Thread: boost bleed off
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spuggy spuggy is offline
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Join Date: Jul 2003
Location: Glorious Pac NW
Posts: 2,830
Originally Posted by Alan L View Post
I have chosen to ignore the faint internal engine noise at the moment
Hmmm. These engines do sound like a bucket of hammers sometimes with the decklid up... But...

But this boost behaviour is consistent, repeatable, with the pedal floored. I wonder about a boost leak.
OK, that doesn't sound like load is a factor then.

Like Goran says, the factory BOV can leak boost if the seals are worn, makes a loud clicking noise, IIRC. There was a good thread on it quite recently.

What is a Dawes?
Proprietary type of manual boost controller.

It may be that the PO fitted an EBC to band-aid another problem - like a boost leak somewhere...

You re-worked the wastegate, so it's probably not that.

My take (and Goran surely knows more than me here) is that boost controllers work well - especially at higher boost levels than commonly used on 930's - by completely isolating the wastegate until boost is at the desired level it is to be regulated at. They thus prevent premature opening of the wastegate and the (very) undesirable early boost bleed off.

You can't run a very soft spring in the wastegate (and do it all in the MBC or EBC), because you still need to control the valve itself, mechanically. Think I read somewhere that .5 or .6 bar (or was that 5 or 6 PSI?) is the minimum advisable lower limit.

And changing springs is a pain, so having the capability to control the gate - even for an extra .1 or .2 bar of boost - without stripping the wastegate out could be a win. You could drop boost if you bought some lousy gas, for example.

I thought about trying an MBC and decided not to. With my C/R, I can't run significant extra boost on pump gas, and I'm really not convinced my wastegate has any leaking issues, so the only possible advantage (that of preventing the leaking of boost prematurely below the cut-off threshold) probably wouldn't be noticable/measurable.

For the sake of $60, I might try one anyway (set to the stock boost level) at some point to see if it improves the area just before boost regulation starts, but I'd also prefer as little stuff on the car to go wrong as possible.

Yes, it does have an unusual throttle response characteristic around 0 boost range. Fun tho.
Ahh yes, the old rubber-band-to-the-horizon-with-1mm-of-throttle feeling when the 3DLZ finally gets it together...

I do miss that a little sometimes.

Not so much now that it's wet, slimy and icy here, mind you. I'm tip-toeing about with the throttle welded shut, and if I'm getting RPMs there's very limited boost involved. Every dollar I spent on that asymmetric-lockup LSD was worth it three times over.
'77 S with '78 930 power and a few other things.
Old 12-17-2007, 11:41 AM
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