The FMU, also know as a “boost dependent fuel pressure regulator,” only increases fuel rail pressure when boost is applied to the reference port on the FMU. This regulator is in the return fuel line and is downstream of the static fuel pressure regulator. The FMU is a simple mechanical device that can be calibrated by changing the internal ring and spacer. Inside an FMU is a piston. The boost pressure comes from the manifold to a fitting on the FMU and applies pressure to a washer sitting on the piston. The larger the washer, the more pressure it applies on the piston. The piston pressure blocks the flow of fuel down the return line. This backup creates a higher line pressure because the fuel cannot freely pass through.
There are two key ingredients to making horsepower: fuel and air. We are going to discuss the most popular methods of increasing the quantity of fuel, to support the air entering the motor under boost, and its relationship to the amount of power you are trying to make.
3 Steps to Delivering More Fuel
Traditionally, there are three most common ways to deliver more fuel to your engine. They are:
- Upgrade Your Injector Scenario
- Utilize the Power of the FMU w/ Existing Injectors
- Use a Computer Programmer to Regulate Fuel Management With Upgraded Injectors
Upgrade Your Injector Scenario
To get more fuel, you can run larger injectors, increase the pressure to the injectors you already have, or add an auxiliary set of injectors. The auxiliary set of injectors usually squirts fuel into the manifold and requires a secondary injector driver to tell the injectors when to fire and for how long. This method is effective for street cars because it lets the car run like normal with smaller injectors. It is also good for cruising because it prevents the motor from overloading with fuel and stumbling. It then allows the second set of larger injectors to give more fuel when you are trying to make power. The major downside is that this method is very expensive because of the additional components required. Furthermore, it can be very difficult to tune because of the wide adjustment range of a completely separate set of injectors. This common dilemma led to the eventual creation of the FMU.
Utilize the Power of the FMU
The FMU is great because it allows the car to run normally on a small injector, but can also increase the rail pressure under boost which, in turn, forces more fuel through the same size orifice. The fuel that the FMU adds has a direct relationship to the boost pressure. The proportionality is usually stated in a ratio, for example 12:1. This means that the FMU will add 12psi of fuel for every psi of boost. (For example, 10psi of boost will add 120psi of fuel pressure.) When all is said and done, this could net a total of about 140psi of fuel pressure which is often too much for a little injector to handle. It is also the reason most people opt for a combination of larger injector and lower ratio of FMU. This is an ideal setup because it allows the same quantity of fuel but at a lower pressure which is more constant for tuning and less fatiguing on the injectors.
Computer Programs and Fuel Management
The third most popular method of increasing fuel is to add larger injectors and use a computer chip to calibrate and control them. This often can cause the car to run great under boost. However, it is often harder to mange when the car is a daily driver or when cold started. Even this method only can supply enough fuel to support a given amount of pressure. Eventually it, too, requires an injector so large that it would not be suitable for any type of street driving – just racing.
This is why the FMU has become so popular. It offers great versatility for street and strip use. You get the ability to support horsepower and still have the street ability of a daily driver. It is also mechanical and not very complex, so there is little chance of having any reliability issues. The final attribute of an FMU, that has make it popular, is its ability to be easily recalibrated (for a relatively low cost) to match the injector choices you make.