Pressure Release (Blow-Off) Valve

Boost pressure and air speeds in the inlet ducting post-compressor increase as the turbo speed increases and rams that air mix in – fine.  When the throttle is lifted, however, the throttle valve snaps shut and leaves this charge air with nowhere to go (ie. into the engine for combustion)

.As a result the intake duct between compressor and throttle experiences a severe pressure spike.  This pressure backs up and will ultimately cause reverse pressure against the turbo compressor.  This load slows the compressor down, reducing the amount of boost available when the throttle again opens.  Additionally, in extreme high boost cases or prolonged or repeated exposure, this reverse pressure can cause compressor, shaft and seal damage or wear.

The solution is to install a valve into this pipe section to vent this excess pressure out of the intake system.

A blow off valve is a diaphragm-type device.  One side of the diaphragm is subjected to intake pressure, the other to plenum chamber (post-throttle) pressure (through a vacuum hose).  When the throttle is open, the pressures between the intake and plenum will be equal, and the diaphragm will equalise in a position where the valve vent is not released.

When the throttle closes the plenum chamber will ditch to vacuum and the intake pressure will skyrocket.  When this happens the combination of pull from vacuum and push from boost pressure will force the diaphragm into a position where the valve is open to vent.  The pressure will vent through this valve until the pressures are again sufficiently close to equalised – generally when the throttle is again opened.

The result?  No turbo compressor back-pressure during throttle lifting, resulting in higher maintained turbo RPM for the shift from closed- to open-throttle, meaning more boost right there waiting.

For momentary lifting, especially during gear changes, this system can be very effective.

A blow off valve vented to atmosphere on an E15ET will bleed air from the intake after its delivery through the AFM.  This leads to over-fueling (running rich), as fuel is injected for air which has been metered but has been vented through the valve before injection occurs.

The solution is to vent a blow off valve back to the inlet tract after the AFM, maintaining air volume in the inlet system, or the other solution….. Use no blow off valve at all ! 😉

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