Quote:
Originally Posted by meanone
The wastegate is used to control turbo speed, not the bov. A portion of the exhaust mass and pressure is bypassed around the turbine so that only enough mass and pressure enters the turbine to spin the turbo to a speed wich produces a desired boost level. If a wastegate wasn't used, and 100% of the exhaust mass was allowed to enter the turbine, the turbo would overspin and there would be no control over the boost level.
If there wasn't a bov in the system the turbo could be damaged when the throttle closes. When the throttle closes the air stacks up and pressure is sent back toward the compressor wheel. The wheel could be stopped and started again very rapidly by this, and could damage the wheel or even break the shaft. The turbo does not speed up when the throttle is closed and there is no bov installed. The reason that the the turbo could possibly speed up when the bov is opened is because of pressure on each side of the turbo. Pressure and mass is driving the turbine wheel wich is driving the compressor wheel. The compressor wheel is pushing air into the engine when the throttle blade is open. When you shut the throttle blade and the bov opens there is no more pressure in the charge pipe, but there could be some pressure in the turbine side. Think of it as a tug-a-war. Two guys are pulling on the rope and one guy lets go letting the other guy go flying. If everything is sized correctly and a w/g is used the turbo shouldn't overspeed when the bov opens because as the throttle closes exhaust mass decreases putting little pressure on the turbine wheel. This is how I understand it. I hope this helps. Ronnie
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When the throttle slams shut, I agree with Ronnie, that the interrupted flow sends the compressor into massive surge without a CBV (compressor BY-PASS valve), a BOV (Blow-off valve), or a CRV (Compressor Recirculation Valve). CRVs are used on EFI systems with Mass Air Meters.
You can also (as Ken C jr mentioned) set up your system to where you can run a oversized Turbo and keep it out of surge by having the BOV open (adding mass flow to the system but wasting it) until the engine goes into WOT and then BOV closes and the turbo supplies the larger air flow to the engine.
Tom Vaught