BRZ: How to Re-scale your MAF using RaceROM Speed Density

Re-scaling the MAF sensor using speed density (SD)

Introduction

In this article we will cover the method of correcting your MAF sensor scaling using Speed Density, it is using the BRZ platform but in other vehicles (like GTR, DIT and 370Z) you can adopt a similar approach.

To start with Mass Air Flow (MAF) is used to calculate Engine Load which then sets a fuel quantity by referencing a target AFR. In the BRZ (and most cars) the fuel quantity delivered is measured and corrected (using AFR sensors) to achieve the target set in the ROM, this means that incorrect airflow estimations will directly effect the fuel trims.

This is called closed loop fueling, and most the vehicles are set up to only use these corrections for light load running conditions, and use a safe open loop fueling map at higher loads where sensors are less reliable.

Calibrating the MAF scale

To adjust the MAF scaling using SD you will need to enable SD mode, with SD enabled the mass airflow (g/sec) will come from the SD map and not the MAF sensor itself but the LIVE DATA, custom logging parameter called MAF Sensor (g/s) can still LOG the output of the MAF sensor (even though the MAF sensor is NOT used for the load input at all).

Although this can be confusing at the start, it’s great for MAF scaling on relatively standard cars. All you need to do is enable SD and you should find the engine runs reasonably well; make a power run and be sure to include the MAF Sensor (g/s) logging parameter.

As you can see below, the ECU is running on SD, the mass airflow input is coming from the SD map (YELLOW line) and the engine is running well. We can also see the MAF Sensor reading below the YELLOW line (the RED line) although the ECU is not using the MAF Sensor.

SD and MAF g/sec Comparison log

By enabling SD we can view and log the effect the new intake has had, and we can simply increase the 2D MAF scaling until the RED line meets the YELLOW line.At the highlighted point (4670rpm) we can see that the MAF scaling (YELLOW line) is slightly higher than the SD calculated Mass Airflow (RED Line) so the MAF scaling could be reduced in this region.

To adjust the 2D MAF scaling set the Mass Airflow (g/sec) value to match the SD value at the MAF Voltage (V) indicated in the log (ensure that MAF Voltage V is logged).

MAF sensor Scaling map

Now we can turn off SD and check the Fuel Trims and measured AFR are on target target. If our AFR is on target and our fuel trims (both long and short term) are withing 5% we have a very close MAF scale. If not is you can try this method again or move to the other methods of MAF scaling in closed and open loop.

If you have a poor MAP signal (large variance due to sensor position etc) you may want to use a Hybrid SD setup. For more information on Hybrid SD set-ups please search view the manuals and knowledge base.

Effects of an Incorrect MAF scale

The Short-Term Fuel Trims (STFT) which are transposed into Long-Term Fuel trims (FTLT) are the learnt AFR correction values and can take weeks to learn and take effect. As they are applied in Open Loop this can drastically change your wide-open throttle AFRs, making the car run very rich or very lean in the long term.

See the next screenshot on an engine running 1.54 bar absolute and an AFR of 14.0: the LTFTs have removed 29% of the fuel volume, and this is why it’s important that your closed-loop FTs are within the region of 5%-10% when in steady state.

In the two maps shown below the LTFT Min and Max values should be set to zero to prevent the FT from altering the injection volume in open loop. Although the Fuel Learning is not fully understood, these maps are critical to the open-loop FT factors; we recommend that wherever possible you check that open-loop FTs are not applied in other modes or conditions.

Keep in mind that your fuel trims while effected by airflow estimations are also influenced by injector scaling/sizing, calibrated compensations as well as other corrections for temperature etc. It is good practice to check your fuel trims after you have made your MAF scaling to ensure that everything is still in order.

Hopefully this will give you enough information to rescale an aftermarket MAF housing with great results. We are constantly trying to update articles and create new ones if you have any questions, please do not hesitate to contact us at [e-mail]