FMU stands for Fuel management Unit. It was first created and applied to EFI engines in the early centrifugal blower and turbo days, making it possible to mass market bolt on, forced induction kits at affordable prices. Without the FMU, it's very doubtful that today's aftermarket performance industry would thrive as it does now. The alternative, adding larger fuel injectors and re calibrated ECU's or other components, can be too complex and expensive for an entry level supercharger or turbo kit.

The FMU's purpose is to raise fuel pressure with boost on a ratio greater than 1:1 in order to make up for a fuel injector that has reached 100% duty cycle, thus preventing the engine from running out of fuel and detonating. By forcing the delta fuel pressure higher (difference between the injector inlet at the rail and the injector outlet at the runner), a small injector can be made to act like a bigger one, to a point.

The original FMU was a pure mechanical device, created to work on the same principal as the fuel pressure regulator, by creating pressure with a restriction of the fuel return line. It has been an effective but coarse tuning tool at best. Due to it's linear response to boost, forcing fuel pressure higher in direct proportion to rising boost pressure, it offers no means to tailor the fuel delivery to the actual injector duty cycle and engine fuel requirements. Normally, a properly adjusted mechanical FMU produces very rich A/F ratios in the midrange (before the injector is actually too small i.e. at 100% duty cycle) in order to be rich enough at the top RPM range (when the injector is definitely too small). Many are those who attempted to adjust the FMU to clean up the midrange, only to find a lean condition and engine damage on top. With the Aeromotive digital FMU, complete tuning of the fuel curve is now possible, however caution must be exercised when adjustments are made or engine damage can result.

The demise of the FMU has been the result of changing fuel system design, where emission control (evaporative emissions created by warming fuel through the fuel rails and back to the tank) has superceded fuel system performance as a priority. With today's "returnless" or "dead-head" fuel systems, there is no place to insert an FMU. This seemingly small thing, the lack of a return line, has created a real barrier to making entry level forced induction available and affordable for the modern day, OBD II, EFI automobile. This is where the Aeromotive "Digital Fuel Management Unit" or "DFMU" comes into the picture.

How the DFMU works can be difficult to grasp but, basically it is a boost sensing fuel pump controller. It runs a second fuel pump, installed after the stock, in-tank fuel pump is plumbed so that during normal, non-boost driving conditions, it is off and fuel from the stock pump simply flows around it, with stock fuel pressure in the rail and stock fuel delivery to the engine. The DFMU control unit has a boost sensing line that connects to an internal pressure transducer, when boost is sensed, the second fuel pump turns on. Because there is no regulator after the second pump, as there is after the first, albeit in the tank, when the second pump starts it forces fuel into the rail at a rate that is determined by how fast the second pump turns. The speed of this pump is determined by the slider settings on the face of the DFMU control box. Positioning this slider at the lowest position runs the pump at the slowest possible speed, building the least amount of additional fuel pressure in the rail. As the slider is moved higher, progressively more fuel pump speed is created, building more pressure in the fuel rail, and delivering more fuel into the cylinder.

Setting up the DFMU for proper engine fueling requires several steps. The first is to establish a correct scale. By this, it is meant that all 5 sliders will be available for tuning within the boost range of the application. For example, some kits will make 5 PSI of boost, some 10 PSI and others 15 PS ( 15 PSI would be pushing the practicality of an FMU or DFMU by the way). With 5 sliders on the box, the ideal scale calibration would involve dividing the total boost expected by 5, then multiplying that by 4, then setting the scalar adjustment so that the 5 slider was activated at the boost point. For example, an 8 PSI kit would calculate as follows:

The scalar adjustment can be made with a reliable, regulated pressure source (CO2 bottle with low pressure regulator works well). The procedure is to apply the calculated pressure from above to the boost reference port. Then, adjust the scalar pot (silver, arrow shaped pot in the lower left corner of the DFMU) so that the light over band 5 just turns on at that pressure. This is the process used by ATI Pro-Charger when the boxes are pre-calibrated to a safe tune at the factory. Note: the scale engraved on the face of the DFMU, around the scalar pot, is not calibrated directly to boost, it is just graduated for reference. The pots themselves are set on the board beneath in the same fashion for each unit. However, particular or exact alignment from one board to the next, in order to ensure that the arrow would point to the exact same scale line, at the exact same pressure on every unit built, is not an assembly criteria. Do not expect that, from one unit to the next, there will be an exact correlation between the position of the arrow and the actual calibration of the scale to a certain PSI of boost.

Once the scalar adjustment has been made, careful tuning can commence. The best approach for the inexperienced tuner (read someone without a wide-band air/fuel meter, a good sparkplug magnifier and the knowledge to interpret both) is to start with the highest possible settings, and work down from there.

Warning: Incorrect adjustment of DFMU scale and individual sliders will result in improper air/fuel ratios in the cylinder combustion chambers. Air/fuel ratios that are too lean for the engines compression ratio, boost level, fuel octane and timing advance will result in engine damage, with possible major component failure. Proceed with caution, make small changes and watch carefully for signs of detonation, before it gets out of hand.

By adjusting all sliders to the top of the scale, the richest possible air/fuel ratio will be created. With an application where no base tune is available from the supercharger or turbo manufacturer, this is the best starting point. The engine should be driven gradually into boost, where the fuel pressure is driven too high at the first slider. Normally, engine acceleration will halt. Gradually bring the slider down until enough additional RPM and boost will light the next slider. Gradually bring this slider down until the same occurs with the next and then the next, until full boost is achieved. At this point, a full rich, acceleration fuel curve is programmed into the DFMU. Note: any changes of the scalar settings after sliders have been adjusted will require a re calibration of all sliders, once the scale is set, further adjustments to it are discouraged unless absolutely necessary.

From here it is advised that professional assistance be acquired if further power (read leaner air/fuel ratios) are desired. The installation of a wide band air/fuel ratio meter, testing on the dyno where careful monitoring of same can occur, along with periodic inspection of ALL spark plugs is conducted. This is the procedure utilized when tuning any high performance racing engine, follow it for best results.

Thanks to Aeromotive Technical Support