Romance is making 700 horsepower on the dyno. Reality is making all that power work for you in the car. The better the car accelerates, the more g-forces are applied to the vehicle, which you feel as an invisible force shoving you back in the seat. One of the effects of acceleration is additional demands on the fuel-delivery system.
Perhaps the greatest misconception that exists in fuel-delivery systems is in the relationship between pressure and volume. To state it simply, pressure is inversely proportional to volume. In other words, as fuel pressure increases, the volume of fuel decreases. We have included a fuel-pressure chart that shows the effect of pressure on the amount of fuel delivered to help illustrate this point. If a fuel pump did not have to fight the forces of acceleration in a vehicle, fuel pressure could be as little as 1 to 2 psi to push a sufficient volume of fuel to the carburetor. But even street cars can muster sufficient power and traction to propel a 3500-pound car from a standing start at 1.4g acceleration, gradually dropping off to perhaps .70g in the first 60 feet. The same acceleration force that pushes you back in the seat is also pushing against the fuel in the line that the fuel pump is attempting to push (or sometimes pull) forward from the rear-mounted tank.
According to SuperFlow’s Harold Bettes, a rough rule of thumb is that the fuel-delivery system requires 8 psi of system pressure for every g of acceleration to maintain fuel flow. You can therefore see that, given a low-pressure 8-psi fuel system pushing against a 1g launch, there could easily be no fuel flow in the system during the time the car is experiencing that g force. The engine is forced to pull fuel only from the float bowl, dropping the float level and leaning the air/fuel ratio. HOT ROD has data-logged a 12.5-second carbureted street car that experienced this situation, producing wildly fluctuating air/fuel ratios from 10:1 to 16:1, all within the first 60 feet at the dragstrip.
For example, Mark Tate’s Fastest Street Car winner pulled consistent 1.28 60-foot times, which equates to pulling at least 2.25 g’s within that distance while maintaining probably no less than 1g of acceleration through the first 100 feet. Given that situation, the car must have at least an 18- to 20-psi system with sufficient volume to handle the incredible fuel-delivery demands. Most knowledgeable car builders take advantage of pressurized flow by placing the fuel pump at the rear of the car. Because pumps are far more efficient at pushing fuel than pulling, mounting the pump in the rear makes optimal use of the pressure pushing the fuel up to the front of the car. Fuel-line material, size, connections and routing also play a role in this fuel-delivery equation.
Obviously, a larger line has the potential to flow more fuel than a smaller line. Rubber line has a far greater resistance to flow than aluminum or steel and can easily be measured as a pressure loss over a known distance. Pressure losses also occur at fuel-line connections, especially in 90-degree fittings. To prevent that, the fuel-delivery system should be designed to limit the number of 90-degree fuel fittings. Ideally, there should be no 90-degree fittings, but if you must use them, use full-flow tube-style AN connections.
For street-driven cars, look for a pump that’s designed for continuous use. Many of the ultrahigh-output pumps are intended for drag racing only and are not designed for long-duration usage. The 45- to 80-psi high-pressure pumps designed for EFI applications can also be used for carbureted cars, but the system must be fitted with a return line to prevent the fuel from being heated under low-fuel-demand situations.
Regulators and fuel filters are also an integral part of the fuel-delivery system. With the advent of the latest high-pressure and high-volume pumps, all of the manufacturers now require that a high-volume filter be placed between the pump and the pickup to prevent debris from damaging the pump. No matter how many filters you employ, be sure to match the volume of the pump with the filter. It does little good to purchase a high-volume pump and then strangle it with a cheap, restrictive filter.
Pressure regulators must also be matched components that should be chosen carefully. Typical mechanical fuel pumps operate at a maximum pressure of 4 to 6 psi and usually don’t require a regulator. But for higher-demand systems, the regulator limits the pressure just ahead of the carburetor or injector at the appropriate pressure. In carbureted applications, 6 to 8 psi is the maximum pressure that the needle and seat can accurately control. Pressure in excess of those levels overpowers the needle and seat and floods the float bowl with fuel.
Most high-performance street applications work well with 10- to 15-psi pumps pushing fuel to one or more regulators that then cut the pressure to 6 to 8 psi. Of course, that assumes that the regulators are capable of flowing the fuel. The original Holley blue pressure regulator, for example, does a fine job, but it’s volume limited and should not be used alone in conjunction with more than one carburetor.
While stock mechanical pumps have been doing a decent job for years, they are hardly optimal—especially for any application above 400 horsepower. The quick solution is to plumb a low-pressure electric fuel pump at the rear of the car near the fuel tank. It will more efficiently push the fuel to the engine-driven pump. Of course, properly sized fuel line, limiting the number of 90-degree fittings and using a high-capacity fuel filter will all assist fuel flow.
Designing a high-performance fuel system isn’t difficult, but it does require attention to detail and a little forethought.
