Aeromotive DFMU

From Nissan 350Z & 370Z Wiki

The Aeromotive DFMU is supplied with the ATI Procharger (regular kit) but it can also be purchased separately and used with other FI systems. Here is an e-mail from the Aeromotive Tech Department, that explains exactly how the Aeromotive DFMU works (based on the ATI Procharger setup).

General FMU and DFMU Discussion

Posted by jesseenglish, who contacted Aeromotive to discuss their DFMU which comes in the ATI Procharger Kit.

We spoke by phone regarding the Aeromotive Digital FMU (DFMU) P/N 16303, it's function and application. You requested that I answer those questions in written form in order that you could share the information with others whom you knew had similar interests.

Your questions were:

1. What are the various adjustments/settings and their purpose?
2. Could there be variation between the scalar set-points from one box to the next, if all were calibrated to the same psi standard?
3. What would be the best way to initially calibrate the DFMU?
4. What would be the best tuning approach for the 'average' user?

Before addressing those aspects, it may be necessary to cover a few basics for the 'average' person before digging too deep into the specifics. In order to best apply the DFMU, it would be necessary to understand it's purpose, which is the same as it's predecessor the FMU. In my experience, few enthusiasts have any idea what an FMU really is, how one works or why it's used to begin with. So, we'll start there.

FMU stands for Fuel Management Unit. It was first created and applied to EFI engines in the early centrifugal blower (and then turbo kit) days, making it possible to mass market bolt on, forced induction kits at affordable prices. Without the FMU, it's very doubtful that today' aftermarket performance industry would thrive as it does now. The alternative, adding larger fuel injectors and recalibrated ECU's or other components, is just to 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, 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 principle 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 of the gear (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. Additional tuning information will be provided below.

The demise of the FMU has been the result of changing fuel system design, where emission control (evaporative emissions created by warming fuel as it flows 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 and regulator. This second 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 the 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 even 15 PSI (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 that boost point.

8 PSI kit would calculate as follows:

1. 8 PSI divided by 5 bands equals 1.6 PSI per band.
2. 1.6 PSI multiplied by 4 bands equals 6.4 PSI.
3. The 5th band should then activate at 6.4 PSI for an 8 PSI kit.

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 a reference. The pots themselves are set on the board beneath in the same fashion for each unit. However, particular or exact alignment from 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 spark plug 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 chamber. Air/Fuel ratios that are too lean for the engine' 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 kit manufacture, 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 setting after sliders have been adjusted will require a recalibration 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.

Any questions about the DFMU or it's use may be directed to me at the phone or e-mail address below. Good luck and thanks for choosing Aeromotive products!

Specific to the ATI Procharger kit

For use with returnless fuel systems only.

The new Aeromotive Billet Digital FMU is a fully adjustable, scalable, electronic fuel pump controller. Used with an external in-line fuel pump, the Digital FMU creates a programmable FMU for use with returnless fuel systems. The Aeromotive FMU senses manifold pressure and automatically controls the external pump, providing the fuel pressure that you dial in. Now you can add forced induction to any returnless fuel system application and maintain the proper air/fuel ratio under all WOT operating conditions.

Referenced to manifold pressure, the Aeromotive Billet Digital FMU controls a supplemental, in-line pump (sold separately), to provide the required fuel flow volume for supercharged engines. Full adjustability allows the tuner to increase fuel rail pressure and injector flow throughout the boost curve.

The unique capability of the digital FMU, unlike traditional, fixed ratio mechanical FMUs, is to permit a custom pressure curve to be developed for each engine application. In other words, proper calibration can create a 2:1 boost to fuel pressure ratio at low boost, going up to 10:1 or more at high boost. Maximum fuel pressure will depend on the in-tank and secondary fuel pump size, along with injector flow rates.

The FMU features 5 independant adjustment bands that are tied to a scalable rotary knob. This allows you to set the boost scale so all 5 bands are usable for any forced induction system ranging from 5-15 PSI. LED indicator lights signal when the FMU is activated and which band it is currently operating on.

The housing of the controller is made out of Billet 6061-T651 aircraft aluminum alloy and is finished with a bright dip red anodize.

How the Aeromotive DFMU Works

The DFMU scale is set by ATI before shipping for a 7 psi boost. The scale controls the range of boost for each slider. Each slider on the DFMU should activate at the following boost levels for the 350Z application.

• First slider covers 0 - 1.3 psi
• Second slider covers 1.4 - 2.7 psi
• Third slider covers 2.8 - 4.1 psi
• Fourth slider covers 4.2 - 5.5 psi
• Fifth slider covers 5.6 - 7.0 psi

You can verify the scale adjustment with a low pressure source and regulator. The 5th slider light should activate at 5.6 psi.

This presumes that you are achieving the full 7 psi boost, so after rough tuning the DFMU, you may want to take a max boost reading at redline. If it doesn't read 7 psi, you'll need to check for leaks or restrictions in the intake plumbing, check your PCV/vacuum lines for leaks, make sure the bypass valve is working properly, and make sure your serpentine belt isn't slipping.

If after checking everything out, you can fine tune the scale by taking the maximum boost you are achieving at redline and divide it by 5, then multiply the result by 4, and that number is the number at which the 5th slider should activate. You can make your scale adjustments with a low pressure source and regulator. For instance, if your max boost is 6 psi, the scale increment becomes 1.2 and the 5th slider should activate at 4.8 psi.

Once the scale is set, you can proceed to fine tuning the air/fuel ratio. Here is where you need to map the A/F with a wideband O2. Your target A/F at the activation of each slider should be about 11.5 (slightly rich) so that it does not exceed 12.5 just before the activation of the next slider. Mine actually holds around 12 from 3,600 rpm to 5,000 rpm, and then a steady 11.6 with a slight upswing to 12 at redline. Remember that these A/F settings are done at WOT (put the pedal all the way to the metal).

Tuning is usually done on a dyno, but make sure your engine doesn't get heat soaked. About three pulls, and then let things cool off for a bit. A heavy duty, industrial fan helps to keep air flow around the engine and aid cooling. If you decide to do the tuning on the road (typically less accurate), you'll need a method to record the rpm and wideband O2 data (boost recording would be good too), and a good, safe, straight track for running at WOT.

I highly recommend a wideband O2 gauge, boost gauge, and fuel pressure gauge as minimum equipment to monitor the health of your system. It's reassuring to know all the time if your system is happy.

An additional piece of safety equipment to consider for any FI installation is the J&S Ultrasafeguard, it's a bit expensive, but a lot cheaper than rebuilding a destroyed engine. Here's a link to their web site:

http://www.jandssafeguard.com/safeguard.html

Some people with the ATI ProCharger have their systems set up with various combinations of larger fuel injectors, the TechnoSquare ECU flash for an FI application, and the Apexi S-AFC II. Check the major 350Z forums for information on these setups.

Do it right, and enjoy all that horsepower safely!