Author Topic: Boosting rpm  (Read 481 times)

Offline metalblue

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Boosting rpm
« on: April 13, 2021, 02:34:10 pm »
This is my second G40 I’ve owned (well I own two but that’s another story) and I’ve got a vacuum/boost gauge fitted to this one (not had one before) and it shows vacuum until about 3k rpm then it starts to boost to around 12psi (not gone above 4K or so for now).  As soon as I lift off the throttle the gauge shows 0 which is what I would expect.   I haven’t really given the car any beans because I’m trying to solve an issue with the temperature sensor not going above basically cold at the moment (shorted gauge, was fine, and replaced both sensors with genuine VW and still no joy so working through that problem - not losing coolant but system doesn’t seem to build up pressure)

Running toothed belts, 65mm pulley, 51mm TB and ported inlet manifold.   Car just feels a little sluggish although it does liven up above 3k but was thinking maybe have a boost leak somewhere? but one of you more knowledgeable types might say boosting from 3k on is right or maybe it’s the ecu restricting something due to seeing the car running cold?

Offline PeteG40

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Re: Boosting rpm
« Reply #1 on: April 16, 2021, 12:47:51 pm »
on a supercharged car boost is always there but builds with revs.

so on idle it should be a vaccum... driving with half throttle you may get some boost (or a reduction in vaccum) as the boost return valve forces more ofthe air into the engine.  When you put your foot flat down the boost return valve closes so all air gets diverted into the engine.  so even at 1000rpm you should read boost with your foot flat down.

then as rpm builds with your foot flat down, so does boost.


Offline metalblue

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Re: Boosting rpm
« Reply #2 on: April 20, 2021, 11:43:49 am »
Thanks Pete, very much appreciated.  I always figured a supercharger should always boost (the benefit of them over turbos).  I’ll see what the gauge says if I bury the throttle, I know I’ve been a little gentle with it whilst I sort the temperature issue out.

I’ll let you know how I get on

Thanks again

Offline metalblue

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Re: Boosting rpm
« Reply #3 on: May 04, 2021, 05:24:02 pm »
Slightly embarrassing it looks like my “problem” was just down to a light right foot rather than the car.  Thing flies once you plant it and the revs are up!   Still can’t get the temp gauge to read above 70F I’ll open another thread on that one.

Cheers again