5250 High Performance Dyno Shop Blog

Turbocharged Performance | Do I need a Blow-Off Valve, and here’s why

[fa icon="calendar"] Jul 21, 2015 7:00:00 PM / by Mike Wiener

Why do some turbo kits not have blow-off valves?Blow off Valve

Recently, I have seen some rotated turbo kits on a few customer cars that were built without a Blow-Off Valve (BOV). I've been told the builder of these kits claims 'a BOV is not needed'.  You need to read this if...

Want more Power? | Turbo kits without a blow-off valve, too good to be true, yes it is, and here is why you need a BOV.

The BOV is a pressure release valve. It's purpose is to release the pressure inside the intercooler and charge piping whenever the throttle is closed.

If this pressure were not vented in some way, a shock wave would form and back up into the turbo, causing it to stall.

This is very bad for the turbo. Damage is usually caused over time, but can occur suddenly and catastrophically. Why have the pressure pulse turn around and kick the turbine blades?  

Imagine we have a 60lb/min turbo (GTX30r) running at full bore. That's one pound of air every second being jammed through your throttle body. I have actually seen this mass of air overwhelm the return spring of the throttle plate, making the motor want to keep going after you let off.

When the return spring does pull the throttle plate shut, that mass of air is like a freight train hitting a wall. Except unlike a freight train, air becomes compressed (air at this pressure will find a way out).

Without a BOV, that freight train of air bounces off the throttle plate and travels backwards, at the same speed it had been traveling forwards, towards the turbo.

turbo Subaru upgradesTurbocharged Performance upgrades | The pressure wants to go somewhere, and it will find the weakest point in the system.

When that mass of air gets to the compressor wheel, it can suddenly stop it. Under the right conditions, this can break the shaft connecting the compressor and turbine. Imagine the exhaust pressure pushing one way, and then the inlet turbine being forced to immediately turn the other way, stuff is going to break.

When the BOV is working properly, the pressure wave is vented and the turbo never stops spinning, or even slows down because the system is working together as it should.  

A BOV, or recirculating valve is an absolutely critical part of a turbo system. Many newer turbo designs even incorporate one into the compressor housing. If you're not sure if you have the proper equipment or if it's working properly, ask us. We can help.

We would much rather share our experience to get you the right subaru performance upgrades of your choice without learning the hard way.  

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Topics: Building Horsepower

Mike Wiener

Written by Mike Wiener