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Tuning the FA20DIT for better fuel economy

5865 Views 7 Replies 5 Participants Last post by  15_Rex
I haven’t seen any information on tuning the WRX’s FA20DIT for better fuel economy, so I thought I’d share a summary of what I’ve learned though my own tuning and testing. COBB’s fuel economy map is pretty disappointing. The only thing it changes is wastegate duty cycle to limit boost to wastegate pressure of 7psi. I’ve done quite a bit of tuning and testing with Accesstuner Race, and my fuel economy is now noticeably improved without reducing engine performance or (hopefully) reliability. Most of these changes will increase fuel economy at the expense of higher NOX emissions.

Lean Burn at Cruising Loads
Running leaner than stoichiometric AFRs results in better fuel economy due to a higher throttle opening (reduced pumping losses), and a more complete combustion of the fuel. The reason few car manufacturers have tuned engines this way is due to emissions. The stock ECU will vary AFRs slightly leaner, then slightly richer than stoichiometric to keep the catalytic converter working well. At lean AFRs the cat doesn’t work as well, resulting in an increase in NOX emissions (but a decrease in hydrocarbon emissions and CO2.) Running lean may result in less fuel contamination of the engine oil, which is a known issue with these engines.

A potential downside of a lean burn is higher exhaust gas temperatures. This is due to the fact that the combustion speed is slower at AFRs leaner than stoic. As the burn is leaned, EGTs increase and peak at an AFR of around 15 to 15.5:1. At AFRs leaner than this, EGTs go back down. A lean burn tune should target an AFR of over 16:1. I have run this engine as lean as 17:1 with no issues. To further mitigate the possibility of higher AFRs, ignition timing should be advanced as well. This will also put the engine closer to maximum brake torque (MBT) for better efficiency. The tune should also only allow a lean burn at low loads to minimize the risk of burning the exhaust valves.

The stock ECU will not allow AFR’s leaner than stoic, so you’ll have to change the O2 sensor calibration to trick the ECU into running leaner. You’ll also need to effectively disable the rear O2 sensor by zeroing out the A/F Learning #3 and A/F Correction #3 tables. If you don’t do this, the rear O2 sensor will correct your AFRs back to 14.7:1. To prevent a check engine light, you’ll need to disable code P2270 – Rear O2 Sensor Biased/Stuck Lean. The alternative to changing the O2 sensor calibration is to run full-time open-loop and change the MAF calibration instead. This is less desirable, since it requires accurate MAF scaling all the way down to idle air flows, and there is a greater chance of running undesirable AFRs that may result in high EGTs. Less consistent AFRs will also make it difficult to increase ignition timing without experiencing knock.

The O2 Sensor Calibration will need to be changed by multiplying all values by a multiplier of less than 1. For a target AFR of 16.9:1, multiply all values by 14.7/16.9=0.87. The five closed loop fueling tables should then also be multiplied by 0.87, except at the load and rpm where lean burn is desired, which will be set at 1.00. You’ll want to modify the one of the columns of the table so that you have a quick transition between lean burn and stoic operation (to prevent engine operation at point in between 14.7 and 16.9:1, where you’ll see higher EGTs). For instance, if you want to be in lean burn at loads less than 1.03g/rev from 1600-2800rpm, modify one column header to 1.02, and the next column header to 1.03. Row headers should then include values of 1550, 1600, 2800, and 2850. You’ll want to monitor AFR knock at WOT to make sure the fueling is richening quickly enough.

You’ll also want to modify the four Closed Loop Fueling Target Base Lean Limit tables to prevent the engine from running lean at coolant temperatures below about -10 degrees C, otherwise you may experience stalling after a cold start. Multiply all values that are less than 1.00 by 0.87.

I’m personally running a tune with a target AFR of 16.9:1, from 1400-2800rpm, at loads below 1.03g/rev. You should be able to stay in this load range while cruising or lightly accelerating in any gear up to faster highway speeds. Ignition timing is set to between 3.5 and 6.0 degrees more advanced than stock in the lean burn range, which is up to 10.5 degrees more advanced than the COBB maps. I started to get some cruise knock at about 1.5 degrees above these timing values. (When tuning for cruise knock, be sure to have the HVAC system off or on front/floor vents only, or you’ll experience false knock when the a/c compressor starts.)

