BoxerFlat4

Just had a chat with someone, who was running a gauze filter and dumpvalve. He was having problems with the dv making funny noises when lifting off, and he mentioned "negative boost" being mentioned.

Would there be anyway for the airflow to reverse itself when lifting off ? Can't imagine it myself, but it's the only explanation I can come up with for "negative boost".

Comments ?

chockymonster

Isn't negative boost what all N/A cars have?
If he means negative as measured on a boost gauge then this is measuring the vacuum in the engine. at full throttle he'll be on boost but with the throttle shut then he'll be showing normal vacuum in the engine.

I'm probably wide of the mark on this one so stand to be corrected lots

marty_t3

Boxer,

Negative boost (i think) is when you lift off the accelerator then put your foot back on it (like during a gear change). If the turbo stops/slows down then it isn't pushing as much air in as the engine "wants".. The engine will suck the air in itself and in doing so it lowers the pressue between the airfilter and the intake (below atmospheric pressure)... hence.. negative boost... until the turbo gets up to speed again...All naturally aspirated cars run with "negative boost" all the time.
If you watch the boost gauge closely during a gear changed you should see it drop below 0 for a split second as the turbo begins to get back up to speed.

marty.

</hiding behind the seat in case i'm wrong/>

BoxerFlat4

Thanks for the replies fellas -

I understand the theory, but why would this cause a dv to start making "strange noises" ? It was a Bailey dv, fitted properly allegedly, yet my mate complained of the noise.

Might have to get some more details....

marty_t3

not sure what you mean by strange noises... the dump valve should make a noise if it's working normally.

<<forgive me if you already know this next bit>>

When the throttle is closed (when you lift off the accelerator) there is a brief moment when the turbo is still spinning and the pressure peaks between the turbo and the untake. This peak pressure is high enough to "set off" the dump valve... the dump valve then lets the air trapped between the turbo and the intake escape... with a dump to atmoshpere valve this is usually noisy. You should only hear it when the car has been boosting and you lift off the throttle. Bear in mind that some dump valves do make different noises (mostly the expensive sequential ones), but most just give off a loud hiss.

Try find out when the noise occurs and exactly what the strange noise sounds like.

marty,

marty_t3

one more thing i should mention ... I've had a K&N cone filter make a fluttering noise on lift off. It actually sounded a bit like a dump valve releasing air.. sound lasted about 1/2 a sec after every gearchange. It was perfectly normal, just air moving through the filter while the pressure equialized. I think the bracket it was mounted to (a 6 inch long, 1 inch wide metal rod) made it sound fluttery rather than just a whoosh noise.

BoxerFlat4

marty -

Thanks for the feedback, the description of the sound I got was a "honking" noise. My mate seemed to think it happened after he lifted off, but before re-applying the throttle.

Probably just have to put it down to "one of those things". Guys going to go for a HKS SQV anyway.

z_chromozone

When he presses the horn does it make a psssk noise, if so I think I've solved the problem.

BoxerFlat4

lol