How do blow off valves work

There is also a school of thought that a blow-off valve can improve turbocharger longevity or, in extreme cases, prevent damage to the turbo. There are two ways in which the majority of blow-off valves work.

Most aftermarket and many factory types use two pressure signals to determine when the valve should open. The other type uses only one pressure signal to open. We will discuss the former type first. How it works: The usual arrangement is to have the pressure side of the valve attached to the pipe between the turbo and the throttle, and a vacuum hose on the top of the valve hooked up to the inlet manifold after the throttle body.

A spring holds the valve shut. When you are on the throttle, the pressure in the turbo piping and the inlet manifold is equal, meaning that the pressure on each side of the valve is the same and therefore cancels itself out, leaving the spring holding the valve shut. When you lift off the throttle, you have high pressure in the turbo piping, and a vacuum in the inlet manifold.

The pressure on the bottom of the valve and the vacuum on the top combine to lift the valve open and release the pressure in the turbo piping, since it can no longer go into the engine. The diaphragm reacts to pressure changes and at a predetermined vacuum it's pulled toward the vacuum source compressing the spring inside the housing.

The spring is connected to a valve that pulls away from its seat and releases the unwanted boost pressure. On some BOVs, an adjustment screw lets you control at what pressure in the intake system the valve is activated.

You can also swap the spring to change the activation point. Blowoff valves are often referred to by diameter, 40mm being a common size. Picking the best valve comes down to the how much boost you'll be running and the physical space available for installation. Not all turbocharged engines are made for blowoff valves.

They might need a bypass aka diverter or recirculating valve instead. They both accomplish the same task but there's a big difference in how they do it. At engine idle Engine vacuum at the top of the BOV piston is trying to force the piston to open. There is no vacuum or pressure at the bottom of the piston.

A vented BOV needs to remain shut in idle state to avoid leaking. To make that possible, there is a stainless spring inside the upper BOV housing that holds the piston closed. Nuke Performance offers three different springs for differences in engine vacuum in order to make it possible to optimize the setup for any engine. At low boost The conditions the BOV is experiencing at low boost is similar to the conditions when the car is in idle state, but there is less vacuum present at the top of the piston, this is because the throttle is slightly more open.

If the correct spring is mounted in order for the piston to be closed at idle state, the piston will also be closed at low amount of boost. At high boost The valve is fully closed at high boost due to the boost pressure at the top of the piston.

Where a blow off valve should be fitted! The more expensive BOVs have much more engineering behind the product which aims to make it more exact and able to perform at higher outputs.

It is very common that a blow off valve will malfunction or that it does not work at all at a higher boost pressure. For different engine setups, different BOV springs are used. If your BOV is open at idle you should change to a harder version. The biggest difference between the V-Band version and the one for hose connection is that the V-Band come delivered with a V-Band welding flange that is used to weld to the intake piping, if you run silicon hose it is not possible to use the flange, then you use the hose connection version with hose clamp.

Skip to content.