more, increase airflow (for a given torque and hence a given idle speed) and that means you need more fuel to get back to your target lambda. This results in longer injection durations and helps move the injector towards a more stable operating region. As a handy benefit, it also gives you a nice big torque reserve, that can be accessed quickly with cycle-by-cycle changes in ignition angle.
Just requoting for truth, I can't put it as eloquently as that but that's exactly what I do to improve idle. Good to have my process validated!Also, if you are running sequential injection, have a play around with moving the injection timing around, you may find that injecting on an already open inlet valve can help idle stability (depending on things like your throttling system and cam overlap etc)
Finally, make sure you are running a lot of spark reserve, i.e. an ignition angle retarded away from MBT. This reduces torque, and allows you to open the throttle more, increase airflow (for a given torque and hence a given idle speed) and that means you need more fuel to get back to your target lambda. This results in longer injection durations and helps move the injector towards a more stable operating region. As a handy benefit, it also gives you a nice big torque reserve, that can be accessed quickly with cycle-by-cycle changes in ignition angle.
Edited by Max_Torque on Wednesday 5th June 16:49
Edited by Richyvrlimited on Thursday 6th June 09:28
Max_Torque said:
Also, if you are running sequential injection, have a play around with moving the injection timing around, you may find that injecting on an already open inlet valve can help idle stability (depending on things like your throttling system and cam overlap etc)
I've never known injecting through an open inlet valve to do anything more than ruin your combustion stability. Optimal (port) injection timing usually starts just at or slightly after inlet valve closing, especially at part load.Still, what would I know...
AER said:
Max_Torque said:
Also, if you are running sequential injection, have a play around with moving the injection timing around, you may find that injecting on an already open inlet valve can help idle stability (depending on things like your throttling system and cam overlap etc)
I've never known injecting through an open inlet valve to do anything more than ruin your combustion stability. Optimal (port) injection timing usually starts just at or slightly after inlet valve closing, especially at part load.Still, what would I know...
BTW: from the above reply, retarding the ignition angle should help with engine cooling! (more heat is lost out the exhaust, and less into the metal of the engine). In any case, your cooling system should not have any issues at idle, as the heat flux is many times lower than for any other running case (because the fuel mass is lower). If you have idle cooling issues it is due to other non-engine related things (like a lack of convection/forced airflow from the radiator etc)
Fair enuf! I haven't played with any ridiculous misfiring engines (except for two-strokes, perhaps.)
I think you're possibly right about the reduced flow reversal with port throttles, due to the lower differential pressure across the head during the valve overlap period, but I think the reduced exhaust residual mass would also play a significant part in combustion stability at idle.
It's a moot point though, since they run better and sound awesome whatever is happening. I do have a box full of Jenveys for my Elise that I should pull my finger out and fit!
I think you're possibly right about the reduced flow reversal with port throttles, due to the lower differential pressure across the head during the valve overlap period, but I think the reduced exhaust residual mass would also play a significant part in combustion stability at idle.
It's a moot point though, since they run better and sound awesome whatever is happening. I do have a box full of Jenveys for my Elise that I should pull my finger out and fit!
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