I’m considering buying one of those self-learning TBI fuel injection systems. I’ve heard that the fuel delivery system is the most important part of the installation so I’ve been reading up on electric fuel pumps. I’ve noticed that companies rate their pumps at different power levels based on how they will be used. Why does it make a difference whether the engine is EFI or carbureted and why does a supercharger reduce the amount of horsepower the pump can deliver?
Jeff Smith: This is a great question. Let’s say a given pump is rated to deliver 800 horsepower worth of fuel. This is a shorthand way of stating the pump’s capacity. The two most important values are the amount of fuel (often now given in liters per hour- lph) and the pressure. Basically, as the a fuel pump increases pressure, volume will decrease. This is just a basic design result of current fuel pumps. Let’s use that example of a pump rated at 800 horsepower. This is not an accurate evaluation because we don’t know the pressure. That’s why companies will state “800 horsepower for a normally aspirated engine with a carburetor.” What this means is the pump is capable of pumping 800 horsepower worth of fuel at probably around 6 to 8 psi. That same pump may drop its capacity to 600 horsepower when used with fuel injection. That’s because EFI requires anywhere from 43 to 60 psi of fuel pressure.
Remember, as pressure increases, capacity decreases.
These recommendations are still somewhat vague because they don’t generally list the specific pressures. The best thing to do to evaluate a fuel pump is to obtain the manufacturer’s official output rating. Sometimes this is in the form of a graph or specific output numbers on a chart. Another important variable for electric pumps is the operating voltage. The best information the manufacturer can give you is the operating pressure, the volume – usually stated in pounds per hour (lbs/hr), and the voltage the pump was operating on. Some companies will try to fool you with high output numbers at zero pressure. This is really just an advertising gimmick because all fuel delivery systems operate at some given pressure.
So perhaps now you can begin to see how pressure and operating voltage affect a pump’s output. Now let’s introduce yet another variable. Let’s say that you have a big block Chevy with a turbocharger and you are going to use a multi-point EFI system to feed the fuel to the engine. First off, this means the system will run at a higher fuel pressure of at least 43 to 45 psi. But there’s a further complication. As the turbo increases output, the boost in the manifold begins to climb. Let’s use a simple case of 10 psi of boost. With a 45 psi of line pressure, this means the injectors located in the manifold must supply the fuel but work against the boost pressure. This means that the effective fuel pressure drops from 45 psi to 35 because of the 10 psi head pressure in the manifold. The simple solution is to boost-reference the fuel pressure to increase 1:1 with boost pressure. This way the fuel pressure at the injectors remains the same. However, this also means that the fuel pump must now make 55 psi of pressure to deliver 45 psi at the injectors, which means the pump is working harder and its capacity at this higher pressure is reduced. So that’s why any supercharged or turbocharged EFI engine fuel pump application will have a lower horsepower capacity than just a normally aspirated EFI engine.
The important point is to match the pump’s capacity to the application and to actually look at the manufacturer’s flow chart rating to make sure that you are getting a pump that has the capacity you need.