Fuel Pump Pressure

[Joe C.]

What is considered normal fuel pressure with standard issue SBC mechanical fuel pump.

[Joe L.]

Re: Fuel Pump Pressure

What is considered normal fuel pressure with standard issue SBC mechanical fuel pump.

Joe----


It varies somewhat by fuel pump and application. For 1965 Corvettes, though, it's as follows:

250, 300 hp-----5.25 to 6.5 PSI

365, 375 hp-----6.5 to 7.5 PSI

[Timothy B.]

Re: Fuel Pump Pressure

Joe,
My 63 with 300 HP has a 4657 AC pump and at idle the pressure is 5.75lbs. This pump has been overhauled with a paragon kit which included the diaphram. I can't remember everything but I think I reused the springs and valves in the pump as they looked fine to me.

[Joe C.]

Re: Fuel Pump Pressure

Thanks for your replies. Joe L. and others stated above, that fuel pressure should be 6.5 - 7.5 psi @ idle. Yesterday, I teed in a combo engine vac/fuel pump compound gauge between the fuel filter and the carb. Here is what I recorded from a short road trip I made with the car today:
Fuel pressure @ approx 800RPM idle: 4.5 psi
Fuel pressure from off-idle to about 3500rpm: 4.5 psi
Fuel pressure @ 5500RPM, third gear, WOT: 3.0 psi
Fuel pump is correct replacement 6440083 without "AC" logo, and without p.n. stamped into casting. It has a small metal tag attached to one of the cover's machine screws, with the GM part number embossed thereon.
It has the "correct" ears, spaced 180* apart, and has a cast aluminum bottom cover.
This pump was installed on the engine when I bought the car. It appeared like it had been fairly recently replaced, so it was one of the handful of parts that were left as-is during the restoration. No leaks, quiet. Maybe it was rebuilt at some point, using the wrong relief spring.
A dyno test of my engine, in June 2006, showed the engine "laying down" at about 5500 RPM. This corresponded to the A/F ratio going from about 12:1 @ 4200RPM, to about 12.75:1 @ 5500RPM. No misfire heard while on the dyno, but possible fuel starvation condition?

Joe

[William C.]

Re: Fuel Pump Pressure

possible, the current over the counter 40083 pumps are a pale imitation of the originals.

[Duke W.]

Re: Fuel Pump Pressure

Pressure is going to vary with fuel flow. If you operate the pump and dump the output into a bucket, measured outlet pressure is zero. The restricted fuel flow at idle builds pressure, but it drops as fuel demand increases - perfectly normal -same thing has to happen in a fire hose with too big a nozzle or too little pump capacity. I don't recommend bigger jets (or more fuel pressure) until it leans out to beyond 13.5:1 at peak revs.

If you have at least 1-2 psi at high revs, I don't think fuel starvation is an issue, and carburetors typically lean out a little as WOT revs increase. Engine power shouldn't drop significantly until you get into lean misfire, which doesn't start until you get leaner than stoichiometric, which is about 14.7:1.

Another source can be ignition problems, Testing the engine under load with a dwell meter will show if the points are bouncing. The dwell angle should not drop off more than about 2 degrees at peak revs, and the timing should be steady from the point of maximum centrifugal advance to peak revs. You can also check for dwell and timing variation by free revving the engine, but this is not conclusive. A "weak sparK" may allow the engine to free rev, but a denser WOT mixture may not consistently ignite if ignition energy is marginal.

Duke

[Joe C.]

Re: Fuel Pump Pressure

Pressure is going to vary with fuel flow. If you operate the pump and dump the output into a bucket, measured outlet pressure is zero. The restricted fuel flow at idle builds pressure, but it drops as fuel demand increases - perfectly normal -same thing has to happen in a fire hose with too big a nozzle or too little pump capacity. I don't recommend bigger jets (or more fuel pressure) until it leans out to beyond 13.5:1 at peak revs.

If you have at least 1-2 psi at high revs, I don't think fuel starvation is an issue, and carburetors typically lean out a little as WOT revs increase. Engine power shouldn't drop significantly until you get into lean misfire, which doesn't start until you get leaner than stoichiometric, which is about 14.7:1.

Another source can be ignition problems, Testing the engine under load with a dwell meter will show if the points are bouncing. The dwell angle should not drop off more than about 2 degrees at peak revs, and the timing should be steady from the point of maximum centrifugal advance to peak revs. You can also check for dwell and timing variation by free revving the engine, but this is not conclusive. A "weak sparK" may allow the engine to free rev, but a denser WOT mixture may not consistently ignite if ignition energy is marginal.

