Thread: New type of BOV
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BoxxerSix BoxxerSix is offline
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Join Date: Jun 2006
Location: Ballston Spa, NY
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Originally Posted by beepbeep View Post
Hmm...interesting. But I still don't get how the bleed helps transient bucking at low load conditions? I mean, if BOV only pops open above 24 psi, it should be shut when bucking occurs as I assume you don't reach 24+ lbs when bucking happends? Thus car would react the same when it comes to bucking with or without the pop-off as bleed is only active at WOT, no?


I haven't quite sorted this out yet myself. The only conclusion I come up with so far is that the valve (controlled by the MBC) is acting similar to a wastegate in that it's opening transition starts well before the actual target boost level is achieved. It may be opening just a minute amount under the low load conditions that it "shifts the compressor map just enough to stay out of surge. this may be due to the fact that the boost signal it receives to open up is a boost only source, directly off the compressor housing. Even in a neutral rev this turbocharger can build abut 7psi of boost, so I imagine that there is some building of low boost at low throttle inputs even though the engine isn't calling for it. I would bet that I could find out by tapping a boost gauge source between the compressor outlet and IC inlet to see if there's any residual pressure in there during cruise/low throttle input. That's where the valve boost source is essentially pulling from.

I still get an ever so slight shudder at VERY low throttle input with engine rpm, say on a long flat cruise, in between ~3hg and ~2psi boost. I think it's just a working area of the compressor map that I just can't "get to" and tune out with this type of setup. This happens once in a blue moon now and is a far cry from what it used to be. It'll never happen out on the track running 10/10ths as it stands now. In all reality I'd say I've tuned this out of 99.98% of the operational area of this motor/turbocharger which to these Evo's owners is a second Christmas day The good(and quite surprising)thing was it had no huge effect on power during the bleed. These cars using this turbocharger fall off up top like a lead balloon anyway.

I'm working on an ported shroud housing right now. If it does what I'm thinking it'll do, in it's own right it should stop this problem altogether w/o the need for the bleed setup I have now. Effectively widening the cross section of the map a bit more to keep the compressor out of surge(in theory)

Goran I really appreciate your insight on this and efforts to help me along in solving it. Not many people are so persistent. It's a good thing Thanks!


Adam Hennessy
Old 01-31-2008, 06:46 AM
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