Turbo BOV (Blow off valve) vs Bypass Valve (Surge Valve).

Endless people, will argue to death, such an example is a fool on youtube (there are a few), clearly can’t comprehend the difference between the two. The best part is, companies make surge valves specifically for belt driven superchargers (and PD Positive displacement) blowers, not as a marketing ploy but as a required function of a supercharger vs. a turbo.

And he goes out of his way to modify a BOV clearly meant for a turbo to mimic something that has already been created… and he can’t understand, it won’t work the same way in the end either.

Perhaps too much welding gas in his time, but why modify (that wont work properly anyways) a BOV when there is a proper item available already. Someone rethinks and tries to reinvent the wheel.

Let’s learn the reasoning, between the two, why each is dedicated for each application and not to be interchanged. This will also clarify the miss-use of the terms.

Even with relatively little increase in engine load at low throttle positions, air is still being pumped by the compressor on a belt driven system, unlike turbo, where boost is non-existent.

With a Belt driven system, this pumping volume is still greater than the engines airflow requirements at idle and low loads, so pressure builds between the throttle blade and the compressor. The added pressure on the compressor blades creates drag on the accessory drive that can make idle control difficult amongst blade stresses, more hp to continue the turn, and lower fuel economy.

A BOV at idle or high vacuum in the manifold will NOT let this pressurized air out where as a surge valve or bypass valve is open during vacuum and part throttle. Even with modification of the spring action of a turbo BOV which is set for a certain pressure level to allow air to escape, you will not get a proper response throughout all engine loads vs RPM. You may make it work for idle, (ex. 35Kpa Vac) but it will not work at 3000rpm with 22% TPS (throttle), real world talk ;)

Leave the two alone and only use the appropriate device for your application.