Do blow off valves serve any purpose besides making the hiss when you shift? how much does one cost?
Blow off valves are used on cars with a Turbo. They act like a safety valve when the pressure gets too high. That's the hiss you hear.
Blow-off valves only work on turbocharged or supercharged engines. It is an intake manifold pressure regulator. I've seen people put them on regular engines thinking they would hear that hiss, but there is no where near enough pressure for them to do anything.
Blow off valve or dump valves are used on cars with large turbos. Their function is to keep the turbo spinning during gear changes or on the over-run. Because they are driven by the exhaust gas the turbo has a lag (delay), it takes time so spin up or slow down compared to the engine speed. The bigger the turbo in relation to the engine, the bigger the lag. By dumping the excess pressure it keeps the turbo spinning reducing the lag when you get back on the throttle. It will have no effect on power on small turbos so all these boy racers you see in civics have wasted their time and money. PS The noise it makes, I think is PTCHOO!
