BRZ/FRS/86/DIT WRX: How to Re-scale your MAF for Intake/Induction Changes


How to Re-scale your MAF for Intake/Induction Changes







It’s very important that the ECU knows the true amount of mass airflow (grams per second) that is entering the engine so it can accurately calculate the correct volume of fuel to inject and therefore achieve the correct AFR that’s shown in the fuel map. The factory fuel map has been calibrated to a stock factory intake and the values in the fuel map are what the vehicle runs when the mass airflow entering the engine is accurate.  Generally when fitting aftermarket Intakes, induction kits or even replacement panel filters, the MAF sensor reading will be altered and the MAF sensor scaling will need adjusting. 

Typically the Inside Diameter (ID) of an aftermarket MAF tube will be larger than the stock MAF tube ID. Most Intakes will change the way the air flows through the MAF tube and thus across the MAF sensor itself. This results in a lower MAF sensor voltage output that is given to the engine ECU for the same mass airflow. This lower mass airflow reading will lead to a lower engine load, more advanced Ignition timing and a leaner AFR. It’s very important that the MAF scaling is adjusted (normally increased) to counteract this problem.   The preferred setup would be making a ‘before and after’ log file showing the MAF volts, mass airflow, engine load, AFR etc for the stock intake and then new intake, you can then cross reference the MAF Volts ‘before and after’ for each RPM and Manifold Pressure and increase the MAF Sensor scaling right hand column (grams) until the same mass airflow reading is achieved with the new intake compared to the old intake. 

Now this doesn’t always happen and quite often the car already has an Intake fitted so the Fuel Trims will be very important and can be used to make the MAF calibration.

While switching from Closed to Open Loop mode the ecu will target 14.7:1 AFR and will ignore any richer target AFR values in the closed or open loop fuel maps.

In the following log file screenshots we have simulated fitting a larger MAF housing, this will mean that for a given airflow the AFM voltage will be lower/less when the larger housing is fitted (as the airflow will be slower). So with the larger MAF housing fitted the Mass Airflow (gms) will be less and therefore the Engine Load will be lower which will mean more advanced ignition timing lookup but also we will have a leaner AFR as the ECU calculates fuel delivery volume based on airflow.

Closed Loop Log File: In this example at 3000rpm you can see the AF Target and AF Sensor #1 are very close at 12:1 AFR, the ECU is working in Closed Loop and the FTST (Yellow line) is currently showing 10% indicating the ECU is ADDING +10% to the Injector Open Time to hit the Target AFR

Open Loop Log File: In this next example at 4300rpm you can see the AF Target is 11.6:1 but the actual AF Sensor #1 reading is 12.5:1, its running lean! 

The FTST (Yellow line) is at zero indicating the ECU is working in Open Loop.

So when the MAF Scaling is out during Closed Loop the FTST will compensate and ADD Fueling to hit the Target AFR, when the ECU is in Open Loop then FTST will NOT work and the AFR will be lean.

The solution is to increase the MAF Scaling in this region and this process is described as follows.

Adjusting the MAF scaling in open loop

Short Term Fuel Trims do not work in Open Loop so MAF scaling in Open Loop needs a slightly different approach.

To make a good MAF scaling we should fill the right hand side of the fuel map with a safe and friendly AFR but simplified to let you see clearly what is going on, we chose 12:1 AFR like shown below, make sure to choose something reasonable and test the lower load and rpm ranges.

If the AFR is 14:1 then increase the MAF scaling (g/sec) at that particular MAF voltage.

If the AFR is 10:1 then reduce the MAF scaling (g/sec) at that particular MAF voltage. 

After you have some of the lower ranges dialed in make a higher power level run and gradually tweak the remainder of the maf scaling range.

With a 12:1 Target then aim to get the AFR within 11.5:1 and 12.5:1 AFR across the board, good tuners will aim for 11.8 to 12.2 AFR but it depends on the time you have available. The first time you make a MAF scaling in this way, it may take a while but once you master it then in future you will be tweaking the MAF curve on every flash. Once the MAF scaling is good and smooth and you have 12:1 across the power curve, then you can profile the fuel map to your preferred AFRs.

MAKE SURE that your Engine Load does not exceed the X axis of the Fuel and Ignition maps, if it does then rescale the maps or use our Example Maps as your base file to start your tuning.


BRZ/FRS/86


WRX


Now in Open Loop (high load), the fuel trim feedback does not work anymore (no Short Term Fuel Trim) but any Fuel Trim’s that had been learned from Closed Loop can be applied in Open loop (shown as Long Term Fuel Trim and AF Learn Values), this is why it’s very important to get your Fuel Trims tight so any Closed Loop Fuel Trims are not applied in Open Loop and therefore affect your Open Loop Fuelling.  The longer the Intake has been fitted then the better as the FT Long Term will show how the ECU uses the FT’s to adjust for the MAF Scaling error, the FTLT that is applied can be used as a good indication of what to do with the MAF scaling. Here is a great example of how long term fuel trim are learned and applied.




The screen shot above shows the Fuel Trims working and actually removing -29% of the Injection Volume on fuel load, this vehicle actually had serious MAF scaling issue combined with a boost pipe air leak. 


Adjusting the MAF scaling in Closed loop

At Idle and light load (Closed Loop condition) the Fuel Trim Short Term (FTST) and Fuel Trim Long Term (FTLT) will typically be adding +5% to -5% to maintain the closed loop target. When we fit a new Intake this will change and the FT will reflect the amount of compensation needed to the MAF scaling to keep the FT tight within 5%.  You'll want to do this for load areas where the vehicle isn't using open loop.


As the DIT WRX has better closed loop control than previous Subaru models (like BRZ/FR-S and STI) we can use this closed loop feedback to help with our MAF scaling.  It's possible to adjust the Fuel Map – Open Loop Lo Det map and set the car to Closed Loop on full load as shown below.  You can see the Fuel Map – Open Loop Lo Det map is filled with 14.7:1 and the Fuel Map – Closed Loop #4 is filled with 12:1.  This will make the ECU use the FT to control the AFR at 12:1 and the FT can be logged and viewed to help adjust the MAF scaling.  This feature should be used with caution and understanding though, we do not recommend running in Closed Loop on Full Load all the time.

    





Adjusting the MAF using RaceROM Speed Density

In this Section 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.




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).




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.  Below is the log of 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 using the information below!



What happens when my MAF hits 5volts?

If you hit the maximum 5 volts then you will get a MAF fail safe condition based on MAP sensor calculation and P0103 DTC. This failsafe condition will occur even if you are running full time SD so ensure that P0103 is disabled so the MAF hitting 5 volt does not cause any issues.  See the Forced Induction section on the MAF Failsafe Load Calculation Multiplier for further important advice.




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