There are several names for blow-off valves (BOVs): they are sometimes called compressor bypass valves, air by-pass valves, or bypass valves. Generally, their purpose is to release pressure from the intake of your turbocharged car after you let off the gas. This release of air creates the famous "psshh" sound turbo cars are so often associated with. Bypass valves (BPVs) do the same thing, however they recirculate the released air back into the intake.

Why would you want a BOV? - BOVs protect against something called compressor surge, which is when the pressure in the intake after you let off the gas sends an air wave back to the turbo, which can damage it over time.

Do not let this make you think you absolutely need one, however. Most turbo cars, such as the Subaru Impreza WRX I will use in this example, have a stock bypass valve that is designed for the levels of boost the car is running normally that will work perfectly. The only reason you would want to upgrade to an aftermarket BOV is if you want to run quite a bit more boost than stock (20psi on the WRX). At this point, the compressor surge can be greater than the stock BPV can handle.

Problems: There is one certain problem that aftermarket BOVs can create. The air that they release has already been measured by the mass air sensor, so when this air is gone after the BOV blows off, the ECU will be injecting the wrong amount of fuel, causing the engine to run rich for a second. This can damage the catalytic converter after time. This can not be fixed with engine management.

Take all these factors into consideration before buying a BOV. Happy modding!