I suspect the question is really "will increasing fuel pressure richen the mixture?" And the answer in a Bosch CIS system is no. If the system is working properly.
The fuel flow rate is determined by the differential pressure valves in the fuel distributor, and these valves always provide a 1.5 psi drop across the inlet pipe to the fuel injectors (in the fuel distribution unit). The Bosch engineers did this on purpose to maintain a constant fuel delivery in the presence of fuel pressure fluctuations from the pump. Even the pressure accumulator cannot damp out all of the pressure spikes.
If you want to richen the mixture, you need to figure out a way to increase the "slit opening" in the fuel distributor for a given airflow. The way to do that is to increase the duty cycle on the full throttle enrichment (FTE) switch or by decreasing the control pressure to the fuel distributor. This is what I, and many others do, to richen the mixture.
On my GP car, I have a system that will adjust the control pressure and can vary it from 30psi to 50 psi at will. This allows me to adjust the EGT from <1200F to >1400F at the twist of a knob while I'm driving.
You can also modify your control pressure regulator by either shimming the warmup strip or by knocking its mounting stud further into the housing (.010"-.015" ought to richen it up enough for you).
If you'd like to learn more how to adjust the CIS and CIS-E systems, I can be reached at
[email protected]. I've played with the fuel system a lot, and am learning more and more as I develop the CIS for my prod car.
Mark Coffin
#14 GP VW Scirocco
#44 ITC VW Rabbit