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.
!! Warning !! Contrary to the above information, installing a blow-off valve on a 1.8t engine may not be a good idea for the following reason:
If you add a BOV to a 1.8t engine, what will happen is that the mass air-flow (MAF) sensor will account for the air and then the BOV will vent it to atmosphere. This means that the fuel injectors will supply enough fuel for the air that was vented to the atmosphere, Creating a very rich condition. This is very bad for your catalytic convertor! Installing the BOV before the MAF will do no good, as this line is not pressuriezed.
Is a blow-off valve necessary with a supercharger?
Blow-off valves are not very commonly used in superchargers. Superchargers create lower boost or p.s.i. than a turbo can. In a belt driven supercharger a b.o.v. becomes a luxury, more than a necessity. If boost is low (5-8 p.s.i.), there is no need for one in a SC setup. They ARE beneficial on turbo systems due to better turbo life and better acceleration without lag.
Regular superchargers can run about 14 p.s.i. maximum with upgraded pulleys. Superchargers are chain or belt driven by the engine, and soon as the engine rpms drop, so does the boost (because the supercharger isn't creating boost at lower rpms). So using a blow-off valve isn't really necessary but can be used to ensure that it isn't running too high of boost. It just depends on what boost the supercharger is capable of producing and what boost won't blow your engine.
You could add a blow off valve to a supercharged engine as it will reduce the amount of horsepower consumed by the supercharger when the throttle is closed (at idle). But this is overkill you really don't need one since power comes from the crank not the exhaust. An oil cooler would be a better investment.
What's the difference between a blow-off valve and a waste-gate?
If you find any of this information to be incorrect, or you would like to add a comment, please contact us!
Credits and Special Thanks to:
Bob Callenius
Automotive Instructor
New England Tech in
Palm Beach, Florida
Jeff Atwood
GTI-VR6 List Member
Matt Racine
V.E.N.O.M. List Member
