Fuel pump pressure

[Jeff P.]

Several year

[Jim T.]

Re: Fuel pump pressure

Curious Jeff, how are you going to measure fuel pump pressure at a high RPM using 3rd and 4th gears? How much fuel pressure do you have at idle?

[Domenic T.]

Re: Fuel pump pressure

Jeff,
Another thing to consider is the needle & seat, there were different sizes (why, I don't know, the larger would work for all models) and the smaller one will not meet the needs of a HP engine at normal pressure. The consequences were starvation at high RPM.

DOM

[Jeff P.]

Re: Fuel pump pressure

^rd and 4^th gears at high RPM. Thissounds a little extreme to me. Theengine was fine at high RPMs in 1^st and 2^ndrd and 4^th also? Thanks to Dominic about the sizedifference in the carburetor needle and seats. I had no idea.

[Joe R.]

Re: Fuel pump pressure

Hi Jeff,

What problem are you having? Fuel pressure varies with RPM, not load on engine. So, you can measure it in your driveway or at redline in any gear with the same results. Who is your engine guy?

Joe

[Michael W.]

Re: Fuel pump pressure

The concern over fuel pressure is misunderstood and over rated. It's 'flow' that keeps the carb bowl full, not 'pressure'. If the pump can keep the bowl full at all loads and speeds, line pressure is irrelevant.

OEM pumps are usually oversized by 50% or so, not sure why your engine guy is making a fuss.

[Duke W.]

Re: Fuel pump pressure

Several years ago I saw a response by Duke Williams (who has got to be some kind of genius) to a question about fuel pressure

Actually if you talk to the guys in the local chapter who know me they'll quietly tell you that I'm not very bright at all.

As previously asked, what problem are you experiencing that leads you to believe you have insufficient fuel pressure?

If fuel pressure is insufficient or the filter is clogged (including the filter sock in the fuel tank) or there is some other restriction in the fuel supply system like a dented or crimped fuel pipe, the engine will shut down at high revs - almost like you turned off the ignition when the fuel in the bowls is consumed. I recall once helping a friend diagnose such a problem and it turned out to be a big dent in the fuel supply pipe that was probably caused by a rock hit.

If you look in the AMA specs and 1963 Corvette Shop Manual you will see a fuel pressure spec (IIRC it's something in the neighborhood of 4.5-6" psi). It can be measured with the temporary gage set up you described, but running this test in third or fourth gear would be very high speed, and it would be tough to find a safe venue for this test other than a race track.

There is a flow test specified in the shop manual -something like one pint in about 30 seconds at idle at zero pressure because you disconnect the fuel line from the carb and dump it into a can. The engine will easily idle for at least 30 seconds with the amount of fuel in the bowl.

The reason why pressure is developed is that the needle and seat is a restriction that causes "back pressure" unless the fuel in the bowl is being consumed so fast that the needle is open more than about a quarter of the seat flow path diameter. The seat itself is a restriction, even with no needle because it is smaller in diameter than the fuel supply line from the pump.

So there are plenty of ways to determine if the pump is performing to spec, but before you start testing the fuel pump, tell us how the engine is misbehaving.

The fuel supply system, and the exhaust, brake, cooling, power steering, and engine oiling systems, are simple fluid flow/hydraulic systems that any mechanical engineering undergrad learns the fundamentals of in the sophomore to senior year.

Duke

[Jeff P.]

Re: Fuel pump pressure

Well first of all let me clear something up. I am not having any problems that indicate that I am having fuel pressure problems, I am sorry if led you folks to that conclusion. It is just that my engine builder seems paranoid about a lack of fuel pressure at high RPM to the extent that he has me worried about it with my freshly rebuilt engine and all. Based on all of the posts I am confident that I don't need to "test the pressure" at 6000 RPM in 3rd or 4th gear. I'm going to drive the car as it is and if I do develop a problem I'll ask for your advice again. It sure is good to have this kind of support from NCRS members! And to Duke Williams, if the guys in your chapter quietly think you are not very bright...then I wish I had a few "not very bright" guys, like you, to talk with about four times a day! Thanks to all.

[Duke W.]

Re: Fuel pump pressure

If the engine doesn't misbehave at high revs, I expect the fuel pump is fine and can't understand why "engine builder" is concerned.

One sure fire way to determine if sufficient fuel is being delivered at WOT/high revs is to locate a shop with a Dynojet chassis dyno with a wide band O2 sensor.

The A/F usually starts out a little on the rich side at low revs, but leans out a bit at the top end. My criterion is that the A/F ratio should not be greater than 13.5:1. If it's over higher than 13.5, usually one or two size larger secondary jets will get it into the correct ballpark.

I've never seen a correct part number/properly functioning OE-type fuel pump fail to supply sufficient fuel to a small block, including a "327 LT-1" configuration that peaked near 290 SAE corrected RWHP at 6500 and was tested to 7250.

A Flint-built L-79 peaks at near 5500 with about 220 SAE corrected RWHP.

Duke