Supercharger blow-off valve explained.

Supercharger blow-off valve explained.

The key to proper operation of the BOV involves the pressure balance surrounding its piston.

By Alfred Bilk

On the left side of the picture (above), the piston encounters boost pressure from the charge pipe coming from the supercharger’s compressor. On the right side of the picture the piston senses pressure or vacuum from the intake manifold, which is positioned below the throttle body or carburetor, plus the pressure from the spring. With the engine stopped, there is atmospheric pressure on both sides of the piston and the spring holds the piston closed.

At idle, the left side of the piston experiences atmospheric or slightly less pressure, as the supercharger’s compressor isn’t generating boost, and the right side of the piston is exposed to the idle vacuum in the intake manifold. At idle, the BOV will be closed; if it was open, it would introduce unfiltered air into the engine. So the spring must be sufficiently strong to hold the piston closed when there is idle vacuum in the manifold, but no boost in the charge air pipe.

When driving normally the compressor builds boost in the charge pipe, which the left side of the piston experiences. Also, with the throttle open, boost is present in the intake manifold below the throttle body or carburetor. So, the right side of the piston will encounter boost in the intake manifold plus the spring pressure, which should hold the BOV closed.

The critical operating point and principal purpose for having the BOV is to protect the supercharger under deceleration—the catalyst for vacuum. In addition to the vacuum occurring in the intake manifold beneath the throttle body or carburetor, there is a large amount of pressurized air in the charge pipe coming from the compressor.

With the throttle closed, this pressurized air cannot gain access to the engine, and the supercharger’s compressor cannot stop producing boost instantly. So where is the pressurized air to go?  Boost arrives on the left side of the piston and vacuum on the right side and the pressure balance is sufficient to overcome the spring force. The piston moves to the right and the valve opens, allowing the pressurized air in the charge pipe to escape. This is the critical function. Without it the pressurized air would stall the compressor and damage the supercharger.

The process becomes more complicated when using a carburetor, which introduces an interesting comparison with fuel injection. On a regular fuel injected engine, the throttle body is usually about the same size as the charge pipe, thus, minimal pressure-drop between the charge pipe and the intake manifold when the throttle is fully open.

Carburetors, on the other hand, have venturii that create restriction, which is their operating principal. But if the carburetor, or throttle body, had enough restriction, the boost in the intake manifold could be less than the boost in the charge pipe coming from the compressor.

With more pressure on the left side of the piston than on the right side, the system is relying on spring pressure to keep the BOV closed. If the pressure differential between the left side and right side of the piston becomes more than the spring can compensate, the BOV will partially open and leak boost.


  1. So, I have a TorqStorm supercharger with a carburetor and at idle I have a slight, very slight, whistle suggesting air leaking through the BOV. Should I tighten the spring (is it adjustable?) to completely seal it while idling?

    • It’s possible you have a leak at idle but more likely a noise from the compressor itself. The best approach to tracing is by way of a stethoscope, which can usually detect precisely from where the whistle is coming.

  2. I have one of your superchargers with a carburetor on a 351W and I love how it runs. I have a 5-speed and my BOV blows off when I leave off to shift. That’s every time, it doesn’t matter if it’s at full throttle or cruising. Is that normal? Is there something I can change?

    • Hello Aurie,

      That’s normal operation. As the throttle plates close, and with air in the charge pipe, the BOV should open.

  3. Have you made a supercharger for the 455 Buick engine yet?

    • Hello Wayne,

      We have explored the Buick option and the factory accessory layout and the obstacles the Buick engine family (and also the Cadillac 500) presents. To date we have not created a system for them and have no plans to build any at this time. Nonetheless, thank you for inquiring with us.


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