A good blow off valve is an absolute necessity for high performance turbo systems. A well designed blow off valve prevents turbine damage by quickly relieving excess boost pressure without sacrificing boost response.
So what, exactly, is this valve thingy and what does it do?
On a turbocharged engine, when the turbo spools up it pressurizes the intake system - from the compressor, past the throttle body and through the inlet manifold, forcing more air into the combustion chambers. This is how a turbo produces more power.
When you take your foot off the accelerator pedal, the throttle body closes - the stream of pressurized air created by the turbocharger is now cut off from the inlet manifold. The only way it can escape is back up the intake stream, surging into the turbo compressor. This reversal of intake charge pulse can put additional strain on the turbo components, as well as reducing the compressor wheel’s rotational velocity. This means that the turbo will take longer to spool up when the throttle is opened again.
A valve placed before the throttle body cures this problem by allowing the pressurized charge to escape the intake system, keeping the compressor spinning and reducing turbo lag. I won’t go into the specific details of how the valve opens when needed, but if you want to know more then email me. Many turbocharged cars come with such a valve from the factory, but they are recirculate valves rather than BOVs - the compressed air that escapes is plumbed back into the intake before the turbo. BOVs vent this air to the atmosphere instead.
So why install an aftermarket BOV? They provide improved throttle response and can hold much higher boost levels compared to the factory-fitted version, and is a bonus because they vent to the atmosphere they provide that sought-after “psssht” sound when shifting gear or free-revving. Plus they look damn cool, if that’s your thing.
The only drawback to this type of valve occurs on cars with air flow meters. Because the engine is venting air that has already been measured by the MAF, the car’s computer adds a little too much fuel for the air that is left, resulting in a rich mixture for a short period of time. This can sometimes result in backfiring, but results vary from one car to the next. This can usually be addressed by proper tuning or you can purchase one of our recirculating BOV's where the air gets put back into the system before the air flow meter.
In most cases, a BOV is fitted to the car’s factory boost hoses, or is attached to a weld-on adapter plate for upgraded intercooler systems. The choice depends on your car’s level of tune, and what your needs are.