[Walt Fricke]

Well, consider this:

There are two pressures: system and control. It is the control pressure that affects the movement of the metering piston, not the system pressure.

Yes, the system pressure regulator (on the fuel distributor, at least usually) can be adjusted with shims. But the control pressure is set by the control pressure regulator. That sits elsewhere and is often known as the WUR. It Rs the WU by lowering the control pressure when cold.

The base (hot) control pressure is set by a spring or springs inside the WUR. You can disassemble the WUR to adjust these (many posts on this and related subjects, complete with pictures, diagrams, opinions, debate). Other things (a bimetallic strip) inside the WUR subtract from the spring system so that when cold the spring lets more "pressure" bleed off, lowering control pressure and richening the mixture. And yes, lower control pressure = richer, and vice versa.

System pressure is the other way around. Charles Probst, in the bible for CIS "Bosch Fuel Injection and Engine Management," Bentley 1989, says the system pressure regulator shims (washers) are 0.1mm thick and change pressure by 0.15 bar/2.2 psi) per shim. While Porsche has specs for these pressures for particular models (and you should at least know what they should be for a 3.3 turbo, and what yours are, before embarking on changes to see what a 3.4 turbo might need), Probst gives typical pressures as 5.2 bar for system pressure. For control pressures, he gives a "typical" one as between 1.4 and 1.8 bar cold, and hot as 3.4-3.8 (though he shows a graph where this would be 2.2-2.5 hot).

Now what is the effect of increasing system pressure? Well, it is increased flow. The injectors see system pressure, as do the slits, so more pressure for the same opening - more flow = enrichment. But Probst warns that this will probably mean you will be too rich at part throttle (where all but race engines spend most of their lives).

You can get around this by using a rising rate system pressure regulator. I tried one of these Micro Dynamics units on my 3.0 and didn't find any seat of the pants change so I took it off. But on your turbo things might be different. And it allows you to adjust system pressure "externally" on the regulator, no need to take apart something so delicate that Probst says if you drop it you need to buy a new fuel distributor.

Anyway, there is a note of caution for you about just equating system pressure with performance, and more with more.

And yes, to see if you are rich or lean at various load levels it would be helpful to make use of a wide band oxygen sensor or a CO tester. Especially a recording one.

Walt Fricke