Learn about Wastegates and Blow off valves.

"Do you know your wastegates from your BOV's"

What are the differences between a waste gate and a BOV or blow off valve?

We find that many people are confused by the two devices and see them wrongly used.

There are two sides to a turbocharger, the exhaust flow side and the intake side.

Wastegates work on the exhaust side and BOV's work on the intake side.

As the exhaust flows through the turbo it spins the turbo generating boost on the intake side.

The Wastegate and blow off valve both control the amount of boost the engine receives but they do it at each end of the turbo.


These control the exhaust flow through the turbo. Some are set to reach a peak and then divert all excess flow back into the exhaust and bypass the turbo.

There are some more sophisticated electronic wastegates around that are controlled by the computer and allow fine control over the exhaust flow depending on what is happening at the engine and taking into account throttle position and load.

As the exhaust gases are diverted away from the turbo and into the exhaust it causes the turbo speed to ease off avoiding over spooling, and lowers the boost produced than would be had the exhaust gases not been diverted.

Blow off valves

These operate on the air intake side of the turbo. When you lift off the throttle the engine cuts fuel and needs much less air. The thing is that the turbo has pressurized a whole batch of air in the intake and this pressure is not going to go into the engine.

It would cause problems having high pressure air in the intake and could blow a pipe or worse still cause the turbo to stall or spin backwards potentially destroying the turbo.

Atmospheric blow off valves dump the air into the air in the engine bay.

Closed loop internal BOV's dump the pressurized air to the filter end of the air intake where it can dissipate or be sucked back into the engine if needed.

BOV to atmosphere will cause overfuelling in a AFM (Air Flow Metered) car due to the EMS adding fuel to match the volume of air that has passed thru the AFM so when you get back on the gas.A recirculating or plum back BOV will prevent this happening.

This will not happen when you have an atmospheric BOV and a MAP sensor controlling the fuel supply.

A blow off valve takes the pressurized air from the intake when it is not required and dumps this elsewhere.

Atmospheric blow off valves create noise, which is probably why most manufacturers dump this air into the low pressure end of the air intake.

Some ECU's dislike atmospheric blow off valves, and note the lost pressure in the intake and throw up an error.

So now you know the differences between a BOV (blow off valve) and wastegate.

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One Response to “Wastegate and Blow off valves”

  1. Sheila says:

    Worth reading. Got the necessary information I needed.

    Thank you for this great piece.

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