Afr reading

[hmdballa219]

Hey guys a little confused on this. After getting my car tuned and having the first tune of the e 85 tune on it, noticing that my afr has been reading about 8.7-9.1 while driving. When I let off gas. And coast. It goes to 20. Then drops back down to the 8.7ish when idling .is there something wrong? Is it the e 85? First time running e 85. Never paid attention to the afr before on this one. It's a 2018 mustang GT with the a10

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[TrevorGT]

This is crazy!!! I was seriously just going to post the same question.

I’ve been using e-85 for 2 years now. Just filled for the first time since last summer and I’m noticing the same thing. I don’t remember that happening last year. 9 ish to solid 20 when coasting.

May have always done this and I’ve never noticed.

 

[ugstang17]

Stoich of E-85 is much different than 93 octane. E-85 is around 9.7:1 AFR stoich where as 93 octane is 14-14.6:1 depending on how much ethanol is in the fuel mix.

Closed loop should be targeting somewhere around that. WOT will be a different story altogether obviously just like 93 octane.

Makes more power, but takes more fuel to do it. If the E-85 is higher or lower this can effect the AFR reading as well. That is why many people carry test kits to verify the fuel especially when they are getting it from a different location. It can vary in octane rating and purity quite a bit from supplier to supplier.

There are some very good books that discuss this in detail.

Any time you are coasting (idle throttle position) a function called decel fuel cutoff (DFCO) takes place and you will see 20.0 because the O2 sensor is not seeing very little burned fuel. It's a form of engine braking for no better explanation. Some tuners will opt not to use this. Some tuners disable this in their tunes and in that case you will see the engine target 14.0 or whatever Lambda 1 is for the fuel is that you are running.

 

[brucelinc]

8.7-9.1 seems awfully rich even for E85 but going to 20 on closed throttle deceleration is totally normal. Mine did it when stock, when on 93 with my flex tune and on E85. My AFR was around 14.2 on E10 and 10.2 on E85.....which is actually E76 currently.

 

[hmdballa219]

This is crazy!!! I was seriously just going to post the same question.

I’ve been using e-85 for 2 years now. Just filled for the first time since last summer and I’m noticing the same thing. I don’t remember that happening last year. 9 ish to solid 20 when coasting.

May have always done this and I’ve never noticed.

Wow that is crazy! Have u checked before or was this the first time really .this is only my second fill up of e85 .never checked the e percent on them, but they are used widely around my area.

 

[hmdballa219]

Stoich of E-85 is much different than 93 octane. E-85 is around 9.7:1 AFR stoich where as 93 octane is 14-14.6:1 depending on how much ethanol is in the fuel mix.

Closed loop should be targeting somewhere around that. WOT will be a different story altogether obviously just like 93 octane.

Makes more power, but takes more fuel to do it. If the E-85 is higher or lower this can effect the AFR reading as well. That is why many people carry test kits to verify the fuel especially when they are getting it from a different location. It can vary in octane rating and purity quite a bit from supplier to supplier.

There are some very good books that discuss this in detail.

Any time you are coasting (idle throttle position) a function called decel fuel cutoff (DFCO) takes place and you will see 20.0 because the O2 sensor is not seeing very little burned fuel. It's a form of engine braking for no better explanation. Some tuners will opt not to use this. Some tuners disable this in their tunes and in that case you will see the engine target 14.0 or whatever Lambda 1 is for the fuel is that you are running.

So your saying on e 85 that is totally normal and nothing really to worry about then? As long as it stays around high 8s to 9.x amounts??

 

[hmdballa219]

Also. Does that mean the higher the E percentage. The lower the afr? Or is that completely irrelevant?

 

[EFI]

Also. Does that mean the higher the E percentage. The lower the afr? Or is that completely irrelevant?

It's definitely relevant. The more ethanol content in the gasoline, the less BTUs available thus more fuel is needed per each power stroke.

Pure 93 is 14.7 stoich, E10 is about 14.1 and E85 is mid to high 9s.

 

[hmdballa219]

It's definitely relevant. The more ethanol content in the gasoline, the less BTUs available thus more fuel is needed per each power stroke.

Pure 93 is 14.7 stoich, E10 is about 14.1 and E85 is mid to high 9s.

Ok thank you for the clarification. So pretty much without looking deeply into it. With my lower afr (being in the high 8s low 9s) means the e 85 is just ehh I'm guessing and not quite 85% ethanol??

 

[AZ18yote]

Normal range. On 91 I'm at 14, on ff which is about e54, I'm at 11.5. At your afr I'd say you have pretty good quality e85. Off throttle typically bounces to 20 regardless.

 

[EFI]

Ok thank you for the clarification. So pretty much without looking deeply into it. With my lower afr (being in the high 8s low 9s) means the e 85 is just ehh I'm guessing and not quite 85% ethanol??

Well it means 2 things really:

The fuel is more like E90 or your tuner has set the stoich to a lower value for more safety.

 

[hmdballa219]

Ok that's awesome then. So it's more of a better thing. Of either the quality or safety aspect. And shouldn't worry about any detonation. Or problems then I'm assuming?

 

[ugstang17]

Datalog the car. Your AFR will be in Lamda. Lambda 1.0 will be the target stoich of the fuel choice you are tuned to run. If you are running an ngage, set the gauge setting to monitor afr1 and/or afr2 they are also monitoring and outputting in Lambda. At Lambda 1 observe your AFR on the LCD display and you now know what your target AFR should be.

Remember that stoichiometric ratio is the optimal point at which all fuel and air in the mixture is 100% consumed in theory. The burn range of the specific fuel will be wider than that. E.G. Gas in general is 14.68 stoich, however its ability to burn goes as rich as 7.5:1 and as lean as 26.0:1. OF course the richer side of stoich leaves fuel left unburnt and the lean side leaves Oxygen left over.

Greg Banish has a great book on tuning. Engine Management: Advanced Tuning. Its a great read and a great reference. Its a little dated now but it gives a lot of good information that you know is solid and not just guess work or opinion. You can pick it up off Amazon. Mine is worn out and highlighted to death.

 

[ugstang17]

So your saying on e 85 that is totally normal and nothing really to worry about then? As long as it stays around high 8s to 9.x amounts??

That is what I would conclude. A little rich is better than a lot lean if that be the case. But this is my opinion.

Pure Ethanol has a stoich of 9.0. E-85 as mentioned has a stoich of 9.7-9.9. If your E-85 has higher or lower content the tune may be designed to correct for that. Lund has this ability on N/A setups. Its referred to as a FLEX tune. If the ethanol content changes from the last time it will sense this and adjust to a more appropriate target table shortly after startup if you let the car sit and idle until it finds the best target table to use. Just learned this from the the guys at FInishline Performance who are Lund dealers. There is more to it behind the scenes in the tune but this is the basic down and dirty.

Your tuner is the only one who can provide you the fact based information on what he is targeting in the tune so that you can then assess with assurance that what you are seeing is correct. Always look to the person who tuned the car for the facts. We can talk stoich theory all day and opine until the cows come home but the tuner can give you the hard facts.

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