Blowoff Valves - 2005 STI

What a Blow-Off Valve Does: Before this can be explained, we must recognize two important factors that are at play in a turbocharged car: i. Anytime your foot is on the gas, your throttlebody is open at a certain percentage, allowing your engine to suck in air. ii. Your turbocharger produces boost under throttle.

Normally aspirated (NA), or non-turbo cars, suck in the amount of air that they need. When you take your foot off the throttle in an NA, it stops sucking air, at least to an extent. In a turbocharged car, when you release the gas pedal (i.e. to shift), there is still pressurized air in the system. When you let your foot off the gas, this pressurized air can build up between the throttle plate and turbocharger. With nowhere to go, the pressurized air can push against the turbocharger impeller, and cause premature wear or fin damage. This is where a blow-off valve (BOV) comes in; when you release the throttle to shift, etc., the BOV opens, releasing the pressurized air and thus preventing damage to the turbocharger (and making that loud noise so many enthusiasts love).