Hi, I'm very interested in car mechanics, and lately I've been curious about forced induction systems. One thing I can't seem to figure out from online resources is exactly how Blow Off Valves work. From what information I gather, there are two types of FI pressure release systems, BOVs, and "pop-off valves". BOVs only release pressure when it senses that there is a vacuum in the manifold, where pop-off valves release air when the pressure between the turbo and the throttle plate exceeds a certain amount.

My question is: how exactly does a BOV sense that there is a vacuum in the manifold? Do you need to drill a hole after the throttle plate and connect a tube back to the valve?

I've also heard of valves that are 'open' all the time, except when it senses positive pressure. I believe this was called a Compressor Bypass Valve. I don't understand how this prevents turbo back-spooling...

Thanks guys! Sorry if got things mixed up, as I am still trying to comprehend much of this stuff.

 

[QUOTE]Originally posted by foolio
[B]
My question is: how exactly does a BOV sense that there is a vacuum in the manifold?

 

Thanks for the link. I just realized that my description of the CBV is exactly the same as a BOV...

Anyways, I wondered about the vacuum hose because I've seen pictures of BOVs without any sort of hose leading to the manifold. I'm guessing those are actually pop-off valves.