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Question Shimming 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, 09:38 PM
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Forced Induction Junkie
 
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Your CIS is a self-compensating system. The metering plate will adjust the fuel delivery due to the increased air flow through it.
I would never fiddle with the fuel distributor to adjust the fuel system pressure. The warm up regulator adjusts the fuel pressure based on vacuum/pressure at the manifold. That is where you should start.
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'85 930 Factory Special Wishes Flachbau
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Old 01-24-2007, 09:09 AM
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Admiral Obvious...
 
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Mine is a 3.4L and I'm running the Imagine Auto fuel head that flows more gas and is a tad more difficult to control without modifying your WUR. Here is my thread on Rennlisst:

http://forums.rennlist.com/rennforums/showthread.php?t=297546

The Factory Workshop Manual is a good place to start for determining how to test your WUR and other fuel delivery components and what the "baseline" (stock) values are. However in the end there is a certain balance that is achieved between your AFRs and WUR fuel pressures and cold/hot starting.

I followed the instructions for making my WUR adjustable (see my link above for the Pelican thread that I used) and since doing so have run "cold" WUR pressures from 2.2 BAR (rich) all the way to 3.5 BAR (lean). And using an Innovate LM-1 AFR meter, I was able to dial my AFRs to 13.3:1 at idle and 11.5:1 at WOT full boost (using the mixture control screw on the air meter housing) each time I tried a different WUR setting. HOWEVER, what I discovered was that even though I was able to achieve the same AFRs for each of these various WUR pressure settings, the car responded much differently. It obviously ran hotter with leaner WUR values and more on the cool side with richer WUR values. Too rich and the throttle response was very sluggish. And just so that we are clear by lean I mean higher pressure gauge readings and by rich I mean lower pressure gauge readings. Another rennlister named Tripster also ran into a similar scenario with his engine...at one point even though his AFRs were perfect, his header pipes would glow red hot after only a short trip. He too finally discovered the balance, I'm told and the car runs very well.

To make your WUR cold pressures drop (rich), you tap the adjustment pin in...to make the pressures increase (lean), you raise the pin.

Sorry this is so long and confusing!!!
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Last edited by sand_man; 01-24-2007 at 11:08 AM..
Old 01-24-2007, 10:52 AM
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Admiral Obvious...
 
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For the ultimate in CIS control, Brian Leask has pretty much developed the standard for adjustable WURs. His has cold, warm and enrichment adjustability and he's been working on a solenoid system to further help reduce our cars' tendency to go VERY rich at WOT. It's mentioned in the link I posted above.
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back in the saddle: '95 993 - just another black C2
*SOLD*: '87 930 GP White - heroin would have been a cheaper addiction...
"Ladies and Gentlemen, from Boston Massachusetts, we are Morphine, at your service..." - Mark Sandman (RIP)
Old 01-24-2007, 11:13 AM
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A fellow Pelacanite
 
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madkow,
I seem to recall that adding or removing shims was an old trick before the cnc'ing of the fuel head mod was thought of.

I am going to get Brian to mod my wur and get his rpm solenoid setup when i get my car back together.
Maybe even a proper fully adjustable, programmable, mappable digital wur may be available by then.

Also 3.3 Vs 3.4 is such a small percentage.
But I agree with all the previous comments.
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Last edited by NathanUK; 01-24-2007 at 01:46 PM..
Old 01-24-2007, 01:42 PM
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Crotchety Old Bastard
 
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You should only need to re-shim your fuel head if your system presure is out of spec or if you are running over 400FWHP. Other than that it doesn't matter what the displacement of the engine is.

System presure and warm/cold/boost control presure are different things. Your system presure needs to increase in high HP applications. Low control presure (WUR) will result in a rich condition, high control presure (WUR) will result in a lean condition. When you reset your system presure you must reset your warm/cold control presures (WUR) to compensate for the change. This may be what happened when your fuel head was rebuilt back to stock.
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Old 01-26-2007, 04:57 PM
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