Inadequate replacement fuel pumps.

[DLevel]

Was wondering how many people ever checked the fuel pressure at wide open throttle under load on the fuel pumps they are selling as replacements for our applications. Build mostly GM engines at my shop but horsepower and fuel requirements are relative. In my experience a 400 + horsepower engine would require a 130 gph fuel pump to stay above 5 psi fuel pressure at peak torque under load and provide a little safety margin. For a 500 plus hp engine I've had to take Carter Strip pumps and convert 1/4 npt inlet and outlet to 3/8 npt while blending out the transitions at those ports in the pump at the same time. That operation there is worth about .5-.7 lbs of pressure which puts me into a comfortable pressure area most of the time.
Was trying to finish the engine for my 70 B and thought that the Carter replacement pump for the 383 hp engine should be more than adequate. Only mods I made was a small hydraulic roller and a bowl blend. Was going to be happy with mid 300 hp numbers but wanted to put engine on dyno and make sure I had no leaks. On initial start up and timing checks it appeared the fuel pressure would be more than good looking at my gauges with more than 6 lbs. Man was I wrong. Made my first short run from 3000 -4500 rpm and data showed pressure dropped to 2.8 lbs. Loaded the engine a couple more times, even moved the pressure pick up point to a spot between the fuel flow meter and the carb instead of the backside of the carb fuel inlet.
Now maybe this pump was just a bad performer but I wasn't going to buy another. Put a Carter strip pump on which now I will have to put adapter fittings on to mate up to my original lines. I would of put the old street/strip pump on but Carter has superceded that to the strip pump now. New pump allowed me to get engine run in and off the dyno even though I was looking for the original fuel pump look. Engine only made 360 hp with a rich carb so I believe it wasn't a lot to ask out of a fuel pump.
Has anyone else experienced these kind of issues with these so called replacement parts ?

 

[lewtot184]

it's a juggling act between streetable fuel pressure and WOT fuel pressure. I modify the 6903 pumps. the carter 4862 pump will provide enough volume but at too much pressure. the 4862 can be regulated somewhat by using a return line and testing return orifice sizes. if I get 4psi at WOT and usable pressure during cruise (7psi or less) I just call it a day. I also "bench" test the pumps for volume and pressure keeping in mind that there will be a pressure loss when the pump heats up. I also due the math on fuel requirements at the carb at WOT. 1/2lb of fuel per hour per horsepower at WOT.

 

[DLevel]

lewtot184, Thanks for the info.
I'm so used to doing mostly engines that only see track time that I get too worried about fuel pressure on a street application like this. I just don't take into account the very limited WOT that the engine might see and the lesser fuel demand required. Feel stupid today after reading your post on that carter M6903 pump. That would of been my first choice for this application after the M3672 wouldn't keep up. But after looking the part up on my Motor State account and reading that it had been superceded to the M4862 I figured that was my only choice. Hell I never thought to look somewhere else but did today and they can be bought anywhere. Don't know why my vendor has that pop up notice on the part but my fault for not investigating farther.
Ordered the M6903 today and feel I should be able to use unmodified for a cruiser like this.
Thanks again.

 

[lewtot184]

all of those carter 6903's and 4862's have small internal passages. the edelbrocks, which are carters, don't. the 6903 is probably the best compromise. don't get lost in the 1/4" vs 3/8" pipe inlet/outlet. the 1/4" is more than adequate but the 3/8" may be better for braided hose fittings. take the pump apart and open the passages up to 5/16" and radius them. I run the body pieces across some emery paper on a flat hard service to make sure they're flat (this is worth a little pressure increase). the 6903 will run about 5-5.5psi hot. to bump that up a little I remove the pump arm and pull out the diaphragm. I shim the spring/stem at it's seat about .060". all this will get you a livable 7-7.5psi. I don't use large needle and seats in carbs anymore that I drive. I started looking at fuel flow numbers and for most street cars a .110" isn't needed. if it's possible to step back to a .101" or less there is better control of the fuel and a little psi increase. of course one should do the math on this and be realistic on how much power the engine is really making.

 

[moper]

Honestly I wouldn't be concerned with 2.5-3psi at the needle at WOT. As you say street engines rarely see full throttle for more than 15-20 seconds. That M3672 has had no trouble feeding a 550hp 496 B wedge, including hard highway use and track time; and a 325hp 383 6bbl street car. Neither car has a fuel pressure gage, but neither have any issues from fuel starvation. I use that pump on pretty much everything that has to have stock look.

 

[toolmanmike]

My buddies 440 had exactly the opposite problem with replacement pumps. 10+ pounds at idle and 7# running. We regulated it to just under 5 # and it runs strong enough to tear the planetary out of the tranny. (Another story) and no more flooding over at idle.