There’s not much, if anything to be gained in fuel efficiency by running lean at idle, however I’m running slightly leaner than stoic at idle, as richer AFRs may lead to more fuel contamination in the engine oil.)

Post-Start Ignition Timing
The purpose of the Post-Start tables is to enable the engine to run more retarded timing for the first 3 minutes or so after the engine is started. This is done to increase EGTs to warm up the cat faster for lower emissions. I would recommend setting this table (Primary Ignition (TGVs Closed) Post-Start Alternate) to the same values as the Main table in the lean burn load range. This will put the engine closer to MBT, and should result in slightly faster engine warm-up. If you’re running a lean burn tune, you may not want to run quite as much advance in the Post-Start Table as in the Main table, as you won’t be running in lean burn until your O2 sensor heats up.

Stratified Fueling
Stratified Fueling Mode retards ignition timing a lot in order to increase EGTs to warm up the cat more quickly. In addition to reducing efficiency, the engine itself probably takes longer to warm up as a result of more heat being lost to the exhaust. Stratified Fueling can be disabled in the Advanced Parameters menu in Accesstuner Race.

Cold Idle Speed
The stock 1800rpm cold idle uses a lot of fuel, and is also often active after a warm start. It also prevents engine braking below 1800rpm (which would also save fuel and improve drivability). Like stratified fueling, a high cold idle speed is required to heat up the cat quickly for emissions purposes (and also to heat up the engine in very cold temperatures). The engine idles just fine at a cold idle speed of 800rpm above 0°C. The only issue with lowering the idle speed is that there is some hidden logic in the ECU that is not accessible with Accesstuner Race. It will occasionally cause the idle speed to jump up to 1800rpm for a second, then fall back down to the target rpm. I haven’t found a way around this.

AVCS Exhaust Cam Coolant Temperature Compensation
In the stock tune, exhaust cam timing is advanced by up to 40 degrees at coolant temperatures below 50°C and loads below 0.58g/rev. Exhaust cam timing advance increases EGTs, so this was likely done to warm up the cat faster for emissions purposes. This timing advance reduces overlap, reducing the scavenging effect, and requires the piston to work harder to pull in air. It likely also increases engine warmup time, as more heat is being lost to the exhaust. To eliminate this compensation, set the two AVCS Exhaust Cam Activation Tables to -100% across all coolant temperatures. This change has a really significant effect on fuel economy during engine warmup.

Ignition Timing Compensation (Intake Temp)
Knock is more likely to occur at higher intake temperatures due to a faster burn rate. The stock and COBB maps pull timing at any intake temperature above 40°C to prevent knock. Conversely, it is possible to add timing at lower intake temperatures to increase fuel efficiency and power. This is only much of a benefit in cooler winter temperatures. Combustion speed decreases with lower intake temperatures, so you shouldn’t have to worry about advancing timing past MBT at light loads, especially with a lean burn tune. I set this table to advance 1.0 degrees at 10°C, and 2.0 degrees at -10°C and below (though actual timing is modified by the Activation table to half of this advance at higher loads). I haven’t finely tuned these values. They are conservative, and you’ll still be more likely to experience knock at higher intake temperatures. If you’re more aggressive with adding timing at low temperatures, you may want to modify the Activation table to exclude idle rpms/loads to prevent an unstable idle.

Idle Ignition Timing
The idle ignition timing can be advanced for better fuel economy at idle by changing the Primary Ignition Idle A and B tables. Ignition timing advanced 3 degrees over stock seems to result in a ~6% reduction in fuel use at idle. Adding too much timing will result in an unstable idle.

