# How accurate is the air-fuel-ratio set by the ECU?

I am analyzing the CAN, OBD and sensor data from my motorcycle (modern bike with throttle by wire, CAN-Bus, etc.) and I plan to measure some of the sensor data, i.e. the intake air temperature. Currently this is a just for fun project.

Now I wonder how accurate it makes sense to measure the sensors. I.e. there are temperature sensors available which are up to 0.1 degree Celsius accurate. But I guess such accuracy is not necessary. I guess that it's i.e. good enough to know the temperature with 1 degree accuracy or maybe even 5 degree is accurate enough. Correct?

At some point the ECU will decide it needs i.e. a air-fuel-ratio of 14. Does it have to be exactly 14.0 and would anybody notice a difference if it would be 14.1 or 13.9? The 14.0 is just an example. It could also be 14.6 or whatever.

How accurate is the air fuel ratio of the ECU and how accurate does it have to be for "normal" engines. For me normal means in this case gasoline engine for street legal cars or bikes which are not tuned to the extreme.

• Do you think ecu controlled engines with fuel injection have a more “accurate” afr compared to an engine with a carburettor? Commented Feb 10, 2020 at 10:01
• @SolarMike: I am pretty sure they could be more accurate. But do they have to be? Long time ago when I had a bike with carburetor it worked at night with low air temperature and at daytime in the sun, maybe 20 degrees Celsius warmer. And I don't think I remember the bike was running better at night or day. Commented Feb 10, 2020 at 10:30
• Modern electronic fuel injection systems are very precise in their measurement of both air and fuel as well as the O2 content in the exhaust. What I think you are missing is that the stoichiometric ratio of 14.7 is not always the best choice for the conditions at hand and the designers of the fuel map have different goals at different parts of the engine's operating conditions. Commented Feb 10, 2020 at 11:30
• The answer will vary per ECU.
– GdD
Commented Feb 10, 2020 at 11:33
• @jwh20: I am aware that 14.7 is not best under many conditions. This is why I used 14.0 as an example. How do you define "very precise"? fuel injection accuracy +/-1% or 0.1%? That's my question. Commented Feb 10, 2020 at 12:29

This is not a "definitive answer" by any means, but I think it gets to heart of the question.

"Does it have to be exactly 14.0 ... would anyone notice 13.9/14.1?"

Most likely, you wouldn't notice immediately.

Combustion is a simple chemical reaction, and like a lot of chemical reactions, the accuracy of the variables is logarithmically beneficial (i.e. the difference between 45% and 50% is much greater than the different between 90% and 95%). So if your "perfect" AFR was actually 14.0563, the driver would likely not see a major difference 14.0/14.1 and may not even notice 13.9/14.2... but the difference would shown over time with precise investigation.

"How accurate does it need to be?"

As accurate as possible.

As we know, there aren't too many variables in the combustion process, but each of them make a considerable difference. The more accurate the EFI system can be, the better burn you can achieve, and the more you can reduce wear and buildup, and better control emissions. As noted above, the impact of each variable adjustment reduces as you approach perfection, but its ideal for all outcomes if you could hit perfection.

Basically, the more precisely you measure, the more you will see any benefits from the fine-tuned adjustments, ad infinitum. As for how precise you actually need to be, that is most likely dictated by EPA (or similar) emissions guidelines. However just like the benefits of adjustments, the emission/environmental impact is logarithmic. So they will set the bar much lower than "perfection".

So I'd say they "have" to be as accurate as they are currently to comply with regulations (which is not to say that they dont require higher precision in the future).

If you want to know how accurate is beneficial from a performance standpoint, you'll probably have to ask manufacturers or companies like pro race tuners that are dealing with ultra-precise benefits. (The prototype race divisions for companies like Audi have literally spent hundreds of millions of dollars on R&D to get maximum benefit out of every possible angle.)

Finally, to use your analogy of the computer CPUs, the difference between a "low/high-quality" ECU is probably more the speed that it samples and adjusts more than the precision. A CPU from the 60's can do the same math as a CPU from today, it'll just take WAY longer to do it.

Again, this is all just a theoretical, logic-based, answer.

As Paulster2 points out, in production ECUs the accuracy is very likely to vary between manufacturers because of the programmers/engineers particular needs/wants. Even, as he pointed out, some may use Farenheit while other Celcius - very different scales. The same is true with the accuracy requirements they request.

I don't have much knowledge when it comes to flashing or custom programming ECUs, which is what I think you're getting at. I would assume that as with computers there will be lower level code areas that can't be adjusted (or at least they would be a HUGE amount of work and effort if they are possible), and therefore you'll need to work within those confines.

• I think this is a pretty good answer. You might want to add, the ECU is only going to be as accurate as it is programmed to be, meaning, it really depends on how the engineers who designed and created the ECU made it in the first place. The odds are, it is going to vary LARGELY from manufacturer to manufacturer. GM might have steps of 0.1 °F while MB might do theirs in steps of 0.1 °C. Because of this, there's not going to be a definitive answer other than the point it's going to be as accurate ad the engineers made it. Commented Feb 10, 2020 at 18:43