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Hey guys,

I would really much like to know whats the fuel pressure on the stock fuel pump is? I've added a rising rate fuel pressure regulator (5m-gte), but of course, this rrfpr cannot rise the fuel pressure more than the pump can deliver - so, what can it deliver? :)
 

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The problem is that flow is inversely related to pressure. As the rrfpr increases the back-pressure, sure it forces more fuel through the injectors, but the flow rate through the pump itself goes down. At some point the fuel flow is less than what is required to support the hp you are trying to make under boost. Mathematically you could figure it out from formulas, but discussions on the mk2 list seem to concur that point is reached somewhere around 75-80psi which assuming a 6:1 rate of gain puts you in the safe range of 7-8 psi of boost. This of course assumes a healthy low-mileage pump. Walbro makes a pump with sufficient flow up to about 90psi IIRC which should be good for up to 10psi. Possibly more with an injector upgrade too but you'd sacrifice idle quality and overall driveablility.

Phil D.
 

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Phil, that is excellent info.!! I think this may also be another case where engineering shines on the MKII. The stock fuel pressure regulator, the restrictor fitting in the return line, and the overpressure safety relief valve on the fuel pump are probably all designed to provide sufficient fuel pressure and flow without loading down the fuel pump excessively from too much backpressure. I have seen the stock pump provide 40 pounds of pressure at idle, but I don't know if that forced the relief valve on the pump to open or not. Hard to say! :?:
 

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Heres a question then Dave...

If the Fuel pump provides a static 40 PSI at idle... why is the injector flow rated at 36?
 
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