Piggyback ECU installation write-up - Page 2 - PeachParts Mercedes-Benz Forum

Blown_M104 · #1 10-27-2005, 01:01 PM

Over the first year of my supercharger install I have looked for better ways to manage fuel control . I’ve tried a Boost proportion fuel pressure regulator and also played around with switching in different resistor values for the Inlet Air Temperature sensor IAT. In the end I was still running lean under boost and leaving a lot of HP on the table due to the wrong A/F ratio. For cost reasons I did not want to go with a new stand alone ECU unit; even most of the Piggyback units out there are rather expensive. After a lot of looking I found the Apexi S-AFC II for just $220 that would work with the stock ECU. (I have the Mercedes 1993 Hot Film Mass Air Flow system on a 3.2L M104).

Basically the Apexi unit wires in series with the Mass Air Flow Sensor input into the stock ECU. Since the ECU uses the output voltage from the MAFS as one of the primary factors in determining injector firing time this is an ideal lever to increase fuel flow. Apexi gives you the ability to multiply the signal from the MAFS at 12 different RPM points by +/- 50% . It has two different multiplier maps that are switched between depending on throttle position. I’ve got map #1 set to all zeros for stock driving, but above 33% throttle open I switch in MAP #2. The numbers for this second Map are based on my math calculations for how much more fuel I need at different levels of boost. It’s also worth noting that I’m using the stock Bosch 0280 155 209 injectors which are rated at 3199cc. My math says these injectors are good for around 325 HP at 6 psi of boost.

After driving the car for a few days I’ve realized I never should have done the blower installation without the Piggyback… the increase in performance is so much greater with the A/F ratio corrected. Also for a normally aspirated car this might be the trick for you; you could change you’re A/F ratio down from the stock 14.7 to a richer mix under heavy acceleration.

The Apexi wires in very easily, one connection to 12V (off the over voltage relay) , ground connection , rpm input wire (I tapped into the Cam position sensor), and then two wires to change the Mass Signal (one to the MAFS and one to the ECU). The wire to the throttle position sensor was tricky to find but I got it (with the help of 300EVIL here on the Forum). Attached is a quick diagram of what the wiring looks like.

Have fun with it.