Technical Information Tuning for Boost with an Air Flow Meter

Overview

This article details forced induction with an Air Flow Meter (AFM / MAF) based calibration

Fueling Process

Very broadly, here is the process the ECU goes through from the measurement of air flow to the firing of the injectors.

  • The ECU reads the AFM voltage.
  • Using the AFM voltage as an index the ECU looks up the intake air mass flow (in grams of air per second).
  • The intake air mass flow of is translated to air mass per cylinder firing (in milligram of air per cylinder) by dividing out the mass flow by the engine speed.
  • From the cylinder air mass the base injector duration is determined.  This is a stoichiometric injector duration  - it will generate a 14.7 air fuel ratio.
  • The base injector duration is modified by the WOT lambda adjustment tables according to load and RPM.
  • The injector lag time / dead time is added to the injector duration to give the final injector duration.

The two most important tables in this process are the AFM voltage to mass flow table and the AFM fuel table relating cylinder filling to injector duration.

Given that all non-stock intake affect the AFM reading (some by 30-40%), if you have non-stock injectors and a non-stock intake, you cannot calibrate both tables at the same time. Here is our recommended process:

  • Calibrate the after market intake first with stock injectors.
  • Install and calibrate the replacement injectors.

If your vehicle is turbo charged you can still run stock injectors by holding the turbocharger wastegate open so that the engine will not see any boost.

AFM voltage to mass flow with after market intake calibration

This is a fairly straight forward process. Just install the intake and datalog the car for a few minutes with a variety of loads and speeds, keeping the throttle under 80% to keep the ECU in closed loop.

Hint - The FlashPro unit datalogs faster than recording from a laptop via USB. Press the Datalog button then download to your laptop when done. The more data there is to analyze, the better the results.

With the data from your run onscreen, select XY graph from the windows menu advanced graph.

Select X axis AFM, Y Axis S.Trim (short term fuel trim), show mean and closed loop. 

  • Adjust the AFM Flow % by the value of the short term fuel trim%. In this case adjust by -4% all mass flow values greater than 50 g/s.

  • Datalog again
  • Repeat the above process until short term fuel trim is under 4% everywhere. The closer to zero the the short term fuel trim, the better.

Note - the stock intake has a + or - 3 % variation.

Graphed in Excel, the stock AFM calibration looks like this. Your graph should have a similar shape.

 

Injector calibration

Large injector calibration is best done with a stock intake. If you do not have a stock intake you must first calibrate your aftermarket intake with stock injectors as described in the section above.

Enter your injector size and opening time in the fuel section. General accepted industry practice is to have injectors flow tested at 43.5 psi (3 Bar)

FlashPro Manager will reduce the injector ms by a proportion of the injector size change from stock.

Datalog the car for a few minutes with a variety of loads and speeds, keeping the throttle under 80% to keep the ECU in closed loop.

Hint - The FlashPro unit datalogs faster than recording from a laptop via USB. Press the Datalog button then download to your laptop when done. The more data there is to analyze, the better the results.

With the data from your run onscreen Select XY graph from the windows menu advanced graph as above.

Select X axis AFM.c (cylinder), Y Axis S.Trim (short term fuel trim), mean and closed loop.

  • Adjust injector (ms) flow % by the value of the short term fuel trim% at the indexed air mass. In this case, add 4% fuel at 300mg and remove 9% at 100-120 mg.

  • Datalog again.
  • Repeat the above process until short term fuel trim is as close to zero as possible.

Note - Datalogged injector ms will not exactly match the AFM fuel table injector ms values as injector opening time (dead time) is not included.