Duke

Fire hose???? Wha?? Please, stay on-topic, will ya'.
In a related stream, my "three footer" days are long gone, too. Don't eat any yellow snow.

Well, I'm going to rebuild the pump, anyway, so's I get the proper pressure. I'll run the same test sans fuel filter, as the filter MAY, possibly have become fouled after running fresh fuel thru the new SS fuel line I installed during the restoration, back in 2003. Your analysis makes good sense. I especially like the "stoich" part. Geez, I haven't heard the word "stoichiometric" not more than a few times since my college days. I can still spell it, though, right? Kinda' reminds me of Avogadro's number (6.02 x 10^ 23). Go to the store and order a mole of potato(e) salad.

One of our "associates" mentioned a weak coil as the possible culprit. This might be a possibility, as I've been using a nice "202" coil that I picked up in Carlisle for 5 bucks, in 2003. No leaks, and full of PCB laden insulating oil. Fresh gloss black paint made her look new. Been running this coil since day one. Again, no perceptible misfire on the dyno indicates that the coil is AOK. But, then again, just trying to establish a good, working "baseline" before the rebuild.

As you know, the distributor, as rebuilt by me in 2002, is very tight. Dwell is pegged at 30 degrees, with no variation at high RPM. I'm running a set of "high spring tension" points, which DID cure a high speed misfire problem with the standard points. There is no end play on the shaft, no wobble in the breaker plate. I was once considering going to a LL Breakerless SE unit, but see no reason to do so, as the distributor is so accurate.

There are 2 more possibilities, which seem reasonable, based on the way the engine acted on the dyno:

1. Engine compression may be lower than optimal, as I cannot induce detonation (total timing is 12 + 25) running 93 PON gas, 4th gear 15 MPH, stand on throttle, hot day.
2. Oil ring issue may be contributing.
3. Possible turbulence @ valve seats, as I did not order 3 angle valve job after I ported the heads.

Joe

[Duke W.]

Re: Fuel Pump Pressure

Don't forget Planck's constant - 6.625 x 10** -34 joule-sec - Planck length, Planck time...

Since head flow is the ultimate determining factor in power production, I think you should have the heads flow tested as they are now. I suspect they won't be as good as Dave McDufford's, which were about 236/203
CFM @ 0.5" lift, 28" H2O depression. Then do a little more massaging as necessary and finish off with a 3-angle valve job with .040/.060" seat widths, and flow test them again as a final check.

Duke

[Joe C.]

Re: Fuel Pump Pressure

Don't forget Planck's constant - 6.625 x 10** -34 joule-sec - Planck length, Planck time...

Since head flow is the ultimate determining factor in power production, I think you should have the heads flow tested as they are now. I suspect they won't be as good as Dave McDufford's, which were about 236/203
CFM @ 0.5" lift, 28" H2O depression. Then do a little more massaging as necessary and finish off with a 3-angle valve job with .040/.060" seat widths, and flow test them again as a final check.

Duke

Exactly!
As I eliminate other causal factors, it looks as if my initial suspicion will be confirmed.
I'll address the pump and the coil before I get out the engine crane. I don't think that they are the causes, but I want 99.9% before I examine the heads. Goal is better than 55/65 int/exh flow efficiency with the stock 461's. If I'm right, looks like they are at 32/42 right now. That's wores than stock. If so, then it just goes to show how important a proper multi angle valve job can be, if a very well ported set of heads can actually flow LESS than stock, lacking a good performance seat config.

Joe

[Duke W.]

Re: Fuel Pump Pressure

55/65 percent flow efficiencies is a lofty goal - doable, but I think 50/60 is a reasonable target. Dave McDufford's EA computed efficiencies are 48.5/59.2.

Here's a summary of EA computed flow efficiency data for the best of the lot based on actual head flow tests. For reference the OE machined head flow efficiencies are about 44/41%. In terms of raw data, properly massaged vintage big port SB heads should show a 10-12 percent improvement in flow on the exhaust side and a dramataic 30-40 percent increase on the exhaust side.

Don Cox: 461-1.94/1.50 ,58.5/66.8%
Joe Randolph: 462 - 1.94/1.50, 60.3/64.0%
Jim Karam: - 461 - 2.02/1.60, 55/59.6%

Unfortunately, I don't have any data for 461X heads, so I don't know if the slightly larger as cast port volume has a significant effect or not.

In any event, these flow efficiencies are excellent considering that the current LS7's CNC machined ports are about 60/62 percent and proves that these 40+ year old heads have lots of potential!

Duke