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    • MADE OF DURABLE CAST AND BILLET ALUMINUM FRAME! PREVENTS PREMATURE BOOST LEAKAGE AND COMPRESSOR SURGE !
    • BY PASS OR BLOW OFF DESIGN, FULLY ADJUSTABLE, SPRING STIFFNESS ADJUSTMENT SCREW ON THE TOP ALLOWS BOV TO WORK WITH MILDLY UPGRADED VEHICLES TO FULLY UPGRADED VEHICLES.
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What is a blow-off valve (B.O.V.)?

A blow-off valve is a valve that monitors the air that is compressed from the turbine compressor (a turbo or supercharger) to the intake manifold. It knows when to pop-off or release pressure through either a spring or electronic pressure gauge. Limiting boost through a blow-off valve is good. The point of a blow off valve is to relieve most if not all of the positive air pressure between the throttle plate and the turbo, otherwise it will have to basically blow through the air-cleaner. This is done whenever the throttle is closed between shifts or when stopping the car (i.e., your foot is off gas.)

The purpose of the blow off valve (other than to make your VW sound like a Porsche 962), is to relieve the back pressure in the intake when slamming the throttle plate shut, as would occur during shifting. Without a place for positive manifold pressure to go, the turbo compressor would rapidly decelerate from the 120k or so rpm that it needs for max boost, to a much lower speed that would cause a serious lag in response when reapplying the throttle in the next gear (i.e. turbo lag). If high boost into the intake manifold is followed by a dead end path when the throttle butterfly closes suddenly, the blow-off valve will open to the atmosphere, thus allowing the compressor to spin freely without much back pressure. It doesn't open until the throttle is closed. We don't want to loose any pressure while accelerating!

How does a blow-off valve work?

A blow-off valve is just a valve operated by a rubber or synthetic diaphragm. Intake manifold vacuum is very high when the engine is spinning fast and the throttle is suddenly closed. It is under these conditions that the diaphragm opens the valve. If it is adjusted correctly, it will not open when driving "normally".

When you close the throttle where is all the air that compressor was pumping going to go? Nowhere. The compressor won't continue pumping air into a closed ended chamber, and since your foot is off the gas there is much less exhaust available to spin the turbine, so the turbo will stop spinning, fairly quickly spool-down.

So in order to keep the turbo spinning we use a blow off valve to just let the air out. Since the turbo has a good amount of mass and it was (just before you closed the throttle) spinning VERY fast, (somewhere between 50K and 100K r.p.m.) we just let the air out and the turbo won't have any work to do and keep spinning.

It is set to operate when you close the throttle, by sensing vacuum in the intake manifold (after the throttle) The valve typically stays closed at all other times. The advantage is that there will be very little or no lag between shifts, i.e.. once you get it spinning, you have almost instant boost when you want it.

My 1.8t has a turbo and no blow-off valve, do I need one?

Every turbo system can use a blow off valve. Basically the higher boost levels demand one. A turbo spools down faster when against 18 p.s.i. than say a low boost system of 5 p.s.i. Older Saab turbos don't have blow off valves or inter-coolers, when the 2.0 turbo engines made 160 h.p. Now we have 1.8t VW engines making 200 h.p. (with a chip), this level of performance DEMANDS a blow-off valve.

You also may gain some longevity to the turbo's life with a blow off valve because it reduces number and severity of spool-up, spool-down cycles.

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