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Join Date: Dec 2006
Posts: 62
Porsche Crest Adjusting Fuel Pressure Regulator

I recently had my Fuel Distributor rebuilt to 930 3.3 specifications. It doesn't run well. I later studied my car's history. According to the receipts, about 2 years ago the car was updated to 3.4 liter cylinders. Now today I got my Bosch CIS tester in the mail. I plan to check out the pressure. I strongly believe that the fuel pressure regulator in the fuel distributor was shimmed wrong on my car!

I was reading that to add pressure you need to add shims and to reduce pressure you need to take away shims. Each (.020 in) shim added increases system pressure by about 4 psi. And each taken away will decrease pressure by 4 psi.

I read that if the CIS pressure is too high then the car will run lean. And if the pressure is too low then it will run rich. Is this true?

According to my specs book my 79 930 needs to run a "control pressure" of 3.65 bar + - 0.20

"full throttle enrichment" 2.9 bar + - 0.20

Does my car still run under these specs or will it be different since it is a 3.4 rather than a 3.3?

Old 01-23-2007, 09:32 PM
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David's Avatar
Join Date: Apr 2002
Location: Houston (Clearlake), TX
Posts: 9,335
The CIS delivers fuel by opening a piston in the fuel distributer and distributing it to the 6 injectors. The piston rises when the flap in the intake opens. If fuel pressure is higher it's harder to open and the engine runs leaner.

Your car may be fine or it may not. The only way to know for sure is to check the air fuel ratio at wide open throttle with high revs and full load. The only way to feasably do this is with a lambda meter on a race track or a dyno. The dyno's much easier.
2014 Cayman S
2011 Cayenne Turbo
1979 930 (475 rwhp at 0.95 bar)
Old 01-24-2007, 04:03 AM
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It makes sense! On startup the WUR does it's job and the during that time I can punch the gas and rev the engine and see no problems. No Hesitation. But as the WUR stabilizes, that is where I see the problem. I totally have to enrichen the engine a lot just to make it run without bucking when I give it gas. Come the next day when I start the motor , it runs too rich!

It sounds to me like I have to take away a shim or two due to there being too much pressure. By lessening the pressure, more fuel will go into the engine due to there being less restraint on the metering plate. I believe I got it now. I will post my results after I test the pressure coming from the Fuel Dist. to the WUR. And if there is too much pressure I will do the necessary adjustments.

Last edited by madkow; 01-24-2007 at 10:52 AM..
Old 01-24-2007, 10:47 AM
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Walt Fricke's Avatar
Join Date: May 2004
Location: Boulder, Colorado
Posts: 6,096
Porsche Crest

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
Old 01-24-2007, 09:49 PM
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Walt Fricke's Avatar
Join Date: May 2004
Location: Boulder, Colorado
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Porsche Crest and one more thing

I see you have the CIS tester, so you can easily measure both system and control pressures. Control you can even measure with the engine running (as long as the hoses don't get sucked into the fan). So if you have the specs you will promptly know if you are in the ball park. I would hazard a guess that with 100ccs more in your engine you might not want control pressures higher, nor system pressures lower than factory 3.3 specs. But these specs are usually given as a range, and for the WUR, shown as a band).


Old 01-24-2007, 09:56 PM
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