# Shimming the relief valve to increase fuel pressure



## Rabbit Farmer (Sep 22, 2003)

I'm getting a new fuel distributor for my turbo Rabbit. The previous owner wasn't sure if he shimmed the relief value to increase fuel pressure or not. Anyone do this? I just need to know what I am looking at when I am swapping over all the lines to the new fuel distributor (i.e. what exactly is shimmed). And, my assumption regarding the adjustment to the flow plate (the allen screw in the airbox) is that I don't need to touch it was the adjustment is on the lever, not the distributor, so nothing should change.
Fuel delivery is key due to boost levels.
Thanks
Steve


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## Southcross (Jul 2, 2004)

*FV-QR*

I still don't undestand why people expect "shimming" to do anything...


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## ziddey (Apr 16, 2006)

*Re: FV-QR (Southcross)*


_Quote, originally posted by *Southcross* »_I still don't undestand why people expect "shimming" to do anything...
Isn't that what the Bentley says to do? Or usually if there's not enough pressure, filters / pumps are to blame?


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## Rabbit Farmer (Sep 22, 2003)

*Re: FV-QR (ziddey)*

Starting doing a little research (couldn't do it early as I was at work)
Menu: http://www.diagnostic-assistan...m#top
Specifically: http://www.diagnostic-assistan...MFI07

_Quote »_15.1.7 Systems Pressure
This is the pressure that is seen within the system between the fuel pump and the metering head. This pressure is determined by the primary pressure regulator, situated within the metering head.
When the required pressure is obtained, the plunger within the regulator lifts off its seat and excess fuel is returned to the tank.
This system due to the nature of its operation will automatically compensate for different fuel demands under different conditions. For example if the fuel requirement is low at engine idle, the plunger will lift and return a greater volume of fuel back to the tank than when the demand is higher, when a smaller amount of fuel is returned.
When the engine is switched off, the fuel pump relay looses the coil negative signals that energise it and the voltage to the pump is removed: this subsequent loss of pressure will cause the primary pressure regulator to close. This action subsequently blocks the return flow to the tank and helps the accumulator to maintain pressure in the system.
The systems pressure is determined by the tension of the spring reacting against the plunger, if a higher pressure is required, small shims can be placed behind the spring, changing it's effective length and increasing the pressure. *A shim of approximately 2 mm will increase the pressure by about 10 psi*











_Modified by Rabbit Farmer at 10:27 PM 7-30-2009_


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## Rabbit Farmer (Sep 22, 2003)

*Re: FV-QR (Rabbit Farmer)*

You mentioned Bentley, so I just checked it. I always thought this was some racer's secret... nope.... black and white.
Section 4: Fuel and Exhaust Systems
Page 44
During manufacture of the fuel distributor, the system is preset by the installation of the correct shims behind the pressure relief valve spring. If the system pressure if not between 4.70 and 5.40 bar (68 and 78 psi) when tested.... you may be able to correct the system pressure by installing different shims. The addition or removal of a .10mm (.004 in) shim will change the system pressure by approx .06 bar (about 1 psi). The addition or removal of a .50mm (.020 in) shim will change the system pressure by approx .30 bar (4 psi).


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## Southcross (Jul 2, 2004)

*FV-QR*


_Quote, originally posted by *Rabbit Farmer* »_I always thought this was some racer's secret... nope.... 


_Quote, originally posted by *Rabbit Farmer* »_
If the system pressure if not between 4.70 and 5.40 bar (68 and 78 psi) when tested.... you may be able to correct the system pressure by installing different shims.


http://****************.com/smile/emthup.gif I usually wind up in some long-winded flame thread of some person saying how it increases fuel flow, and me explaining that it doesn't work that way


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## Rabbit Farmer (Sep 22, 2003)

*Re: FV-QR (Southcross)*

It is all about pressure.
I took apart the old fuel distributor to determine how many shims (or thickness) was used so I could transfer it to the new fuel dist.... there were none. Better yet... it was completely gutted inside. No plunger, to spring, no shim(s).... nothing but the spring load section that is connect to the threaded cap. Huh? Figured it out... it was removed as the external FPR is doing that job... don't need more than one (plus the warm up regulator is going a little there also with holding pressure, etc.).
My turbo Rabbit still doesn't run properly, but this did fix some of it. I think it is electrical so it is time to dive into the limited wiring harness (8 wires?). Four wires to go the tach and back out to the brain. I'm speculating that the tach is having issues. Unsure how to test it as I am not a wiring guru (more like a wiring idiot.... ID-ten-T rule does apply)


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