by-pass valve for what? by-pass valves are used in all kinds of situations - you could have a by-pass valve on a heater, you could have one on a cold air intake, you could have one on a supercharger, you could have one on a turbocharger, you could have one on a brake system, you could have one on engine oiling system - generally speaking a by-pass valve would be used where you wanted a certain amount of whatever you had (for instance turbo boosted air) going somewhere (into the intake) and any excess would by-pass the system and go somewhere else (into the atmosphere) - in the case of a cold air intake, it would not allow water to be drawn thru the intake into the engine, if the air intake happened to be in a puddle (exaggerating)