What is an FMU?
Horsepower is a result of two key components: air and fuel. The supercharger itself, whether a centrifugal, roots, or twin screw, really only provides one of the two major ingredients for making more power. Each supercharger kit is a complete system that increases both air and fuel flow into the engine. A supplemental fuel system upgrade must complete the package.
The FMU Explained
There are several methods used by various supercharger kit manufacturers to deliver supplemental fuel to the engine under boost. An FMU, or “Fuel Management Unit”, is the chief component used for one of these methods. An FMU is often referred to as a boost-dependent fuel pressure regulator. The FMU is essentially a variable fuel-pressure regulator that automatically raises fuel pressure as boost rises.
Depending on the capabilities of the stock fuel pump, a booster pump may be used in conjunction with the FMU. The FMU is downstream (after) of the stock regulator. As boost pressure begins to rise, the FMU starts restricting the flow of fuel returning to the gas tank. Like a garden hose, if the flow is restricted, the pressure increases. The increase in restriction results in an increase in the pressure of the fuel being delivered to the factory fuel injectors. Higher fuel rail pressure enables the fuel injectors to deliver more fuel in the same amount of time than they do at the static stock fuel pressure.
The FMU is calibrated precisely for each supercharger system – a rise in fuel pressure equals a directly proportional rise in boost. The ingenious simplicity of the system means that no computer recalibration is required. Without the FMU, the stock fuel system would not be able to maintain an air-to-fuel ratio low enough to prevent a lean condition. FMU-based systems are the most popular with supercharger kit manufactures.
Other Types of Supplemental Fuel Systems Used With Supercharger Kits
Some supercharger kits take a different approach to supplemental fuel supply. One of these alternate methods, to an FMU-based approach, uses an auxiliary EFI computer. This computer is connected to one or more separate fuel injector(s) installed just before the intake manifold. The auxiliary injector(s) work like a TBI to provide additional fuel to all cylinders. These systems do not require an increase in fuel pressure over stock and, therefore, the fuel flowing through the factory injectors is not increased.
On most supercharger systems, booster pumps are not needed unless the supercharger kit manufacture determines (through testing) that the stock fuel pump is not able to provide enough volume to supply both the factory and auxiliary injectors. These kits do not require recalibration of the factory computer.
The most effective way of compensating for the additional fuel required under boost is to replace all of the factory fuel injectors with higher-flowing ones. This method requires recalibration, or replacement, of the factory computer with a new fuel map appropriate for the new injectors. Replacing all of the fuel injectors is expensive and labor intensive, thus making this fuel system upgrade the least popular among supercharger kit manufacturers.
It should also be noted that some engines are designed with proprietary fuel injection that makes swapping out injectors impossible. Just like the others, supercharger kits getting this fuel system treatment may require a booster pump or replacement of the stock pump depending on the application.
Conclusion
So, that’s the bare-bones of an FMU. In the next installment, we’ll get into more the more detailed and technical aspect of this darling of the Fuel Management program – the FMU.
FMU’s that are available:
For specific information about different FMU’s, start with our FMU page and select an application.