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ECU Tuning leaves a lot to be desired, both in-game and IRL


Hello all. Today, I decided to do an experiment to see just how effective ECU tuning is in-game, as well as how it translates to the real world.

Now, this isn’t a study of how much horsepower you can gain on turbocharged engines by simply ‘reflashing’ the ECU. Instead, this is a study of how large a difference you can make with a car’s cruising fuel economy by simply changing parameters in-game that would equate to nothing more than reprogramming the ECU on-the-fly while not driving in a spirited manner.

My experimental subject today will be this 5-door RWD hatchback, targeting the Family Sport demographic.

(Yes, the fixtures on this model are acting up a bit, that’s why this is open beta)

Every aspect of the car will be +3 quality.

The engine that I will be testing with is this 3.0L I6. It is N/A, DOHC 24V with VVT/VVL, and AlSi block/heads. It runs on premium fuel.

The first setup will be the engine in its ‘SPORT Tune’ configuration, with AFR set to 12.5 and ignition timing set to 100. It makes 273.9 hp @ 7200 RPM, and 241.9 lb-ft @ 4600 RPM.

When placed into the car, this is the fuel economy that the car returns:

27.4 MPG average, 33.9 MPG @ 75 MPH, and 50.7 MPG @ 43 MPH. That’s about average fuel economy for a car of this size with that much power.

Now let’s make a quick tweak to that engine. All I did was drop the AFR all the way down to 15.0, and tuned the ignition timing down to where the engine won’t knock. These are all things that an ECU is capable of doing on-the-fly without manual recalibration completely necessary.

As you can see, power and torque took a big drop by reducing the amount of fuel injected, as well as cutting back ignition timing. 245.3 hp and 216.4 lb-ft torque at the same RPMs; 10.5% reduction in power and torque. However, average fuel economy jumped by a significant margin, from 22.9% to 31.8%; a massive 38.9% increase in efficiency. Peak power and torque are meaningless when you’re just cruising down the highway, so the massive efficiency bonus is incredible, simply by adjusting AFR and ignition timing.

Now let’s look at the car’s fuel economy with this new state of tune:

Absolutely massive increases in fuel economy can be seen here. 38.8 MPG average, 47.3 MPG @ 75 MPH, and 73.6 MPG @ 43 MPH! Those are increases of 41.6% for the average, 39.5% @ 75 MPH, and 45.2% @ 43 MPH. Sure, straight-line performance is reduced, but if you’re just commuting in ECO mode to get the best gas mileage, the increase in efficiency is certainly worth it.

To summarize, by altering only AFR and ignition timing, which an ECU can do on its own almost instantaneously, you can have maximum performance when you want to drive spiritedly, or unbelievable fuel economy when just commuting or grabbing groceries.

So the real question is, why don’t cars in real life and in-game take advantage of such tuning to have both fuel efficiency and performance? Why must we make this compromise if the computers in our cars are able to make this drastic of an efficiency change while only sacrificing a small amount of maximum performance?

Thank you for reading, and I hope you enjoyed this quick experiment as much as I did.


Holy shit that’s efficient


Interesting stuff! To me it looks like there is one particular issue here, and that is that your sportiness rating didn’t take enough of a hit for how much you detuned it. That might be the bigger picture issue that needs addressing.

IRL you can make sporty family cars REALLY efficient. My father had a Saab 9-5 Aero with stage 3 tuning pack, beast of a machine, but he had it tuned to run super lean and would get below 6L / 100 km (39 mpg) out of it. That did negatively affect reliability though as there were weird fuel cut issues and such, don’t quite remember.

Anyway, it is not only the tune itself that would be changed though, but also stuff like the injector size and tuning.
Thanks for the little investigation, I’ll look into a potential sportiness nerf for lean AFRs.


