One of the most ignored areas of building and maintaining a race car is the fuel system. The fuel tank, filters, vent system, and fuel line size all must be designed to work together to supply the carburetor or fuel injectors with the correct fuel volume and pressure for the engine demands.

Fuel Pressure The fuel pump must be able to supply the proper fuel pressure and enough volume to the carburetor in order to supply the correct air/fuel mixture for the engine's demands. Based on my experience, the fuel pressure needed for a race vehicle equipped with a carbureted engine is 5.5 to 6.5 pounds under all load conditions.

Always measure the fuel pressure at the carburetor inlet. If the fuel pressure goes lower, the fuel level in the carburetor's float bowl will drop too low, causing the air/fuel mixture to go lean. The lean fuel mixture can cause engine damage. If the fuel pressure increases, the float, needle, and seat will not be able to properly control the float level. If the fuel level in the carburetor's float bowl goes too high, the air/fuel mixture will go too rich and cause the engine to miss and foul the spark plugs.

We have done tests by using an exhaust gas analyzer for detecting changes in air/fuel mixture that occur as the fuel pressure changes. When the fuel pressure goes beyond 7 pounds, we can observe the fuel mixture going rich as the vehicle is driven over the normal imperfections that are in almost any track. When the fuel pressure drops below 4 pounds, we can observe the air/fuel mixture shifting lean, especially when we are driving at wide-open throttle/high-load conditions.

Fuel Pumps The two most common methods of supplying fuel to a carburetor are with a remote electric fuel pump or a mechanical fuel pump mounted on the engine. A mechanical pump can do a good job, but if there is a problem of space or heat in the engine compartment, an electric pump mounted away from the heat near the fuel tank may work better. No matter what type of fuel pump is used, it must be able to supply enough volume of fuel to keep the pressure at 5.5-6.5 pounds at all driving conditions. A high-volume or race-electric fuel pump must have some kind of bypass/return fuel pressure control system that keeps the fuel flowing through the fuel pump in order to cool it, or the fuel pump will fail. Many electric fuel pumps have failed due to the pump overheating because a standard non-return fuel pressure regulator was used. A standard pressure/volume electric fuel pump may not overheat, but it may not supply enough fuel volume/pressure for the engine's needs at full throttle.

Both mechanical and electric high-performance fuel pumps are rated for flow of a certain number of gallons per hour of free flow (zero pressure). The flow of that same fuel pump will be less when the fuel pump is maintaining proper fuel pressure. The important thing is that the fuel pump can deliver enough fuel to the carburetor while maintaining proper fuel pressure.

One of the performance aftermarket electric fuel pumps is rated at 97 gph free flow (zero pressure), but it only flows about 25 gph at 6 pounds of fuel pressure (this is about enough fuel for a 300hp engine).