Thread: EFI's fuel pressure/GPH

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The question is for mainly EFI injection systems on SBC's but I would think it would in some degree effect carb'ed SBC's as well.

Fuel pumps have two ratings.One is pressure and the other is GPH.I am saying GPH is the volume the fuel pump delivers.

So I am thinking that pressure of the fuel is more related to HP output of a engine.

And I am thinking that GPH or volume of fuel is more related to the torque output of a engine.

Am I correct in those assumptions?Comments please.

its like hp and torque are related--you can't have one without the other

In fuel pumps pressure is ditated by what kind of system you have--carbs needle/seats will only handle up to 5 or 6 lbs reliably and you need a pump that will supply the power demands of your engine-----of course in any system the plumbing has to be big enough to supply the 80, 100 gallons per hour rate

in efi systems---most of the oem throttle body systems work on 20 +/- lbs and the individual port systems will be upwards of 40 to maybe 60 lbs

Like Jerry says, the two are inter-related, not independent. Every pump will have a pump curve, which is where the pump performs relative to volume and pressure - increasing volume on the X axis (vertical), increasing pressure on the Y axis (horizontal) with the curve falling off from left (0 pressure, highest volume) to the right as pressure increases. The pump will have a point of maximum efficiency, but will operate anywhere on the curve - at any given pressure (system resistance) you will have a specific volume flow for that pump in new and clean condition.

Originally Posted by rspears

Like Jerry says, the two are inter-related, not independent. Every pump will have a pump curve, which is where the pump performs relative to volume and pressure - increasing volume on the X axis (vertical), increasing pressure on the Y axis (horizontal) with the curve falling off from left (0 pressure, highest volume) to the right as pressure increases. The pump will have a point of maximum efficiency, but will operate anywhere on the curve - at any given pressure (system resistance) you will have a specific volume flow for that pump in new and clean condition.

Jerry,
That makes sense.My thought is volume helps crate torque because of the lower rpm range it occurs and pressure helps hp because it is ,if I am correct,helps with hp because the speed it delivers the fuel at higher rpm's where generally hp occurs.But that now doesn't make sense because as you say,the volume is somewhat a constant threw up the RPM range.True enough that hp is a derivative of torque.

The CPI systems uses pressure in the range of 58 to 65 PSI with GPH in the range of 50GPH.

Now this isn't just a exercise in mental questions.The Astro CPI in tank pumps do have a limited life expectancy.We have found a good priced in-line pump that has more than enough pressure,but is 10 GPH less than the spec of the O.E.M. in tank pump.I was wondering what effect it would have??.And how that would effect a V8 swap in the Astro vans using a SBC 350 or 383.

Originally Posted by 1gary

Jerry,
That makes sense.My thought is volume helps crate torque because of the lower rpm range it occurs and pressure helps hp because it is ,if I am correct,helps with hp because the speed it delivers the fuel at higher rpm's where generally hp occurs.But that now doesn't make sense because as you say,the volume is somewhat a constant threw up the RPM range.True enough that hp is a derivative of torque.

The CPI systems uses pressure in the range of 58 to 65 PSI with GPH in the range of 50GPH.

Now this isn't just a exercise in mental questions.The Astro CPI in tank pumps do have a limited life expectancy.We have found a good priced in-line pump that has more than enough pressure,but is 10 GPH less than the spec of the O.E.M. in tank pump.I was wondering what effect it would have??.And how that would effect a V8 swap in the Astro vans using a SBC 350 or 383.

Pressure is a base requirement for fuel injection and is the most important value. The volume requirement comes into play at WOT, so it really depends on how your system is going to be expected to perform. I would think you need to look at your injector size/flow rate and determine what the true demand is for your pump. Comparing vehicle OEM values, which are for a small V-6 application at best, to your needs with the 350/383 is a moot point. What is the fuel demand of your new engine (injector max flow X 8)? Regarding your "good priced in-line pump", that pump has an output curve which relates fuel pressure to fuel volume across the full operating range of the pump, from 0psig to peak pressure for that specific pump. If the pump cannot provide the volume flow that you need to feed your injectors at the given pressure they need then it is not the right pump. This is why EFI systems have recirculation lines - you need enough pump to meet peak demand, and you will be recirculating fuel when in cruise mode. Do you want twice your peak demand?? No, because now you will be recirculating more fuel than necessary, adding heat to your tank volume all the time, and you may get into a vapor lock situation in some situations.