Questions from beginner car enthusiasts on where to get blow off valves for normally aspirated cars are not uncommon as they don’t know what blow off valves are for, and how they work.

It is not possible for you to install a blow off valve onto a normally aspirated car, they are for forced inducted engines. No matter how much you like the sound.

Normal engine operation.

Let’s ignore how turbochargers work and just assume it works since this post is not about turbochargers. The engine receives the turbocharged air through the intake. That’s assuming the throttle is open. Now what happens when you lift your foot off the pedal? The compressed turbocharged air has no where to go. It hits the closed throttle plate and goes back up the intake manifold the opposite direction. Back into the turbine.

This high pressure air hitting the turbine blades from the wrong side of the turbo slows down the spinning turbine, thus reducing boost. When you press the accelerator pedal again, the turbo has to slowly spin up again to gain maximum boost. This is bad. Very bad. The reverse surge of turbocharged air could also damage the turbine. This is also bad. Very bad.

Air is released. Pardon the noobness of the diagram!

This is why blow off valves exist. It releases turbocharged air coming down the wrong direction of the intake manifold out to the atmosphere. You get longer turbocharger life, quicker spool, and better transient response. Of course, there is the issue of the stock standard ECU expecting air but not getting air, and pumping too much fuel in, but that’s a different issue.

For cars with that problem (usually because of the usage of Mass Airflow Sensors) there is a different type of blow off valve that recirculates turbocharged air back into the intake instead of releasing it into the atmosphere. Sounds a lot less cooler though heheh.

Recirculating blow-off valves send air back into the intake

You can’t put a blow off valve on a naturally aspirated car because induction (the sucking of air into the engine) is done using vacuum. The engine itself sucks air in, instead of the turbine pushing compressed air in. When the throttle is closed, no air is being sucked in, and there is no boost bouncing back into a non-existent turbocharger turbine. There is nothing to “blow off”.

So that’s what blow off valves are for, and that is why you only put them on turbocharged cars. So if you want that cool sound that turbocharged cars have whenever they shift gears, you need a turbocharged car.