how do i choose what gph to get on a mech fuel pump?
my other question is, how many gph is my stock pump and is it good enough for what i want to run?

hoping for 400 rwhp 350ci that i am currently putting together.

-750cfm mech sec dual feed holley(currently checking out qft ss-750 or Q-750AN)anyone think i should try the 650 instead
-vic jr
-afr195street elim
-355ci 10.5to1
-xr280r solid roller

i like that the stock pump uses a return. i have run a plain jane carter mech fuel pump with no return line and no regulator after the pump in the past. is my stock pump the same as the carter but with a return? what is the pressure rated at for the stock mech fuel pump?are there major problems with running a mech fuel pump with no return line?

-doug

 

The stock pump for these cars will support 400 HP EASILY, if all else about the fuel system is working right. (no blockages fro 100-yr-old crusties in the lines, good fuel filter AFTER the pump NOT before, etc.)

Carbs don't work off of fuel PRESSURE; they work off of DELIVERY. If the carb's fuel bowls would stay full with .01 psi of fuel pressure, then .01 psi is enough. There's no need to get all worked up about how many PSI the pump can make. A good indicator of having "enough" pump, is if the fuel pressure DOESN'T DROP OFF at the end of a run; i.e. if it's 5 psi at the start line, and still 5 psi at the lights, it's enough. The ABSOLUTE VALUE of the pressure isn't near as important as, that it DOESN'T DROP OFF. 5-6 psi is usually PLENTY, which is about what a good-quality replacement stock pump such as Carter or Airtex puts out.

A common cause of high-RPM fuel starvation is the fuel pump drive rod. Remember, the cam PUSHES the drive rod DOWNWARDS against the FP spring, which creates the "suck" stroke that draws fuel TO the pump; then, the SPRING in the FP must not only FORCE that ridiculously stupid heavy rod back UP (against gravity) INTO the block, BUT ALSO at the same time, pressurize the fuel. A lightweight drive rod such as this one http://www.summitracing.com/parts/MOR-65750/ will make a HUGE difference in the fuel pump's efficiency.

 

Stock fuel pumps are usually in the range of 65-80 GPH. A typical performance replacement will be about 110 GPH. And in good working condition either should be sufficient to feed a very healthy engine. Like was said above, it's more about REAL volume delivered than pressure.

Most mechanical fuel pumps never achieve anywhere near their rated capacity becuase of restrictions in the factory fuel line system. The LONG DRAW from the tank to a block-mounted pump can cause headaches in a hard-launching car. As with all pumps, they do a much better job PUSHING fuel than they do DRAWING fuel. And when you add in the extra resistance of a hard launch with g-forces trying to pull fuel away from the pump back to the tank, you can run into problems.

I've had problems outrunning even very large pumps on cars running as slow as high 13s, just because the factory lines were so restrictive on the draw side. Add a low-pressure electric helper pump back by the tank (Holley Red) and suddenly the exact same car can run in the 11s without the slightest dip in fuel pressure anywhere in the run- still using all stock factory lines!

 

I had a 350 built similarly to what you have and I used a 110gph Holley pump, the one that uses the clockable bottom half. I never had one problem with that pump - great unit.

 

The Carter M6626 is rated at 120 gph, and has a return. Fits our cars (carb'd is a direct install - EFI to carb will require lines to be modified).