fuel pressure arithmetics

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08-09-2005, 08:49 PM

george7941

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fuel pressure arithmetics

I was getting 8psi fuel pressure at idle with the stock lift pump and also 8 psi with a holley blue installed as a pusher with the stock pump bypassed. However when I install the two pumps in tandem (series) I am getting 25 psi at idle and am also getting hard starts. How does 8psi + 8psi = 25psi. I expected 16psi. What is going on?

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08-09-2005, 08:53 PM

george7941

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oops! I forgot that my fuel pressure gauge underreads and so the pressures are actually 10, 10 and 28psi.

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08-09-2005, 08:58 PM

Shovelhead

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10 + 10 = 28

Must be the same math that the oil companies are using to set the price at the pump.

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08-10-2005, 09:21 AM

HOHN

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LOL@ Ed.

It's simply efficiency. Just like when you talk about twin turbos. One turbo @25psi feeding another @25psi should give 50psi total, right? Nope-- closer to 70+.

Your hard starting is a result of the high fuel pressure. Keep it under 18 or so and hard starts will go away.

jlh