Blow Off Valve Information

Blow-Off Valve Info
Author: Dan Hurwitz
Page: 1 Last Updated: August 7, 2003
This article aims to educate the Evo-driving public about compressor bypass valves and blow-off valves.
A compressor bypass valve (CBV) is a vacuum-actuated valve designed to release pressure from the intake tract of a turbo car when the throttle closes.It re-circulates the air back into the intake before the turbo inlet, but after the airflow sensor.
A blow-off valve (BOV) does the same thing, but the released air is vented into the atmosphere.
Purpose of a CBV:
The reason most turbo cars have a CBV is that when the throttle closes and the intake system is under pressure, the high-pressure air entering the motor will bump into the closed throttle plate, and (in the absence of a CBV) a pressure wave will travel back to the turbocharger.The result is that the compressor wheel will stall (a phenomenon known as “compressor surge”) and slow down very quickly.This is hard on the bearings and decreases the turbo’s lifespan, but it also means the turbo will take longer to spin up the next time the throttle is opened.
With the CBV in place, the pressurized air in front of the throttle body is released, and the turbo can freewheel happily.When the throttle opens again, the turbo will have a head start and will get up to speed faster, since it never stopped spinning in the first place.
Blow-Off Valves: Pros and Cons
A BOV does the exact same thing as a CBV, but the vented air is released to the atmosphere, causing a loud “PSSSSHT!” venting noise that is distinctive and attention-getting.Many turbo car owners (and people who wish they had turbo cars) love the noise.
The downside of releasing the air to atmosphere is that it has already been metered by the mass air sensor (MAS), and when it blows off the ECU will be injecting the wrong amount of fuel into the cylinders.(The MAS reads ten air units, say, and the ECU tells the injectors to squirt the right amount of fuel for ten air units.But then five of those air units are vented off to atmosphere, and only five make it to the cylinders, and five more have to be drawn from the outside air, through the MAS again) so the engine temporarily runs extremely rich, meaning too much fuel is injected into the cylinders.
This temporary rich condition isn’t usually that harmful, but it can cause bucking or hesitation on lift-throttle.If the condition is really bad it can eventually foul spark plugs and even clog the catalytic converter.(Unburned fuel on the cat burns very hot, and too much of it can melt the cat).
You can’t vent a stock CBV to atmosphere because the spring setting is too soft, and at idle or part-throttle it will open, which will confuse the MAS and cause rough running, stumbling, and stalling.A stiffer valve (like most aftermarket valves) will stay mostly closed at light vacuum, promoting better behavior when the valve is vented.The downside of stiffer valve spring pressure is that the valve may not open when releasing the throttle from light boost.This can cause some compressor surge (it sounds like a fluttering air noise), and can hinder turbo spool when shifting at light throttle angles.
CBV/BOV Tuning
Adjustable valves have a provision for changing the spring tension of the valve, usually in the form of a screw or a stack of shims for changing the spring preload.Customers often ask us how to set these.The short answer is that you want the valve as softly sprung as possible, while still stiff enough to hold the boost pressure you want to make.If set too soft, the valve will not close correctly the car will idle badly and bog when the throttle closes.If the valve is set too stiff, you will see lots of compressor surge, and the valve will not open at anything less than full pressure, and it will close too early, which defeats the whole purpose of having the valve.Use your boost gauge and your ear to figure out exactly what spring tension your car likes.Another handy tool is a hand vacuum/pressure pump, which you can apply to your stock valve and your aftermarket one to compare their behavior.Start with a spring tension that makes your aftermarket valve behave roughly the way your stock one does, and adjust from there.
BOV Maintenance Notes
We field a steady trickle of tech questions about jamming or sticking BOV’s.The nature of a BOV means it is exposed the elements under the hood, and that means sand, grit, road dust, and other debris can get trapped in the works of the valve.Most aftermarket valves can be disassembled and cleaned.If your valve is jamming or doesn’t seem to be working right, carefully take it apart – caution, the spring pressure is pretty strong! – and clean all the parts.Lubricate the valve with whatever substance the manufacturer recommends, and reassemble.We find this solves 90% of all BOV problems.Some BOV’s – like the Blitz model – come with an optional filter specifically for preventing this type of problem.
Our Recommendation
The best choice for drivability on the Evo is a CBV, which returns the vented air back to the intake.The stock valve works pretty well, although its plastic construction means it may leak a little at high boost levels.If you are going to run higher-than stock boost, you may want to run an aftermarket valve with tighter and/or adjustable spring pressure.
If you really want the noise and attention of a BOV, go for an aftermarket unit.Be prepared for some occasional stumbling and part-throttle compressor surge, but it’s nothing most people can’t live with.For a lot of our customers, the sheer thrill of that noise every time they get off the gas is worth any minor issues.
Mach V Motorsports
edneo said:
have a question.... so will having a blow off valve do any harm to the car in the long run?
think the BOV is supposed to protect ur turbo.. no harm to the car in the long run, only harm to the environment

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