Don't want to seem like the historian but there is a 70 year old man who has been racing for 40 years who sat me down one day and gave me the skinny on the BPV/BOV. Here is the story.
The current usage of the BPV is a beneficial side effect of its original intended purpose which had nothing to do with the turbo or spool or emissions. Back in the day when cars didn't run fuel injection there rose a problem. See, turbo's used to be carburated draw through designs, that is, they were positioned such that the compressor was fed by the carburetor. Essentially the carb was in front of the compressor inlet and the turbo outlet ran right into the intake manifold, non-intercooled. The intake path had to be short as possible since everything post plate at the carb would be under vacuum. So let's say these pioneers want to increase the boost and intercool the charge. Now the intake path is getting alot bigger and all of it under vacuum until the plate opens. The guys realized they would hit the gas and sit for several seconds before the car even began to move. The bigger the IC, the more pipes, the worse it was. None of these cars needed a BOV but they did need some throttle response. Attempts were made to use a secondary plate between the turbo/IC and the manifold which helped but was still not optimal. Many aftermarket supercharger setups can still be bought utilizing a dual TB setup, one before and one after the charger for this reason. This again is greatly exacerbated by the addition of an intercooler. To be clear here, this is not turbo lag. This is throttle response. Turbo lag is a product of exhaust flow and turbine weight.
So in order to fix this someone says, "Hey, lets put the carb back on the manifold and push it through. That way we won't have that vacuum and throttle response will be normal." Sure enough they did and all was right with the world, until, they let off the throttle and the car died. What happened? Well, when the throttle plate closed the air still being pumped by the turbo, in a state of surge btw (Which is what causes the noise of not running a BOV, surge being actually defined as trying to create a boost pressure for which you have not enough airflow to support)....surge, would flow into the carb and blow the gas out of the bowls, drying them instantly and stopping the car dead. Problem.
How do we deal with this? We need some sort of valve that opens up under vacuum and vents that pressure off to keep it from blowing the fuel away.
Hence, BOV.
The added benefit and its current application is that the uncomfortable noise of the compressor wheel in a state of surge was also gone. This made the car much more reasonable to drive on the street. The Grand Nationals didn't have a BOV. Up until the 80's I don't know of any car that did but I am sure there is one. BOV's are however a fairly recent invention and any fuel injected car does not need them. I'm not making any claim about spool or anything because I have not seen data on it but I imagine it is another additional side benefit.
To make another analogy.
Porsche has estimated that due to the mechanical stresses placed upon it's LeMans race cars, the stresses of a single mile on the track are equivalent to 1000 miles in a street car, their words not mine. These LeMans cars travel roughly 2-4 thousand miles over 24 hours, without changing oil, non-stop, banging gears and shifting more than most people do in a lifetime. They run crazy setups, with insane turbo systems, and no BOV. So it's a mechanical equivalent 2-4 million miles on a street car, all at once, no BOV and no turbo failures due to lack of one. The idea that BOV's are saving the turbo is a nice way to sell BOV's, but has no data to back that claim. Even if it did, it is still an unintended side benefit of its original use. Now they are used to keep the noise down, another non-intended side benefit.
And that is how it was told to me by a guy who built one of the very first ones by hand. So no, you don't need one.
/brox
