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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
What am I looking for on the post you mention?

I am talking about the intake side, not the exhaust. Bleeding exhaust off through the wast gate is how we maintain our boost at the level we want. If your wast gate is to small or restricted you would go into an overboost condition. This can be a problem with the Turbos with an oversized compressor wheel and an aftermarket header system that has a poorly designed WG system.

Some race cars do have provision to bleed off boost on the intake side under throttle as a safety feature or as a guarantee that they do not exceed a mandated boost level by the race org. No production Porsche Turbo dose this.

EFI motors are different from CIS. They can shut off there fuel based on values programed into the map depending on throttle valve angle, intake vacuum, and rpm. Thus, when the throtle is closed at any rpm, fuel flow is stoped.

With EFI they can use blow off valves on overrun to atmosphere. However, Porsche still chooses to vent this pressure back into the intake even on its EFI turbos like the 993 & 996.

CIS is always flowing fuel in a steady stream. (Continuous Injection System) The amount is determined by the among of air flowing over the metering plate and pushing it open. If you bleed off any intake air after the fuel metering sensor and before the intake valve, the motor can not figure this our and will be sending to much fuel.

Like wise, if you develop an intake leak after the metering system and before the turbo where extra air is sucked in, the motor will be getting to much air and will go lean.

If the leak is developd in the intake after the throttle plate the motor will be lean pre-boost as air is sucked in under vacuum, and will go rich on boost as air escapes.

I do not know how you can find one but there is a factory digram of the C2T's plumbing. It better indicates how CIS is plumbed and works.

A digram from a 930 should have the same basic thing. The CBV is just different.

When I had my Turbo set up I got instant .2-3 bar boost off idle and .5 bar off cruse upon acceleration. Also boost stayed at full boost between sport shifts. It did not have to build back up.

The goal for a responsive 930 is in the eliminating all restriction possable after the turbo in the exaust. Keeping the turbo spinning. Getting the metering plate out of the way fast. And getting the best timming and A/F values you can.
Old 12-08-2008, 06:27 PM
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