Car tuning tips:Blow off valves or dump valves"

In a turbo engine the pressures build up as air is compressed and forced into the engine. The turbo is driven by the exhaust gases so the faster the engine goes the more air you get forced into the engine. This is fine until you lift off the accelerator. You engine speed does not instantly change and you are not asking for fuel to go into the engine where does the compressed intake air go?

 This is the job of the recirculating valve also known as a dump or blow off valve.

Well you certainly don't want it to go bang, which it would if the air was not released. Enter the Blow off valve. This effectively vents the air out of the intake area and most OEM recirculating valves put this air in to the air intake upstream from the filter where it does no harm and is actually ready for the next press of the accelerator.

If this process takes a while you have a situation where the turbo compressor is trying to compress air beyond its design. This caused the turbine to spin more slowly and creates drag on the exhaust. In a high performance engine this could easily damage the turbo. Aftermarket valves can handle much more pressure and respond more quickly than the OEM ones. If you remap a turbo engine you should also replace the recirculating valve.

The dump valve or blow off valve vents this pressurised air into the atmosphere rather than discretely back into the intake beyond the turbo and filter. This gives the option of a noise as it does this and there are plenty to choose from. Some variations include a whistle, a clatter, a pop, a pfffst noise like a bus brake and variations on these themes.

Do not confuse a blow off valve with a screamer pipe or a waste gate control these work in different ways and have more to do with the cars exhaust flow through the turbo than the intake flow.

This entry was posted under air intake, engine, turbos. Tags: blow off valves, BOVs, dump valves, screamer pipes, turbo
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