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Weldon Fuel Pump - Let It Flow

Taking The Mystery Out Of EFI Pumps

Bob Mehlhoff Jan 19, 2007

Today's high-tech musclecar is fed with an EFI system that helps create the ultimate performance car. With everyday combinations allowing more power, the ordinary EFI fuel system has proven very flexible. But whether your ride is a fourth-gen Camaro with modified factory equipment or a '65 Chevelle with an aftermarket EFI system, being able to fuel your motor correctly is critical to performance. Among the benefits a good EFI system provides are a big torque curve, easier starting, and improved fuel distribution at part-throttle.

To do this well an EFI system demands proper fuel pressure, adequate fuel supply, sufficient voltage, and free-flowing fuel lines and fittings. This month we're looking at the general requirements unique to an EFI system. Overlook one of these steps when building or maintaining your fuel system and chances are you won't be taking full advantage of your available horsepower.

Pump You Up
It takes fuel to make power, and to move fuel from the tank to the engine requires a pump. A fuel pump does not produce fuel pressure but produces fuel flow. Consequently, as a fuel system's pressure goes up, the fuel pump's volume will go down. On a modern EFI system, a bypass regulator-correctly matched for a given flow volume-establishes a controlled restriction.

Before selecting a fuel pump you'll need to know how much power you'll be making and the amount of fuel required to support it. This is best done on a dyno, but if this is not possible, make an estimate on the high side. Don't forget to add horsepower to your figure for supercharged applications. You'll also need to know the available voltage at the pump under engine load and the pump's published flow volume at that voltage. Most electric fuel pump manufacturers have this information.

Remember that the fuel pump must be wired correctly to achieve optimal perfor-mance. This means using heavy-gauge wire (typically 10-gauge), a relay, and a good ground. Yes, higher voltage at the pump terminals increases motor torque, resulting in increased fuel-pump flow volume. In testing, an electric fuel pump at 80 psi produced a 40-percent increase in volume when voltage increased from 12 to 13.5 volts. Wire a high-dollar electric pump incorrectly, and the pump's performance will suffer due to insufficient voltage.

There are two elements critical to an electric fuel pump's ability to support flywheel horsepower. The first is the pressure at which the fuel pump is required to deliver fuel volume. Second is the horsepower consumed by the engine accessories ahead of the flywheel. Higher fuel pressures created by boost-reference fuel systems, typically found on forced-induction EFI engines, load and slow down electric fuel pumps, which reduces available fuel pump volume.

Keep it Regulated
Unlike a typical carburetor regulator, which controls pressure between the regulator and the carburetor, on an EFI system a bypass regulator controls pressure between the bypass regulator and the pump. This means the bypass regulator should be placed after the fuel rail(s) whenever possible. If your system is a dual-rail, be sure to employ a Y-block to split the supply line before entering the rails and run individual lines from the opposite end of each rail to each inlet port on the regulator. A setup like this serves to provide constant fuel pressure and flow volume through the fuel rail and to the injector inlet.

Sump Your Tank or Go Cellular
If you'll be making no more than 500 horses, chances are your stock tank may work well if the rest of your fuel system is properly designed and maintained. Keeping your stock tank allows you to have a larger capacity than most fuel cells, a standardized mounting position, an easy-to-reach cap, and a well thought-out ventilation system.

If you plan to make over 500 horses, though, there are several reasons you should consider installing a correctly engineered sump and the appropriate fittings or completely replacing your tank with a racing fuel cell. Remember, a stock pick-up assembly-even when used with a high-performancein-tank pump-may become too restrictive for high-horse-power applications. Some builders remedy this situation by replacing the in-tank fuel pump/pickup assembly with a larger fabricated unit, which feeds an externally mounted, high-flow fuel pump.If this is done be sure to keep the tank more than half full during hard driving. Common problems associated with using fabricated pick-up assemblies in stock (unmodified) fuel tanks are pump cavitation, vapor lock, varying fuel pressure, exaggerated pump wear, and lean conditions during both low and high loads. Additionally, many stock tanks do not have the proper internal baffling or fittings to correctly install adequately sized return lines.

With a good sump correctly installed into a stock tank, the restriction of fuel flow through the sump box and to the actual pick-up point is drastically reduced. Thisis because a sump offers a much larger reservoir that holds enough fuel to support lots of power (with some sumps over 1,000 hp) at wide open throttle and ongoing inertia forces in many directions. This can be accomplished while holding fuel at the pickup during cornering and braking maneuvers, with as little as a quarter of a tank.

Fit to Flow
Fuel lines should have an easy-to-follow, unrestricted path on both the feed line and the return line. Depending on the application, you'll want to use at least AN-8 line (11/42-inch id), and for engines with more power than 600 horses consider AN-10. When making your feed and return line remember the return line should be no smaller than the feed line, so the bypassed fuel has an easy return to the tank. All feed and return lines should have smooth paths without hard 90-degree turns.

At low speed the regulator and return line combined must flow almost all of the fuel pump's volume. If not, the resulting fuel pressure is considered a false-high. This means pressure is out of the regulator's control and will drop to the actual regulator set point as the engine goes under load (WOT). A good test for correct regulator and return-line selection and performance is to check if the pressure will adjust at least 5 psi below the desired base pressure, with the vacuum line disconnected.

10 Things Not To Do With An Efi Fuel System
We talked to Weldon Pump and got the lowdown on some of the more common mistakes builders make. Read it, learn it, and take advantage of the following tips:
1. Inadequate fuel tank (minimum 5 gallons for drag, 15 gallons for street/strip)
2. Improper fuel tank vent (or lack thereof)
3. Incorrectly sized feed and supply fuel lines
4. Excessive use of 90-degree fittings (cast-type or sharp-machined) on inlet or outlet of fuel pump
5. Too small a fuel filter (micron rating and dimensional size) on the inlet or outlet of fuel pump
6. Improper electrical grounds and connections-lack of an electrical relay to power the fuel pump
7. Bypass regulator in wrong location (or plumbed incorrectly)
8. Restrictive bypass (return) line
9. Incorrectly sized fuel injectors
10. Incorrectly sized fuel pump (bigger not always better)


Barry Grant Inc.
Dahlonega, GA 30533



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