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E10 and afr

Understanding AFR

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Hello all,

I was was wondering if anyone has compared Recreational non ethanol fuel to high octane e10. The stoichometric number for e10 is 14 to 14.1 and non ethanol fuel should be 14.7. My ecu command afr is 14.69, 99% of the time. This means my engine is running lean most of the time, because it is expecting fuel with a 14.7 ratio, does it not? The car definitely seams to run better on non ethanol 90 than on e10 93 octane. My car is absolutely not flex fuel enabled. 2010 vw jetta 07k 2.5l NA. I was also wondering if there has been any long term studies of e10 effects on non flex fuel engines. Also, I am wondering what would the reprogramming of command AFR to 14.1 in a closed loop system do to horsepower and also to the health of the engine?

Hope this finds all of you healthy and we'll.


What are you using to ' know' the AFR? Meters/gauges read the lambda and convert that to an AFR number. This means that the stoich' lambda 1.0 will be displayed as 14.7:1, regardeless of the fuel used and the true AFR ratio.

Just using an elm device and an app. My pre cat o2 is a wide band bosch. So are you saying the 14.7 is just the app telling me the number or is it the Ecu? If I understand you, then you are saying that my ecu is measuring lambda, not stoichometric. I do have a command afr pid in the options ......... I maybe slightly confused.


You’re quite right that stoich for E10 would be around 14.1 or 0.96lambda.

There will be some flex ability engineered into the hardware to take leaner combustion but if you have the functionality to reduce target AFR then give it a go. Alternatively you could use a global 1.04 multiplier or add 4% to the high load sites in the base map maybe a touch more. This should help recover a little performance.

The lambda is reading 1.0, the ecu receives that value and it corresponds to an AFR (for petrol/gasoline) of 14.7:1. If you use E10 fuel and the AFR is still 14.7:1, then the lambda is still 1.0, which is stoich' for that fuel blend - most modern vehicles are self calibrating for injector, pressure, etc. variations to ensure they meet emissions over their service life (some regulations specify a minimum distance) and I might expect it to be correcting itself.

If you have the option to alter the target AFR in software, the ECU will alter the fuelling to meet that AFR - but, again, it will be to the lambda that corresponds to the requested AFR. This will NOT be the ACTUAL AFR!

If it makes it easier to understand, I may be able to find a chart that has AFRs and lambda values for a selection of different fuels - nope, can't find the one I wanted, but I suggest reading these as primers. - note while the stoich' AFR for the fuels is different, the lambda remains at 1.0, and so an AFR meter calibrated for petrol/gasoline will read the same 14.7 for all those fuels.

Thank you both for the posts, I thought I was correct. Thanks for the articles they are some awesome resources. I belive my ecu is assuming 14.7, since vw says non ethanol. I don't have a way to change the value yet, but I will be getting some things in the future and also a starter pack. I will keep running non ethanol until I can make my 2.5 flex fuel (then I can use e85.)

Stay healthy,


Uh, yes, that's the way I read it - lambda 1.0, gauge displays that as 14.7:1 due to calibration, actual 14.04:1 as that's the ratio that gives that lambda value.

Some gauges have the option to display lambda - if possible I'd change to that and get used to it, as alcohol percentage can vary significantly from the nominal values for pump fuel.