Modular Head Shop - Bypass Valve / Blow-off Valve

Bypass Valve vs. Blow Off Valve

Why do you need one?

On a boosted (usually turbocharged) engine, pressure has to be released when the throttle body closes. Without the pressure release, pressure will skyrocket on one side of the throttle body, which can damage plenty of components. If the pressure forces its way back into the turbocharger it can cause compressor surge that can kill a turbo. If pressure stays on the throttle body, it can bend or shatter throttle blades or shafts. Most of these outcomes lead to engine damage as well. 

What is the difference?

Bypass Valve:

A bypass valve vents extra boost pressure back into the inlet when the throttle body closes.

Blow Off Valve:

A blow off valve vents extra boost pressure to atmosphere when the throttle body closes. This is what causes the enjoyable 'psssssshhhh' sound that people tend to enjoy with boosted applications.