Ford Tuning: Injector Scaling


One of the most common modifications that require recalibration of the ECM are changing injectors and changing Mass Air Flow (MAF) sensors.

For the rest of this article, we’re going to assume that you’ve already read the articles explaining basic MAF operation and a model for injectors.  We’re going to discuss how to properly change the tune to compensate for new fuel injectors.

You should also take a look at the article on MAF Calibration as they often go hand in hand.

About Injectors on Ford ECMs

Ford uses the concept of injector slopes, breakpoints and battery voltage latency adjustment to cover the behavior of injectors.   Slopes represent the flow of the injector at high and low pulsewidths.  Breakpoints determine the pulsewidth required to switch from the low slop to the high slope.  Returnless fuel system cars add additional compensation tables related to fuel rail pressure.  When changing injectors, it is best to have a complete set of test data.

  • In many cases, injectors purchased from Ford Racing will include all of this information.
  • If you’re using a larger OEM injector (Cobra, Lightning, etc.) you can generally obtain valid data from the OEM calibration in which the injectors were used.  Example SXH1 08 GT500 48#, 01 Lightning CUX2 for 42#, 03 Cobra AMZ2 for 39#, 98 Cobra AOL3 for 24#, etc.
  • If you don’t have complete test data, you can make do.  You will need a wideband.  Recommended procedure:
    1. The rest of this procedure assumes you have a SOMEWHAT sane MAF transfer function.  If your MAF transfer is jacked, you may need to adjust, retune MAF then readjust a few times to get things properly aligned.
    2. Start with the data of the injector closest in size and design to the one you are using (slopes, inj latency, etc.).  If you can’t get any good data on other injectors, then your stock ones will do.  We will call this the “old” injector.
    3. Figure out what the injectors you are installing are rated for (i.e. 24#).  Remember the size of you old injectors (i.e. 19#).  Divide your NEW rated flow by your OLD rated flow.  Make sure your injectors are rated at the same pressure.  24/19 = 1.26 in this case
    4. Multiply both the LOW SLOPE and HIGH SLOPE by the value from above, in this case 1.26.
    5. Set your target AFRs / Open loop targets to a a UNIFORM value.  (i.e. 12.5 for a NA car)
    6. Do a WOT pass on the car.  Observe AFR.  Adjust BOTH high and low slope until actual AFRs resemble the target AFRs you have set up in your tune.
    7. Repeat #6 until the car is as close as possible to what you are commanding.
    8. Let the car idle.  Turn off closed loop if necessary.  Observe AFRs.   Adjust latency (battery voltage table) so that observed AFR is close to commanded AFR.
    9. Drive the car at low – light throttle.  Hopefully, Observed AFRs will be close to commanded AFRs.  If so, skip ahead to #11
    10. If observed AFRs differ significantly from targeted at part throttle, determine how badly they are off.  If they’re really far off, re-adjust in order to get things as close as you can.  After this, make SELECTIVE adjustments to the MAF transfer function at idle in order to achieve targets at idle while maintaining proper operation at light throttle.
    11. Once you have a preliminary set of slopes, latency values it is time to tune battery voltage tables.  First, observe battery voltage and AFR while IDLING.  At idle, the injectors are open the smallest amount of time so changes from battery voltage have the largest effect.
    12. Next turn on headlights, blower motors, brake lights, EVERYTHING you possibly can to put an electrical load on the motor.  Observe changes in battery voltage and AFR.  Make adjustments to the injector battery table in order to compensate for fluctuations.  I.e. if the car goes lean when you turn on the headlights, INCREASE the latency value at the voltage that the ECM reports with the lights on.
    13. Once you have the engine operating in a more consistent AFR range under electrical loan, rev the motor up and make sure that you don’t go too rich when battery voltage increases as a sanity check.
    14. At this point, you’ve probably done a more thorough injector calibration than most tuners will.