Proper Pump Mounting
Jesse Powell: If you talk to most hot rodders, they’ll probably tell you that big external pumps can’t be driven on the street for extended periods of time without vaporlocking because they run too hot. Here’s what’s actually going on. When you buy an Aeromotive pump like an A1000, you get a very detailed set of instructions. They state that the pump must be mounted below the tank or equal to the lowest level of the tank. They also state that the tank must be sumped, and to run an AN-10 feed line from the tank to the pump. If using a pre-pump filter, it is recommended that you use nothing smaller than 100-micron stainless steel. The reason for this is because fuel is no different than any other liquid. Think about your radiator and cooling system. It is under pressure because that pressure raises the boiling point of the coolant. The opposite is true if you put a liquid in a negative pressure environment or a vacuum. The boiling point is actually lowered. A fuel pump obviously pushes fuel to the engine at a desired pressure, but it also sucks fuel to itself. This suction creates a negative pressure environment of vacuum. So, when you use a pickup tube instead of a sump, the pump is no longer gravity fed. The area from the pickup tube to the inlet of a pump is in a vacuum and therefore it’s a place where the fuel can boil quickly. In most fuel system in most cars, fuel is cycled through the system. It goes from the tank to the engine and then is returned. With a pump like an A1000, the entire content of the tank is cycled every four to five minutes. Each time it cycles, the fuel picks up heat from the engine bay, the exhaust and the pavement getting hotter and hotter. Eventually the temperature of the fuel reaches a critical point in which it will actually boil in the pickup tube because of the negative pressure and artificially lowered boiling point. What then starts to happen is that the pump begins to cavitate because it is pulling the fuel to a vapor instead of pulling a liquid. The pump relies on the liquid fuel to keep it cool and lubricated. Eventually the pump or engine will quit because it cannot get fuel. If run dry long enough like this, the pump will vaporlock.
Avoiding these issues is very simple. If you are going to use an external fuel pump, make sure you follow the manufacturer’s recommendations carefully. In our case, it starts at the tank. You need to gravity feed your pump. A properly designed sump creates a new low point in the tank, ensuring fuel remains in that sump at all times, even until the last bit of the tank is emptied. Then, mount your fuel pump lower than the tank or level with that sump. That will keep the pump gravity fed or keep positive pressure on the pump inlet. Once again, remember that positive pressure raises the boiling point, so this set up will be much more forgiving. Also, make sure you use the right line size and a proper filter.