The most common way of figuring fuel volume needed from a fuel pump is in this manner. A typical engine requires 0.5 pounds of fuel per horsepower, per hour. A gallon of fuel weighs about 6 pounds, so an engine producing 300 hp will require 25 gph of fuel delivered at 6 pounds of fuel pressure. (300 hp x 0.5 pound = 150 pounds of fuel; 150 pounds 6 pounds = 25 gallons of fuel per hour.)

VaporLock and Return-Style Fuel Systems Vaporlock is created when fuel in the fuel lines, fuel pump, or the carburetor fuel bowl overheats and then boils. When this happens, the flow of fuel to the engine will cease. If you have a vaporlock problem, about all you can do is open the hood and allow the heat to dissipate so the fuel can again be supplied to the engine.

The best way to prevent this problem is to try to route all the fuel lines away from heat sources such as radiator hoses, transmission cooler lines, and the exhaust system. Most engines have their mechanical fuel pump mounted in a high heat area, at the front of the engine, very near the radiator hose and right where the radiator fan is blowing the hot air from the radiator. A partial cure could be to use insulation or place shielding around the fuel pump to keep the heat away. A mechanical fuel pump has to suck the fuel from the fuel tank in the rear of the vehicle and pump it up to the carburetor under pressure. If the fuel in the lines or fuel pump gets too hot, the fuel will boil, causing the vaporlock problem.

A way to keep the fuel cool is by using an electric fuel pump mounted near the fuel tank that will pump the fuel through lines routed away from any heat sources to a bypass fuel pressure regulator mounted at the carburetor. The fuel is returned to the fuel tank through fuel lines again routed away from all heat sources. This type of system uses a high-flow, high-pressure electric fuel pump that routes the fuel up to the carburetor and fuel pressure regulator, at which time the pressure is reduced to the 5.5-6.5 pounds of pressure needed by the carburetor. The unused fuel is returned to the fuel tank. However, many racing sanctions do not allow electric fuel pumps.

The use of a high-pressure/high-volume mechanical fuel pump can also work well with a bypass/return fuel system as long as the mechanical fuel pump is not mounted in an area that has too much heat. If the mechanical fuel pump is in a high heat area, the use of an electric fuel pump mounted in a cooler area near the fuel tank may be necessary.

A bypass/return fuel system allows the cooler fuel from the fuel tank to keep flowing through the fuel lines, just like a factory fuel injection system. This avoids fuel boiling due to the fuel sitting in the fuel line in the hot engine compartment, as a standard non-return fuel system tends to do. The best fuel line routing keeps the fuel lines away from heat sources such as the radiator hoses and exhaust heat.

Fuel Lines
Using proper fuel line size and low restriction fittings is a very important part of proper fuel system design. The minimum recommendation for fuel lines is 31/48 inch. You should avoid 90-degree fittings because they restrict fuel flow. If the fuel lines are too small, the engine may starve for fuel. If a high-performance electric fuel pump is used, the pump can be damaged if the fuel pump supply system allows the fuel pump to starve for fuel. If the fuel lines are too big, you may lose fuel pressure during hard acceleration.