Why turbocharged vehicles need to run a bypass/blow-off valves (CBV/BOV)?

The turbocharger is moving large amounts of air while under boost, and this pressure is being delivered to the engine while the throttle is depressed. As soon as the throttle is lifted the butterfly closes in the throttle body preventing the pressurized air from entering the intake manifold. That dense charge air now has nowhere to go. Because the engine is still moving exhaust gases and the inertia of the turbocharger is turning the turbine and compressor wheels, the air literally stacks up against the throttle blades.

Pressure builds up within the system’s piping extremely quickly and once it reaches the compressor wheel the air actually flows back out of the compressor inlet and limits the compressor from rotating. Because of the air reversing direction the pressure drops allowing the wheel to spin and once again raise the pressure. This process of up and down pressure and air reversal is surge and the result is the fluttering/barking sound that comes from the turbo. Surge is very detrimental to the turbocharger, resulting in various stressed being placed on the unit. The compressor wheel has high loads placed on it as it tries to move against this wall of air, and as high pressure air finds its way behind the compressor wheel. Compressor surge can result in the compressor wheel spinning on the shaft, shaft breakage, bearings to have unnecessary wear and potentially cause contact between the wheels and housings.

The compressor bypass valve/blow-off valve (CBV/BOV) is placed after the turbo and before the throttle body to help eliminate the pressurized air. Vacuum controlled, the CBV/BOV receives a signal from inside the intake manifold or just after the throttle body, pulling up on a diaphragm, causing a valve to open inside the unit. The pressurized air that is discharged is then routed back to the inlet of the turbocharger. It is important to note that the true design of a CBV is to do just what the name implies; to bypass the air, or move it from one location to another. In most instances by routing the bypassed air to the inlet of the turbo, throttle and boost response are greatly improved. In the case of a BOV the air is vented to atmosphere and still does a good job protecting the compressor from surge and help to assist in maintaining the speed in which the turbo shaft is turning.

A CBV/BOV can also only be used on applications that have good vacuum sources after the throttle blades and thus cannot be used in draw through carburetor applications. The CBV must also be installed so that the discharge air from the CBV is routed back to the inlet of the compressor in Mass Air Flow (MAF) vehicles, or placed after the MAF sensor on a MAF Blow-thru setup, so that the computer does not measure inlet air more then once. If vented to atmosphere after the MAF sensor, the vehicle will not run correctly due to incorrect air/fuel ratios.