Blow-off valves are design for one thing:  to prevent damaging compressor surge to the turbo.  By venting excessive boost due to an abrupt throttle closure (i.e. shifting at WOT), this prevents all that boost from backing up into the compressor section of the turbo.  If it wasn't for the BOV, that boost would hit the compressor wheel and cause damage to the thrust plate on the turbo.  Most people can associate the whistling or whooshing sound of the BOV, but that's not it's primary function.  This is also a tall-tale sound that your car is equipped with a BOV (and probably a pretty potent turbo or serious boost).

There are a lot of BOV kits available to the FC.  Most of the major Japanese aftermarket tuners offer their BOV's and kits.  This includes Trust/GReddy, HKS, and A'PEXi.  These manufacturers also offer their universal BOV's with no adapter pipes for custom installations.  These universal BOV's with the appropriate mount flanges are designed for custom installs, especially with cars with long, exposed intercooler pipes.

The stock BOV (or CBV as Mazda calls it) is only efficient with the stock boost levels.  If you're running a larger turbo or more than 10 psi of boost, we recommend to go with a larger (aftermarket) BOV to prevent damage from compressor surge.

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