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Fuel System - How To

This story has all the math and goodies for your next fuel-pump install so you'll do it once and get it right.

By Barry Grant Fuel Systems, Photography by

If you have been following the CC/Rambler project, you might have noticed that we've made 480 hp and run 12.06 at 112 mph using a completely original fuel system. It has the crappy stock 31/48-inch pickup in the tank with original fuel lines and a Jeep Cherokee mechanical fuel pump. It works just fine, so we are going to fix it.

The next huge step for the car has always been nitrous oxide. We geared it high so we could add 1,000 rpm in the traps and bench-built the car with 150-, 250-, and 500-shots so we could visualize the schrapnel bouncing out of the 904 transmission. Good times. We also have a gnarly Tim Hogan intake manifold that we've been trying to install on the car in between differential explosions, paint jobs, transmission deaths, and drinking beer on the couch on Sundays instead of working on the car.

Before we can plumb in the happy gas and the corresponding timers and such, we have to rig up a fuel system that will both feed the existing engine and be upgradable as we put more power to the ground. The fuel systems we are going to address in this article are for carbureted cars, because that's what we think most of you guys are using. We are also going to talk a little theory so you can apply what we've discovered here to your street machine. Check it.

Fuel PumpsThe very first thing you are going to need to know is how fast you want to go. If you don't care about going fast, you need to know how much horsepower you're making or want to make. If you don't have access to an engine dyno, you can rely on Desktop Dyno from Comp Cams or a similar program to get into the ballpark. We have, and it works.

The 370-inch V-8 in the Rambler makes 480 hp at 6,000 rpm, so we can fit that number into a handy formula, 2 (flywheel hp x brake-specific fuel consumption (BSFC) / 6) = gph, to calculate the needs of the engine in gallons per hour (gph). This formula is an industry standard that takes into consideration g-loads and friction losses in the lines.

What is BSFC? A gasoline-powered, naturally aspirated engine uses roughly 0.5 pounds of fuel to make 1 hp for 1 hour at WOT. The calculation is

BSFC = fuel lb/hr
  uncorrected brake hp

Obviously you are going to need an engine dyno with a fuel-flow meter to get both parts of the equation, or you can guess using a rule of thumb developed from thousands and thousands of previous dyno runs. For the purpose of selecting a fuel pump, the BSFC range varies from 0.38 for an extremely efficient racing engine with great cylinder heads to 0.65 for an engine with a turbo or blower. If you take the average of these two numbers, you get 0.515, which is why everyone uses 0.5 for this calculation when a dyno sheet isn't available.

We used the formula and ended up with 80 gph as the requirement for a fuel pump: 2 (480 flywheel hp x 0.5 BSFC / 6) = 80. We checked the lineup of mechanical fuel pumps from sources like Holley and found that the pump with an advertised rating of 80 gph actually flowed slightly less than 40 gph at 6,000 rpm and 4.5 psi. This is because the advertised number is based on zero-pressure free flow and, as Sesame Street taught us years ago, as pressure increases, flow decreases. Considering that a high-performance engine with a carburetor requires 6-8 psi, this is clearly not enough pump. Instead, we looked to Holley part number 12-327-13. That pump is rated at 130 gph free flow and pumps 85 gph at 4.5 psi and 6,000 rpm. We know this because these pumps are accompanied by a flow chart in the Holley catalog. Using the chart, you can see how much fuel the pump is actually capable of pumping at a certain pressure and rpm.

By Barry Grant Fuel Systems
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