How are mechanical fuel pumps rated for there flow? This pump
(www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=86605&prmenbr=361) is rated @ 170gph free flow at 8 PSI. Doesnt the previous statement contridict itself (free flow at pressure.)
The reason I ask is I have built a supercharged BBC and am looking for an easy way to feed it.

Lets say I want to be able to feed 1000 hp (not that I am going to make that much I just like overkill).
if one gallon of fuel weighs about 6 gal then the above pump puts out 1020 lb per hour
(170 gal* 6 lbs/gal= 1020 lb)

pounds of fuel per hour= target HP* BSFC
600lbs=1000 hp* .6 BSFC
1020(170gal)=1700 HP*.6 BSFC :eek:

Now I realize that for a blow through carbureted fuel pressure must rise 1:1 with boost. So boost refrenceing (apply boost to the top of the pump diaphragm) the pump will make it put out the pressure needed.

Now my second question is how much does the volume drop off on a diaphragm type pump when pumping higher pressures? :confused: (say 15 lbs boost+ 7 psi fuel pressure= 22 psi needed)

Sorry about the novel

Rule of thumb when it comes to fuel pumps, and most pumps in general: as pressure increases, flow decreases.

I would not tempt 15psi of boost on a big block with a boost referenced mechanical pump. That's asking a lot of the pump in terms of pressure and flow. Mechanical pumps were never intended to make this kind of pressure and I would really have reservations about the longevity/durability of the diaphragm at 22 psi of pressure seen under boost. I know other people have run this kind of setup and get away with it, but they are really pushing their luck IMO.

Your best bet is a high flow electric pump, but unfortunately that dictates all sorts of other changes-- running new feed and return lines, a return style regulator, and sumping/baffling your stock tank or buying an aftermarket tank. It really is the best way to go IMO, but definately not the cheapest.

I had excellent luck with my SX fuel pump when I ran it with my blowthrough carb. I used a -10 (5/8") from the tank to the inlet filter and a -10 from the filter to the pump, then -8 (1/2") from the pump all the way up to my Mallory return regulator, and then -8 all the way back to the tank.

With a carb and the relatively low fuel pressure at idle/no boost conditions, you need a return line with very little restriction or else you'll have the pressure stack up in the return line and you may not be able to dial your pressure low enough-- i.e. if you use a 1/2" feed line and a 5/16" return line the backpressure in the return line may be 9 psi, which is as low as you will be able to set your fuel pressure. With my 1/2" feed and 1/2" return lines I had 3 psi of backpressure if I turned the regulator all the way down, so I dialed it up to 6.5psi at idle which was adequate.

I have had other friends have excellent luck with Aeromotive pumps as well. Try to stick to a "flow-through" style pump as their motors are cooled by the fuel and will live a longer life than external-motor style pumps (like a Holley blue or red pump.)

Troy

One other thing to realize with a boost-referenced mechanical pump is that when you come off boost there is a second or two that the pressure between the pump and carb is still real high; this can cause a brief flooding condition, although usually it's just an annoying "stumble" and sometimes isn't felt at all.

A friend made nearly 700 rwhp with a carb and boost referenced Edelbrock 6-valve pump; ran very good but an electric pump and return regulator is a much better plan.

Jody

Thanks for the insight guys. But does anyone know how fast the flow goes down as pressure increases on a diaphragm type pump?

Also does anyone know how the mechanical fuel pumps are rated (pressure or free flow)?

But does anyone know how fast the flow goes down as pressure increases on a diaphragm type pump? Of the very few plots of flow vs pressure I've seen for a mechanical pump, they're more or less linear (somewhat straight line). It would really take a flow test to characterize (with any accuracy) a particular pump.

Also does anyone know how the mechanical fuel pumps are rated (pressure or free flow)? Usually, for marketing, flow is rated at the max flow possible, i.e. free flow or zero pressure. Pressure is usually rated at max presure or zero flow (dead-head). This gives you two points that you could plot and guess at an intermediate operaring point (connect the two with a line). In rare cases, pumps are rated with a specific flow @ specific pressure.

thank you!!