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The Boost Pressure Control (BPC) valve is used by the ECU to control the wastegate to regulate boost. The ECU measures the boost with the Manifold Absolute Pressure sensor (MAP).
The By Pass Valve (BPV)is controlled by the vacuum/pressure from the Throttle Body (TB). When the throttle is closed, the negative pressure opens the valve. When on boost, pressure combined with the spring pressure holds it closed. When you are on boost and then close the throttle, a manifold vacuum is created which opens the valve to release the pressure. This prevents a pressure and flow wave from 'bouncing' back from the TB to the turbo compressor. That backflow can create shock waves in the compressor which can do two things. 1-The compessor wheel can be become more than one piece of metal. 2-The turbo rotor also looses a speed suddenly. So releasing the pressure allows the turbo to maintain speed better across shifts.
A BPV routes the air back to upstream of the turbo, and will be down stream of a MAF is fitted (C900's, Y7's not-T5s). A BPV is open at idle, so it must not be vented directly to atmosphere as this will alow unfiltered air to get into the system. A Blow Off Valve vents directly to atmosphere and is closed at idle or throttle overrun. The BOV makes lots of noise and is good for frightening and startling people and animals. A BOV might have a performance advantage of faster boost recovery across gear shifts, but only an issue if you have an agressive driving style to take advantage of that. There is no HP or peak torque advantage.
If you install a Manual Boost Controller (MBC), the OEM Bosch plastic BPVs start to make stupid noises and this prompts many to replace with a metal piston type BPV. At higher pressures, some Bosch BPVs suffer mechanical failure.
The Forge BPV is a good direct fit. TurboXS makes a small BOV (type S) that will install right where the BPV goes. You need to request that they supply a 25mm hose adapter, as it will otherwise come with a aluminum and steel weld-in adapters. A screw driver install is nice to have. You would then remove the 25mm return hose and block off where it went.
The valves will operate faster if the face of the piston is against the boost pressure. When flow starts, the impulse of the turning flow will act on the piston face and make it open faster. Thats a fact. You may find instructions for orientation that are contrary.
I don't see any point in replacing an operational OEM BPV when running a stock boost control system.
Installing a K&N will not increase the boost settings, but will reduce boost droop at high RPMs, which increases HP. The turbo will spin up a bit faster with the K&N.
See the link for a disussion of intake issues for a T7. Some of this will apply to a T5 ECU if you prudently read between the lines.
posted by 208.24.179...
, Fri, 11 Oct 2002 16:15:22
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