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Thread: dumpvalve problem?

  
  1. #1
    anothadave
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    dumpvalve problem?

    i noticed that when i accelarate and then lift off the gas the dump valve blows off almost straight away and the accelaration is instantly lost, it sometimes makes the car very jerky when driving. my mechanic said that quite a lot of modern turbo cars have an on/off feel to them and he seems to think that its relatively normal. but i had a 200sx years ago with the same type of dumpvalve and that was much smoother and not as abrupt when i lifted off the gas. it was running much more boost too.. (im running standard boost on the astra)

    ive been driving a v6 for many years now so i suppose im used to a different driving style and was wondering if this is a normal characteristic of the Z20let or could it be the dump valve? i notice on other threads that most people arent impressed with the bailey dump valves. i had one on the 200sx for years and ive found it did the job perfectly fine so if this is how the Z20let works then ill stick to the bailey and ill just have to get used to the jerkiness. but if not then ill shop around for another dump valve!

  2. #2
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    mine does this and due to it being my first turbo thought it was normal
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    Snipervxr's Avatar
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    The dumpvalve will dump when the pressure goes negative thats how they work... acceleration is lost because you lift off the accelerator thats how they work.

    i dont see any problem, of course it depends if the dumpvalve has been fitted correctly.

    the jerkyness probably just down to your manner of driving
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  4. #4
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    astra_dave's Avatar
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    have you disabled the rear bcs?
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    the rear bcs and vac box should soften the jerkyness, so if that's bypassed that will make it worse than it was

    but i've bypassed both of those and i don't find it jerks enough to worry about... if i let off suddenly, the jerk it significant, but driving normally it's fine...

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    when my dv was leaking the car was very jerky, and had a lot of lag between gear changes at high rpm. stuck a new one on and all the problems were sorted.
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  7. #7
    anothadave
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    i dont have any lag, just that when i slightly lift off the pedal i can hear the dumpvalve go off and i can noticeably feel the loss in power and the car slows quite quickly. its been quite a few years since i had a turbo car and im sure that my old one didnt do that. but like i said, ive been used to v6 power for many years now, when i lift off the accelerator the car still has forward momentum and will gradually slow down. the astra is more abrupt and immediately slows down.

    as far as i know the previous owner correctly bypassed/disabled the factory valve etc. they seemed to know their onions lol. from the sound of it then ive just got to adjust my driving style...

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    sounds normal to me, lifting off the pedal and the dump valve going of slightly is just the excess boost been released
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  9. #9
    anothadave
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    it was more the immediate loss in forward momentum that was bothering me, as soon as it goes off im jerked back by the loss of acceleration. but im assuming thats because the power from the turbo has been vented by the dumpvalve and the engine is basically just running on its own.

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    well, yeah, you're goinf from 10psi of boost to 20-hg vacuum so there's gonna be a massive loss of force in the engine

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