It’s like déjà vu –
In the same way that government legislation nearly killed the muscle car in the late 60s and early 70s. Emissions mandates once again threaten to put the kibosh on the horsepower numbers from big displacement engines.
But today, manufacturers are cleverly keeping the pony numbers up in thriftier engines by using forced induction systems, like superchargers.
In general, superchargers don’t come with a blow off valve. And in general, they don’t need blow off valves.
However, centrifugal superchargers use blow off valves. But they’re used as bypass valves, not blow off valves.
Let’s investigate this further.
How a Supercharger Works
Superchargers and turbochargers are types of forced induction.
A supercharger – like a turbocharger – is an air compressor that increases the amount of oxygen that goes into an engine. The more oxygen that goes into an engine, the bigger the combustion.
This helps the engine create more horsepower.
Forced induction helps a smaller engine make as much power as a bigger engine. It also helps an engine use less fuel.
The biggest difference between a supercharger and a turbocharger is that a supercharger is belt driven and a turbo is powered by exhaust gasses flowing through a turbine.
How a Blow Off Valve Works
An engine is a big air pump.
A blow off valve is a device that helps regulate air pressure within an engine by allowing air to escape when needed.
Doing this protects the engine from damage. Blow off valves are mostly used in automobiles with a turbocharger.
But whether you’re talking about a turbo or supercharger, both induction systems are designed to force air into the engine to create more power.
But what if the engine doesn’t need the power from the compressed air? Where does all that pressure go?
Enter the blow off valve.
How a Bypass Valve Works
Like a turbo, as a supercharger builds boost it creates heat.
Forced induction needs a way to release pressurized air when it’s not needed.
But if pressurized air has no means of escape when it’s not needed, it’ll grow and force the pressurized air back to its source.
Bent throttle shafts, bent throttle blades and other bent engine components.
A bypass valve is a feature more common with superchargers the way a blow off valve is more common with turbochargers.
A bypass valve allows the pressurized air from the supercharger a place to escape.
But unlike a blow off valve the pressurized air doesn’t escape into the atmosphere. Instead, it recirculates the air back into the intake keeping pressure consistent.
Blow Off Valve vs Wastegate
Blow off valves and wastegates are more associated with turbochargers than superchargers.
The difference between a blow off valve and wastegate is in their function and placement relative to a turbocharger.
A wastegate is used to control the speed of the spinning turbine within a turbo. As such it’s connected to the exhaust side of a turbo system while a blow off valve is connected to the boost side.
Connecting a Blow Off Valve to a Supercharger
Remember that a supercharger is mechanically driven, whereas a turbocharger is exhaust driven. And as we said in the introduction, only some superchargers need a blow off valve.
The reason why is because since a supercharger is driven by the crankshaft, boost drops as engine speeds drop.
Supercharges equipped with blow off valves are called bypass valves.
The function is different from a blow off valve as we’ve explained. For one thing, a bypass valve connects to the intake side of an engine.
Controlling Boost on a Supercharger
Another reason why most superchargers don’t need a bypass valve is because the supercharger won’t make boost at partial throttle.
That’s why you don’t hear the engine whine until you put your foot to the floor and open up the throttle.
In the case of superchargers that come with bypass valves, when the engine is at idle or cruising speeds there’s a vacuum that keeps the bypass valve open.
The supercharger is still producing boost. Just not enough to be forced into the engine.
Blow Off Valves and Sound
Blow off valves were sought after feature for cars with turbos in the early 2000s – a guy I knew had one in his Subaru WRX that made driving exciting.
I didn’t hang on to my Eagle Talon long enough to install it on my car. The great thing about blow off valves is you can adjust how loud or quiet you want them to be.
They are a crowd pleaser at any meet up or race track.
Blow Off Valves and the Life of Your Engine
Blow off valves and bypass valves are designed and engineered to improve the performance of a turbo or supercharger.
As such they won’t harm your engine and should actually prolong the life of the turbo or supercharger.
However, if the blow off valve is set to do its thing at higher boost levels, this can cause premature wear to the turbo.
Turning a Supercharger On and Off
If you watched Mad Max 2 and you saw Max Rockatansky activate and deactivate the Weiand supercharger on his 1973 Ford Falcon XB Pursuit Special.
I’m afraid to have to tell you that this was all done through the effects of movie magic.
It’s like how Michael Knight was able to hit the Turbo Boost button to make KITT jump over obstacles in television’s Knight Rider – it wasn’t real.
Sometimes it’s fun to just suspend our disbelief and roll with it.