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Name: Stefan M.
Status: Student
Age: 16
Location: N/A
Country: N/A
Date: July 2004

What does a blow-off valve do in a car?

Well, several eons ago when I was sixteen, I recall being blown off by a number of girls while in a car. However, there was no valve involved, just the phrase "...not if you were the last guy on Earth" or words to that effect.

The positive crankcase ventilation (or PCV) valve is sometime referred to as the blow-by valve because the PCV system scavenges the gases that "blow by" the piston rings and end up in the crankcase. Cars built prior to the late 1950's had a "road tube" which would simply exhaust these gases to the atmosphere. However, under some conditions these gases could build up and reduce performance and even damage the engine, as well as pollute the atmosphere. So, the PCV system was developed to use engine intake vacuum to suck these gases out of the crankcase and recycle them through the engine. The PCV valve regulates the amount of vacuum applied to the crankcase and is a common maintenance item because the gases that flow through it tend to leave deposits that clog it.

Less commonly, turbocharged engines have a wastegate that exhausts excess pressure to prevent overcharging the engine. This is also referred to sometimes as a blow-off valve. Those who hot-rod turbocharged engines like to replace the stock wastegates with ones that provides higher peak pressure. This provides instant horsepower but may also provide instant engine destruction when taken to excess. (My brother once had a Mitsubishi Starion modified in this way that whistled like a party favor when the valve actuated. I remember this noise clearly because I generally had my eyes closed and was braced for impact when hearing it.)

Andy Johnson

The blow-off valve should not be confused with a waste-gate. The waste-gate is a device that by-passes the turbine wheel of the turbocharger, limiting the shaft-speed of the turbocharger. Therefore, limiting the boost (pressure) that the compressor generates and keeping the turbocharger from over-speeding.

A blow-off valve is mounted in the intake plumbing between the turbocharger compressor and the throttle plate. The blow-off valve is a second safety measure against the turbocharger over-boosting and damaging the engine.

The blow-off valve is more commonly used to keep the turbocharger spinning when the throttle plate is suddenly closed. When the turbocharger is generating maximum boost pressure at full throttle and then the throttle is suddenly closed, compressed air coming from the compressor slams against the throttle, generating extremely high pressures that travel backwards to the compressor stopping the compressor from spinning. When the throttle plate is again opened, the engine must spool the turbocharger shaft again. The effects of this high pressure can also be very damaging to the turbocharger.

Brian Wright
Washington University in St. Louis Formula SAE Racing

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