There is a lot of mystery and speculation around how the Porsche 911 Turbo CIS fuel injection system works. The reality is, it's not that complex. There are two fuel pumps, one front and one rear. As long as those fuel pumps are working correctly, and pushing hard enough to keep the system pressure within spec, everything else is controlled by your Warm Up Regulator (WUR). Your WUR has basically three settings. When you first start the engine for the day, and everything is stonecold, the WUR is said to be in Cold Control Pressure mode. That setting is based upon a specific value as defined by a graph developed by an engineer in the mid-1970's. So, if you know your ambient temperature (this is why you see a thermometer in most Porsche shops) you can cross reference that number with the required pressure setting and adjust the WUR. Once the engine is up to normal operating temperature, so is your WUR. At that temperature the WUR adjusts the fuel pressure to a specific number. There are two different numbers depending on model, but basically somewhere around 3.6bar will get you in the correct ballpark. The final WUR setting has to do with the decrease in fuel pressure required to richen the mixture under boost. At this point, you basically want the WUR to decrease from 3.7bar to about 2.9bar. This decrease in control pressure actually allows more fuel to pass through your fuel injectors, thus richening the mixture. Perhaps the most complex thing about the CIS fuel injection is realizing that higher control pressures flow less fuel through the injectors and less control pressure flows more.
In the video below you will see the fuel pressure gauge stay constant until you hear the whine of the turbocharger as second gear comes in. At that point you will see the fuel pressure drop and the corresponding air fuel ratio (AFR) number will go down to show how the system richened the mixture just the way it was designed to do back in the 1970's!!!
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