blowoff valve

99FbPSi · 01-14-2002, 09:33 PM

whats a blow off valve?
does it make a sound when u shift gears?

turbo2.3 · 01-14-2002, 09:44 PM

Yes. It is used on turboed cars. When you lift off the throttle, pressures bulids up in the throttle body. It provides a way to get rid of excess pressure.

PNG ®² · 01-14-2002, 09:45 PM

Another classic for the FAQ!

A blowoff valve (also known as a BOV or compressor relief valve) releases the pressure surge in the intake system between the turbocharger and the throttle body when the turbocharger is producing boost and the throttle is suddenly closed. That's why a BOV makes noise during a shift. It's triggered by the vacuum signal created in the intake manifold when the throttle closes between gears.

There are two main reasons BOV's exist. The first is to relieve the backpressure so that the turbocharger doesn't suddenly slow down between shifts, thus reducing lag between gears. The second is because that sudden surge when the throttle is closed can be very damaging to the turbocharger, especially if the intake happens to be the "right" length to reflect the pulse in such a way as to reinforce it.

There are two main kinds of BOV's, as well: Vented, and recirculating. Vented (or "atmospheric") BOV's dump the excess pressure to the outside world, while recirculating ones redirect it back into the intake side of the compressor. Every factory turbo vehicle that I can think of equipped with a BOV has a recirculating type. Cars with MAF-type engine control systems require it, because otherwise air that has been metered never makes it to the engine and it runs very rich every time the BOV cycles. Plus, not everyone thinks the sound of an atmospheric BOV is cool... Personally, I think it announces, "Hey, look at me throwing boost away! I'd rather make noise than power!" :p

turbo2.3 · 01-14-2002, 09:58 PM

*bows down to the all knowing PNG*

wow, man you are good

Lil Racer · 01-14-2002, 10:06 PM

Do superchargers need blow off valves?

99FbPSi · 01-14-2002, 10:07 PM

can u use blow off valves without turbo like on a regular civic?

Scott · 01-14-2002, 10:24 PM

Quote:

Originally posted by AzNxRaCeR
can u use blow off valves without turbo like on a regular civic?

you can buy a fake blow off valve from companies like vis. but why in the hell would you want to do that? a blowoff valve is just used for turbo cars, after reading what it does for a car, why would you need it on a n/a car?

a bov is made for a purpose rather than a new way to show off something your engine cant back up.

PNG ®² · 01-14-2002, 10:31 PM

In chronological order:

Thanks, T2.3 :p Would you believe that 2 years ago I didn't know a BOV from a LSD?

LR: Yes, centrifugal superchargers can use a BOV. Vortech supplies them with certain kits, IIRC. A centrifugal supercharger is basically the compressor half of a turbocharger driven by the crankshaft instead of an exhaust turbine. Roots or Eaton type twin screw superchargers don't use a BOV, both because there's no place to put one (these types of SC's mount directly on the intake manifold) and because their design isn't particularly sensitive to surge.

AzNx: No, you can't. They market an electronic gizmo in Japan that simulates the sound of a BOV on a naturally aspirated car, but I can't imagine anything lamer. It's the automotive equivalent of putting baseball cards in the spokes of your bike to make that motor noise.

99FbPSi · 01-14-2002, 11:48 PM

is it possible to have a blow off valve on a non turbo car???
would anything bad happen to ur car?

Lil Racer · 01-15-2002, 01:15 AM

Quote:

Originally posted by AzNxRaCeR
is it possible to have a blow off valve on a non turbo car???
would anything bad happen to ur car?

Did you not just see what PNG so clearly posted for you??? By the way thanks, PNG. I had been curious about that for a while. That'll be the icing on the cake for my car, one of these years...

smp534 · 01-15-2002, 01:20 AM

Quote:

Originally posted by PNG
It's the automotive equivalent of putting baseball cards in the spokes of your bike to make that motor noise.

I use to do that.......hahahaha

PNG ®² · 01-15-2002, 01:21 AM

My pleasure, LR...

Strangely enough, my truck (which is a factory turbo) didn't come with a BOV or compressor relief valve. Now that I'm replacing the engine, I'm going to add one, but not for the sound or to keep the turbo spooled (having an automatic transmission means the throttle stays open during shifts - just another reason why autos and turbos are a match made in heaven). I'm getting one so that my turbocharger will last another 150k miles...