If I remember correctly, AFR with modern ECUs is not intended to be be an exact ratio but rather to represent something similar to the maximum AFR allowed by the ECU. So cars in real life and in-game do actually reprogram themselves to run in high AFR mode when driving hard and low AFR mode when getting groceries. But when you set the maximum AFR to a low ratio, you are intentionally limiting the maximum output while the minimum output is the same for both. Which sounds like the reasoning behind @Killrob’s intended sportiness nerf.

A comment about that though - would it be possible to give a high AFR a buff, instead of giving a low AFR a nerf? I know that, effectively, they are the same thing, but it feels different - one is a punishment, while the other is a reward. Or perhaps the AFR scale just needs to be changed to give more octane with higher AFRs.


There were a few cars (early 90’s Civic I think for one) that did tune to lean burn mode when in light cruise, supposedly the LS engines have a non activated lean burn mode too. The problem is it is much harder to clean NOX out of the exhaust which is created much more with lean burn engines. Which is one of the reasons they came and went. One of the others is with the drive by cable the driver would wind up pressing the pedal further for the same power which caused quite a few complaints as there was a obvious loss of power when it kicked in and a lag before it kicked out when the driver pressed the pedal.

Switching to lean burn may come back with drive by wire as the ECU can adjust throttle position independent of what the driver is doing with the throttle pedal.


Or as the throttle will disappear altogether, which is already happening AFAIK (Fiat’s MultiAir engines, which are a step towards camless ones).


One thing that I did notice is that the responsiveness dropped from 73.5 down to 48.5, which gave a big boost to drivability, so much so that the drivability increased more than the sportiness decreased. (Drivability +2.9, Sportiness -2.7)


That’s the opposite of what my test showed. My testing showed that changing the maximum amount of fuel injected also affected the minimum amount, which caused a massive change in cruising efficiency. If the AFR and ignition timing were idealized, you should be able to have the maximum power output of the 11.9 AFR and 100 timing when driving aggresively, but the efficiency of the 15.0 AFR and retarded (NOT in the offensive way) timing when under cruising, light throttle, or maybe even a specifically programmed and switchable ECO mode where you can force reduced max power in order to get max efficiency.


Thank you. That is a big part of my design philosophy: getting the most performance as efficiently as possible.


Here is a wiki article

They kind of glossed over the negative side of lean burn

@b0bsaget007 you can quote multiple users in one post by highlighting the text you want to quote then click quote and it will include it in your post.


I know, it’s just that I had posted each quote as I read each post, and I didn’t feel like editing them all into one post.


Very efficient, but even in sport tune the power output isn’t incredibly impressive to me. Nice job, though


Emissions in general are just harder to clean since catalytic converters require some unspent hydrocarbons in order to do their job, though this is not to say you are wrong. You are absolutely correct that in particular NOx becomes a big problem! Thanks to the higher temperatures created by lean burn, nitrides and nitrates form in much greater quantities. These factors are why even fuel injected engines typically run slightly rich i.e. ~14.4:1.

I think this is a very large reason we aren’t seeing this kind of tech. Humans are much better at adaptive control and often know things the computers don’t. Computers are bad judges of context which is why traction control can be turned off (on most cars that is to say), manual gear selection exists even on automatics, and why I hate the stupid CVT on my mom’s 2009 Subaru Outback because it feels like you’re trying to drive a rover on Mars via messenger boy. I think my operating systems professor summed it up best:

"Computers are just extremely fast idiots. And they’re blindly obedient, too."

Perhaps though with all the new sensors on cars and the recent advances in deep learning, sensor fusion, and drive-by-wire tech, we’ll see dynamic mode switching on ECUs handled more seamlessly. Still, I’m not holding my breath.


Honestly it’s mostly because AFR is actually a really bad way of describing it. That slider is more accurately “How agressively do you want to enrich stuff to try and always get the best power/response, or lean for the best economy”

It does represent a much more dynamic system in the case of EFI cars, and that’s kind of simulated by the much better power/economy compromise you can get with them in game.

I think if we designed it again we’d come up with something more sensible than AFR for that slider, but it works reasonably well for now, if maybe in need of some balance tweaks with regards to its effect on final car stats :slight_smile: