Don:
A couple things to consider. First, the S1 Pikes Peak rally car I saw at Monterey '99 had a cheapo plastic bosch valve routing the dump to 4 smaller hoses aimed radially (quite deliberate in target) at the turbo intake blades. The car didn't have the injector in the EM (could have been removed), but audisport was at least "hoping" that the blow effect would do something (I doubt much either).

Secondly, a BPV (I use BOV to describe the absolute boost pressure device downstream of the t-body in the IM)vented to atmosphere must be shimmed/cranked closed at idle, or it is a leak by definition. The problem with using BPV open to atmosphere is it causes rich fuel spikes because just at liftoff, the "fart" creates a quick air flow thru the MAF. This is pretty well documented with the mitsus and Supras as WJM pointed out. This can cause some interesting bad side effects to O2's, cylinder walls and cool looking flames burning up cats (depends on size of turbo, FI etc. Most likely not a problem with your "application"

IME/O there really isn't any harm in putting the BPV blowoff back into the intake tract (anywhere between the MAF and the turbo inlet), keeping it as metered air really has no downside that I can think of. Put another way, if your argument for the shims is that it's a leak otherwise, at somepoint (vented to atmosphere) it becomes one. Logic would tell me that you'd want that "leak" at idle before you'd want it coming off WOT.

Regarding the BIG valve. My understanding is that air pressure and flow is strictly a cross-sectional area argument, you actually could get even more air out by using two smaller valves. My thinking is that you could find some benefit (especially in racing) by putting in two smaller valves, and staggering the spring preload. Specifically, have a light vacuum BPV and a full lift vacuum BPV. Have you tried this?

Thanks for taking the time out to share your practical secrets of speed. Most interested in your thoughts on the above.

Scott Justusson
'87 T44tqw
'84 RS2URQ project
'83 Urq