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Thread: A poser for the turbo experts...

  1. #101
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    The IAC valve maybe? How much this is allows through I wouldn't know though.

  2. #102
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    There will still be air going in through the idle bypass pipe, but not a lot. That pipe is only ~15mm(?) internal diameter, it's not big.

  3. #103
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    Yeah that's the one I meant.

    I'd say Clive's empirical argument is the most convincing. Do we know what the AFR reads with a recirc, I'm guessing not 10/off the scale

  4. #104
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    Quote Originally Posted by dan_t View Post
    OK I agree with this... Throttle closes so the injectors are cut. Now if the injectors are cut, who cares where the air goes? Its never going to run rich if the injectors are cut as there will be no combustion at all.
    As Ben send though, if you DON'T close the throttle completely the injectors don't shut down.

    Anyway, I'm not sure where the statement about it being dangerous came from. It's not dangerous, I used to run it no problems and that includes the Cat being fine. However it's not ideal.

  5. #105
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    Quote Originally Posted by dan_t View Post
    OK I agree with this... Throttle closes so the injectors are cut. Now if the injectors are cut, who cares where the air goes? Its never going to run rich if the injectors are cut as there will be no combustion at all.
    so, an idling engine has no combustion at all?

  6. #106
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    Quote Originally Posted by PR200SX View Post
    so, an idling engine has no combustion at all?
    The IAC valve keeps the engine running at idle with throttle plate closed

  7. #107
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    Quote Originally Posted by PR200SX View Post
    so, an idling engine has no combustion at all?
    It only cuts over a certain speed, around 2000rpm. Below that they'll keep firing.

    Hook up consult and coasting down hills you can do 300+mpg

  8. #108
    Guest PR200SX's Avatar
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    Quote Originally Posted by drifter View Post
    The IAC valve keeps the engine running at idle with throttle plate closed
    I know, I was quoting someone elses post.

  9. #109
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    LOL, I'm totally confused now

  10. #110
    Guest PR200SX's Avatar
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    most modern injection engines use VERY little fuel while engine braking (no throttle, slowing down) but there will always be combustion, there is always a spark, just VERY little fuel.

  11. #111
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    Which is why I think my old man is a muppet because he knocks the car out of gear when slowing to save fuel It uses more fuel to keep the engine running at idle than it would coasting with the throttle closed.

  12. #112
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    Quote Originally Posted by BenTaylor200 View Post
    Yes, because it isn't trapped, it's recirculated back through the compressor of the turbo turbo (which will have slowed down, but not stopped) and into the engine.

    It becomes a lot more noticable when you start running a bigger turbo with a lot more boost. You notice it more when you're coming off the throttle slowly (throttle plate and dump valve are open at the same time), if you're going through the gears really quickly its not so much of an issue.
    when i turn my boost controller up i notice it more...

    Quote Originally Posted by stu_ View Post
    Yeah that's the one I meant.

    I'd say Clive's empirical argument is the most convincing. Do we know what the AFR reads with a recirc, I'm guessing not 10/off the scale
    as i said... im more then happy to put a re-circ on (just need a bit of pipe) and ill do a before and after video of an AEM afr gauge hooked up to a wideband sensor....

    ill probably just end up leaving the re-circ one on.... but im a " chav " and like the noise

    science (physics)... never did enjoy the theory side/arguments ... but liked the experiments (mostly the burning things ) with some hard hitting facts there in front of you... if you get what im saying
    Last edited by Clive; 04-01-2010 at 18:10.

  13. #113
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    What reading do you think you'd get on your wideband if it was in a vacuum?

    Now lets say the atmos valve has just gone off. There is now no oxygen in the intake as its been released. The engine is still turning over and the exhaust is still scavenging. Is it not possible that this causes a vacuum in the exhaust?

  14. #114
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    Quote Originally Posted by PR200SX View Post
    so, an idling engine has no combustion at all?
    We was talking about overrun

  15. #115
    Guest EvilScotsman's Avatar
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    yup.

    I didnt insult any one, and I was drinking like everyone else, I just wanted to know why it happens.

    When I worked out the percentage extra fuelling that seems to occur, it was such a tiny amount that it seemed insignificant. My whole point was everyone says that the air shouldnt be vented to atmosphere as engine damage will result, and I asked why.

    No one was able to give a proper explanation except to say something along the lines of "it just does".

    Now, it seems like any question that cant be answered, and has nothing to do with bodykits or kouki lights or summat is taboo? Otherwise why did I get flamed to death by almost everyone here except a couple?

    And since some of you seem intent to make stupid childish remarks about me being scottish, as if there is something wrong with me already because of that, there is no point in discussing anything with most of those who replied.

    A playground-style mauling where everyone jumps on one guy for asking something they couldnt answer is not a discussion.

