Pump Up The Volume Part 1 -- Technical Article -- Chevy High Performance Magazine

Building The Proper Fuel System For Carbureted Applications

By Bob Mehlhoff

Photography by Bob Mehlhoff, Courtesy of the Manufacturers

Today's high-performance engines are able to deliver horsepower numbers unheard of 10 years ago. With better-flowing cylinder heads, ignition systems, and camshaft profiles, the potential for huge power is almost endless. Combine this with the strides in chassis development and you have a car that not only makes lots of power, but can get it to the ground with higher g-forces. This month we'll take a hard look at selecting proper fuel pumps, filters, lines, and system components for a carbureted system and follow up it next month with similar principles for fuel-injected appli-cations. Now, whether your forte is drag racing or road racing, a good supply of fuel fed to your engine at the right pressure (under all driving circumstances) is critical to achieving the ultimate performance.

Choose the Right Pump First
When choosing a pump, you'll first need to determine how much power your engine is developing (under full power) at the flywheel and how much fuel will be required to support it. If you don't have actual dyno results, you can estimate your power. For most carbureted systems on street engines developing under 450 horses, an off-the-shelf performance mechanical fuel pump installed to a properly maintained fuel system should work fine. Get into higher horsepower numbers, or add nitrous or a supercharger to your engine, and you'll need a properly installed, significantly stronger electric pump.

You'll also need to know your engine's fuel efficiency, commonly referred to as BSFC (Brake Specific Fuel Consumption), the maximum fuel system pressure, and the pump's flow volume at that pressure. Lastly, on electric pumps you'll need to know the available voltage at the pump under engine load and the pump's flow volume at that voltage. This is very critical; wire a high-dollar electric pump incorrectly, and the pump's performance will suffer and the engine could run lean.

In the absence of actual dyno information, you'll need to first determine how much horsepower will be produced and the amount of fuel required to support it by estimating engine horsepower on both the high side and low side of BSFC. Most gasoline engines use less than 1 pound of fuel to make 1 hp for 1 hour, so expect the BSFC number to be less than 1. Different performance combinations, power-adders, and even fuel-octane ratings and engine tuners will have a great impact on BSFC. In the world of electric fuel pumps, many are flow-rated at zero pressure. Be careful here because your system works under pressure, and pressure will drastically affect how much fuel flow your pump delivers. When shopping for a fuel pump, be sure to look for fuel-flow ratings at the pressure you need.To estimate your engine's BSFC, use the following guidelines. It's important to note that the best method of establishing actual BSFC is through proper flywheel dyno-testing.

* Naturally aspirated engines are generally most efficient with a BSFC between .45 and .55 lbs/hp/hr.
* Nitrous combinations typically use a little extra fuel and often develop a BSFC from .5 to .6 lbs/hp/hr.
* Forced induction engines are often least efficient, and BSFC ranges from .6 to .75 lbs/hp/hr.

Using 500 hp as an example, we'll show the fuel requirement for two engine efficiency combinations:

(HP x BSFC = pounds of gasoline)
500 hp x .5 BSFC = 250 pounds of gasoline.
500 hp x .75 BSFC = 375 pounds of gasoline.

Since a gallon of fuel weighs about 6.2 pounds, we find that in our first example, 250 / 6.2 = 40 gph (gallons per hour) and 375 / 6.2 = 60 gph.

Fuel Pressure & Volume
The relationship between fuel pressure and volume is inversely proportional. As fuel system pressure rises, the pump's volume decreases. In addition, the force of a car moving forward (g-forces) and the friction the fuel encounters in the system thorough lines and fittings may also impede the delivery at the carburetor. In order to supply fuel in the most efficient way, you'll want to use adequately sized lines (typically 1/2-inch id) and fittings without sharp bends. As a rule, a 90-degree fitting is equal to adding several more feet of line into the system (about 10 extra feet of line). This means that it's better to use two 45-degree fittings to negotiate a turn instead of one 90-degree fitting. When routing your fuel line, be sure to keep the line away from exhaust components and moving suspension parts, and never route a fuel line through the interior of your car.

Remember that the fuel your engine needs has to be delivered past the fuel pressure regulator and the needles and seats, and the fuel delivery system competes against g-force and friction in the system. Also, make sure your fuel pump is mounted where gravity will help draw fuel to the inlet. Ideally this is at the rear, lower side of the tank. If this isn't possible, at bare minimum you'll have need to mount the pump near the tank, since electric fuel pumps are better pushers than pullers.

Measure at the Fuel Pump Terminals
Simply having power connected to an electric pump and being able to turn it on does not guarantee the pump receives the correct amount of voltage. Wire size, wire unions, and a proper ground are all equal and critical elements of a properly operating fuel pump. Most electric fuel pumps are designed to operate at 13.5-14.2 volts DC. To illustrate this, testing has shown that an A-1000 Aeromotive fuel pump at 80 psi will see a 40-percent increase in volume when voltage is raised from 12 volts to 13.5 volts. A good fuel pump electrical system will use a 10-gauge wire (sourced from the alternator charging stud) and routed to a 30- to 40-amp relay near the pump. Equally important is a good ground system wired to the frame and onto the engine block. A secondary ground--again, properly installed--to the underbody is also a good idea. To check your fuel pump's voltage, use a multimeter, and with the engine and pump running, measure the voltage output at the alternator. Next measure the voltage at the pump. If you find more than a .5-volt drop, inspect your wire size, connections, and ground system. If you do run across wiring issues, best to rewire the entire system.

Think of the voltage to an electric motor as fuel pressure to an injector; more pressure in equals greater volume out. Higher voltage at the fuel pump terminals increases motor torque, resulting in an increased rpm and flow volume for a given pressure. Again, you may have the best and most expensive fuel pump available on the market, but if it isn't properly wired and installed, it'll reduce your fuel system's performance.

Setting Fuel Pressure
Your system's fuel pressure should be generally set between 6 and 8 psi (measured at the carburetor) for a street engine (higher for a race engine). Keep in mind that fuel pressure is not fuel volume. Your pressure reading is only an indication of the level of restriction in the system.

Choose & Position the Right Filters
Fuel filter type and placement are critical to achieving the proper fuel pressure and volume being delivered to the carburetor. A high-flow, fine-element fuel filter should be used between the fuel pump and carburetor on the pressure side, not between the tank and the pump on the suction side. Between the tank and the pump you'll want to run a coarse-filter screen no finer than 100-micron. This is because as a pump pushes, it also has to pull, and when a pump has to pull too hard to acquire fuel through a restrictive filter, a vacuum or low-pressure area develops at the inlet. To be sure of the specs for your fuel filter, always check with the fuel pump's manufacturer for a recommendation. A more restrictive filter on the suction side of the pump may fail to flow the full volume of the pump, which can result in cavitation at the pump inlet.