Blow Off Valves

On gasoline engines, blow off valves are plumbed between the turbocharger compressor outlet and the throttle body. They prevent potential damage to the turbocharger due to a phenomenon known as compressor surge. When the engine transitions from boost to closed throttle, pressure can build up between the turbo and throttle plate. This surge can rapidly slow the compressor wheel, resulting in accelerated wear and possible damage to the turbocharger. It also causes turbo lag when the throttle is re-opened and the driver has to wait for the turbocharger to spool up before it can begin making boost again.

The main components of a BOV, sometimes called a dump valve, are a housing, spring, and piston. The upper portion of the valve references intake manifold pressure via a hose. At idle, the blow off valve is held shut by spring pressure. Under boost, both spring pressure and manifold pressure continue to hold the valve closed. When the throttle is closed, manifold vacuum helps lift the piston against spring pressure, allowing boost to escape to atmosphere. In some applications, blow off valves recirculate boost pressure back to the inlet side of the compressor; these are often referred to as diverter or recirc valves. With many blow off valves, additional springs are included to adjust the opening to a desired amount of manifold vacuum. Depending on the vehicle or engine management used, blow off valves can also be electronically controlled, with no vacuum line to the intake manifold.

In applications with a centrifugal supercharger, a blow off valve performs the same function, allowing pressure to escape when the throttle is closed but the compressor is still creating boost. It can also function as a supercharger wastegate, limiting maximum boost.

While they are traditionally used on turbocharged gas engines, blow off valves are also used on high-performance diesel engines to vent excess boost pressure under deceleration, when the engine is slowing rapidly.

Blow off valves are often classified by size in millimeters, with 50mm being common size. Universal blow off valves often mount with a V-band clamp, and a weld-on fitting may be included for installation. They can also mount with a hose barb, 3-bolt flange, or OE-style mounting for factory replacement units. Shop a huge selection of blow off valves from Summit Racing, Turbosmart, TurboXS, Precision Turbo & Engine, Vortech, and more.