    I posted some figures to show that the overfuelling that occurs is very small, and pretty much insignificant, and asked anyone who could say in technical terms why it IS harmful. No one could refute the info, so instead resorted to insults about being me being scottish, saying I must be someone they used to dislike on here using a different name, and it all just got ugly and unpleasant.

    I think most of you just showed exactly what kind of people you really are, and only a couple of folk actually saw what I was getting at. Read all of my posts again, and tell me where I MYSELF insulted anyone....then count how many times someone made a personal reference to ME that was insulting.

    Nah, I think I will refrain from posting anything and just read what info I need from now on.

    Cheers for the welcome lads.

    (certain persons excepted, the genuine punters - you know who you are.)

    Kilt-voice "off" now.

    Oh wait, there's no such thing as a kilt-voice. Tosser.

  16. #116
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    It's not going to cause any engine damage (except perhaps the cat, but I've never heard of one dying because of it) but it does make it go lumpy for a second when you come off the throttle; for example when you accelerate up to motorway speed in 5th and then lift off to keep your speed steary, you get a big jerk with no DV.

  17. #117
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    Quote Originally Posted by EvilScotsman View Post
    When I worked out the percentage extra fuelling that seems to occur, it was such a tiny amount that it seemed insignificant. My whole point was everyone says that the air shouldnt be vented to atmosphere as engine damage will result, and I asked why.

    No one was able to give a proper explanation except to say something along the lines of "it just does".

    I posted some figures to show that the overfuelling that occurs is very small, and pretty much insignificant, and asked anyone who could say in technical terms why it IS harmful. No one could refute the info.....
    I don't think people believe it is harmful though which is why no one posted up evidence of it being so. Well, I can't remember it being implied that way other than the cat, which imo is not an issue either.

    As I posted twice earlier, it's not harmful, just not ideal

  18. #118
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    Funnily enough I get that jerk in 5th just the same way, and now i've looked I cant spot the dv, maybe mine's missing but there isnt any obvious bungs either. I just assumed it was slack in the drivetrain of which there is plenty in the worn subby bushes etc. Interesting...

    Anyhoo, cheers for the discussion, thats all I wanted to know. I have an apexi stainless cone kit fitted to my audi quattro turbo (2003 model) and the factory dv chuffs away nicely thru the cone filter since it dumps into the intake just behind the maf on the 20v motor.

    I'm going to fit the same model apexi kit to the sx, and wonder if it will cause the same effect without changing the factory dv.

    I know that having the dv deleted completely is bad for the turbine due to backflush turbulence etc, not a great idea unless its a race engine and you chuck turbo's away whenever required

    btw I heard some guys on YT running screamer pipes last night - sheesh that noise would do your head in on a daily basis eh? lol. Sounds like one of the Undead Wraiths from harry pothead flying down to scoop your soul out

    Last edited by EvilScotsman; 05-01-2010 at 12:20.

  19. #119
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    The standard dv is under the fusebox, just behind the battery. Recircs through the pipe across the top/front of the camcover back into the pipe just in front of the turbo.

    As for reducing the life of the turbo - S13s didn't have a DV and they don't get issues. S14s with no DV don't shat their turbos as soon as the dv is removed... The recirc is mainly to get around the overfuelling and to make it quiet, not prolong the life of the turbo. As for an atmospheric dv, that's just to make lots of noise.

  20. #120
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    Quote Originally Posted by EvilScotsman View Post
    When I worked out the percentage extra fuelling that seems to occur, it was such a tiny amount that it seemed insignificant.
    This topic is very interesting to me since I have first hand experience of this over fueling with vta dv. And I would also like to know the technical answers behind this.

    In my experience some setups (including mine) this insignificant extra fuelling you refer to can even stall the engine when coming from boost to idle. In my case it was cured with recirc dv. And yes I have a tuned chip for _my_ car.

    This problem can arise with the combination of many things including a bigger turbo, vta dv and a maf that also reads the air flow backwards (z32).

    If I had to guess (and this is purely speculation because I'm not technically savvy and I could be talking out of my arse) why this over fuelling after throttle shut happens when using vta dv is that something happens with the idle air system when vta dv opens in throttle shut. Is it that the air which was ment to be used for idling after throttle shut is sucked out of the system via vta dv, hence making it overfuel? Now if you have vta dv which opens very easy you worsen the situation. When it's set to hard (or no dv at all) you worsen the turbo chatter and maf possibly reads the air coming back thru from the turbo again, hence making it overfuel on partial throttle shut, which again isn't ideal.

    So I have figured there are downsides for an easy venting dv, hard venting dv and no dv at all, which made me recirc my setup and I'm never going to any other system again on maf setups.

    Unfortunately I have no technical data or calculations to back these assumptions but my recently installed wideband lambda, which gives me nice readings (not rich) now on gear changes and coming to off boost/idle. Shame I didn't have it when I had the problems.

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