Post-Start Enrichment
There’s not a whole lot of fuel savings to be gained here, but every bit helps, and excessive post-start enrichment likely contributes to fuel contamination of the engine oil. I found the enrichment to be excessive with the stock tune and stock MAF scaling. After properly scaling my MAF at low loads, the stock Base and Decay Rate table values were pretty much perfect. Still, most of this enrichment is not required outside of idle conditions. The RPM Compensation table can be changed to reduce enrichment by at least 75% above 2000rpm without affecting drivability, though I haven’t yet tested this at temperatures much below freezing.

Warm-Up Enrichment
Again, there’s not a lot of fuel savings to be gained here, but you can reduce these values to zero at higher coolant temperatures without affecting drivability during engine warmup. In lean burn this enrichment does nothing for drivability anyway.

MAF Scaling / Full Time Closed Loop
A properly calibrated MAF sensor will ensure that the engine is not running rich at high loads. Any custom tuning should include a properly scaled MAF. Tuning the engine for full time closed loop will make MAF scaling a non-issue however, and may result in more accurate fueling.

Lowering boost can significantly increase fuel economy at higher loads. This is what the COBB fuel economy map does (but in a much too drastic way in my opinion). Lowering boost allows higher throttle openings (lower pumping losses) at a given power level, more advanced ignition timing that is closer to MBT, leaner AFRs, and less exhaust back pressure. Because of these factors, power is not affected as much as you might expect. Personally, I have one map at 19 psi for summer, and another at 14 psi with a smoother powerband that I use for winter hooning.
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I haven’t done any really scientific before and after testing for fuel economy, but my last tank has been 31mpg US measured (35mpg displayed) in all city/freeway driving in cool weather, included some WOT pulls for tuning, and with all trips shorter than 30km.

I’m not an expert. If you’re going to tune your engine in the way that I have, you should probably have some tuning experience, and even then you may destroy your engine. Lean burn tuning is more involved and takes more time and datalogging than tuning for power at WOT. Some of these changes are easy to implement and don’t require any fine tuning, while still having a significant impact on fuel economy on short trips (Idle Speed, Stratified Fueling, Post-Start Timing, and AVCS Coolant Temp Comp for instance).
HOLY **** !
I thought I was a mpg nut.

Let me know how this all works for you. I have 34.5 showing on the cars ODO mileage and whatever is in my 'fuelly' tag below. I also have gotten 37.something actual mpg on one tank when taking a highway trip once.

(oops, sorry for hitting the language filter :(
You are using some different terms from what I'm used to.

My understanding of Stratified Injection is using the direct injector and valve event to control combustion with throttle blade fully open. This reduces pumping losses. It is not used in USDM due to the more stringent NOx requirements.

Subaru does employ a pseudo-atkinson thermodynamic cycle by delaying the valve close event. This creates a lower dynamic compression ratio than expansion ratio permitting more energy extraction from the combustion event. EGTs will be lower by definition of this process. I'm not sure you would even be able to control this logic at this point, but be careful with valve control until you fully understand Subaru's strategy.

Sent from my SAMSUNG-SM-G890A using Tapatalk
Thank you, Rex_15, for thoroughly making my head hurt. :eek3:

On a more serious note, thanks for sharing!
What are the EGT differences between stock, Cobb, and your calibration?
You may be right about the definition of Stratified Fueling. I'm using that term because that is how it is defined in the Accesstuner Software. The stock ECU has 2 idle modes, "Stratified" and "Homogeneous". Each mode has its own ignition timing, post-start, and warm-up enrichment tables. I'm not sure how exactly valve timing is different, as it doesn't seem to be accessible. The fuel mode is automatically changed from Stratified to Homogeneous as soon as the throttle is applied at all, or after the engine is partially warmed up. The Accesstuner feature simply uses the stock Homogeneous mode fueling at idle at all times. I don't see how this could be dangerous. Stratified fueling mode increases fuel consumption at idle by 50-100%.
What are the EGT differences between stock, Cobb, and your calibration?
I honestly have no idea as I have no EGT gauge. In theory, EGTs should be lower at 16.9:1 AFR than at 14.7. Lean burn is limited to lower loads and rpms in my tune in order to hopefully keep EGTs at safe levels even if this is not the case. Timing is also advanced quite a bit, which will keep EGTs lower.
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