Fuel Pumps and the Fuel Supply

High pressure fuel pump

High-pressure roller-type fuel pump

A high-pressure roller-type electronic fuel pump is required to supply the fuel and the fuel pressure to the fuel rail and the injectors. The fuel pump must be large enough to supply more fuel than the maximum amount that the engine may require to ensure that the fuel pressure remains adequate at full throttle and at maximum RPM. Remember that when you alter the fuel pressure regulator or use a risings rate fuel pressure regulator to increase the fuel pressure in your EFI system, you also need to increase the fuel flow rate or fuel flow volume of your fuel pump as it has to pump against higher pressures. When the pressure increases, the fuel pump operates against more resistance and hence it operates at a slower pace. However, make sure that you don't increase the fuel flow rate too much or else the fuel will be traveling to the fuel rail and back to the tank too often and will pick up heat. Heat has a detrimental effect on the octane rating of the fuel! Therefore you should not exceed the required fuel flow rate by more than 33%. You can use the formula hp × 7.3 to find the required fuel flow rate in cubic centimeters (cc) per minute that will give you a 33% over supply of fuel.

Increase the Fuel Flow Rate

You can increase the fuel flow rate by installing a larger fuel pump or by installing a second fuel pump. Installing a second fuel pump is the best solution for a modified street car as it allows you to wire the second pump separately so that it is only in use when extra performance is required. More often than not you'd be using a street race car for cruising. Under these conditions you would not need a high fuel flow rate, in fact, it would be best not to have a high fuel flow rate under these conditions as it will just increase the amount of fuel that circulates to the fuel rail and back to the tank. This will expose the fuel to heat, which is not good. You can wire the second pump to only operate when it is required by installing a pressure sensitive switch on the intake manifold, or by installing a microswitch on the throttle body.

The two pumps can be installed in parallel with each pump having its own fuel pickup, or in series with one pump pumping into a second pump. When you install two pumps in parallel, you increase the fuel flow rate; but the two pumps must be of equal capability. Here the fuel flow rate will be the sum of the fuel flow rate of the two pumps. When you install tow pumps in series, you increase the fuel pressure rather than the fuel flow rate. This is because the second pump is being force fed by the first pump and fuel pressure is increased between the two pumps. Hence the second fuel pump does not need to increase the fuel pressure by much to meet the fuel pressure set by the fuel pressure regulator. However, the fuel flow rate does not increase by much and will only be approximately 120% of the fuel flow rate of the largest pump.

The Fuel Pickup

The fuel pickup should also get some consideration, especially on a modified street race car with a steel fuel tank. On such vehicles the fuel level in the fuel tank can drop below the fuel pickup during hard cornering or heavy acceleration and heavy braking and can cause a momentary drop in fuel pressure and engine performance. The solution is to have a secondary fuel pump supply a small fuel reservoir or swirl pot that feels the primary fuel pumps. The fuel reservoir is a simple cylinder with an fuel inlet from the secondary pump at the side, a fuel return line at the top and fuel pickup lines to the primary pumps at the bottom. The fuel reservoir should have a capacity of at least 1 liter and should be mounted low, beside the fuel tank. With a fuel reservoir, the primary fuel pumps will always have a fuel supply even under hard cornering of heavy acceleration and braking conditions.