Yep, I own Corky's book plus MacInnes' as well.
It's pretty hard to use either one to build a case for the use of a blow off valve in our applications. I don't see a reference to a bov by Corky, but it may be there, and MacInnes mentions the use of one on the intake to control boost rather than using an exhaust wastegate.
I don't put too much stock in Turbo magazine as it is aimed at a different segment of the market with different set ups, volumewise, than ours. (and I admit to being cynical to magazine tech as it is usually just an infomercial for its advertisers
)
With regard to surge, I cut this off a web site.
"The compressor blades are analogous to a wing. As they move through the air, they direct the movement of the air according to their aerodynamic design. If an airfoil is operated at an angle of attack beyond its design limit, the flow on the low pressure side of the foil separates instead of following its contour. This condition is known as "stall". The surge line indicates where the pressure differential, air velocity, and compressor rpm conditions combine to define the aerodynamic limits of a particular compressor. If operated at or beyond this line, the compressor stalls and surges in and out of stall until external changes are imposed to keep the compressor away from this zone."
Technically, that makes you right.
When the tb blade is closed, the pressure differential across the compressor wheel drops and the wheel begins to stall. Even though the wheel is decelerates rather than trying to accelerate, velocity does take it left of the surge line or stall line. On the other hand, it is the reverse phenomena of what occurs when trying to spool a large turbo at light throttle.
Now, the damage resulting from compressor slam/stall is still the subject of discussion. As stated above, 360 deg thrust bearings and the large intake tract of our cars cushions the surge, particularly in the deceleration mode, and it is not a problem. If there is a problem, it would seem to be in the acceleration mode when the blade is open slightly and the compressor wheel is trying to jump out of the housing. A conventional blow off valve can do nothing in this case which is far more violent-at least on the cars I have experience with.
The argument for bov's sounds really good. It's downfall is that those guys that build modern day turbos for our particular set ups don't find the predicted damage when they open the turbos. Perhaps they put ethics behind "good business" practices, but, I really don't think that is a general rule.
