Running a different fuel stoich??

[chops44]

So I was thinking about how it’s quite common among forum members to run their tank of 93 low, pour in 5 or so gallons of race gas and hit the track. So, race gas with a stoich value of let’s say 13.4 was added to a car that’s tuned to a 12.1 afr (.82 lam) on 14.7 stoich pump gas. Lets see if I have this correct. Since the current tunes written for pump gas with a 14.7 stoich that’s targeting a 12:1 afr you’re not going to see the true afr of the race gas on your data logs/afr gauge. To figure what we should be seeing on our gauge, take the target afr/wideband stoich x the race gas stoich and that gives us our afr....12/14.7x13.4=11afr....So the tunes commanded afr of 12:1 is 1 point leaner than it should be on race gas. Yes our cars can make plenty of adjustments but I can see this being an issue, especially on a high hp car pushing the factory limits . There are several unleaded race fuels that have a lower stoich, each with various chemical markups. At what point do we say “ok, better safe than sorry, let’s get a tune for said race gas?” I can analyze this to death, but there are people smarter than me that probably have the answers lol. This is a pretty common practice, so I’m curious. The target afr’s I’ve used are just an example, so please don’t tell me they aren’t optimal. If I’ve dicked this up, feel free to drop the knowledge on me.

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[Jay-rod427]

You are correct, and either people get wrong final afr for the fuel, buy fuel as close as possible to pump gas stoich, or have a tune written for the blended fuel with the proper stoich

 

[TX-Ripper]

Lambda 1 = stoich

.820 is a .180 enrichment of 1.0 lambda

Doesn’t matter if your fuels stoich afr is 18:1 or 14.7:1 or 9.85:1 all = lambda 1.0

Our cars do not command or target an air fuel ratio...

only lambda

 

[Avispa]

Lambda 1 = stoich

.820 is a .180 enrichment of 1.0 lambda

Doesn’t matter if your fuels stoich afr is 18:1 or 14.7:1 or 9.85:1 all = lambda 1.0

Our cars do not command or target an air fuel ratio...

only lambda

Indeed, that's what the oxygen sensor detects. It doesn't know what fuel ya got in the tank.

 

[markmurfie]

Just because all the tables in the tune are in lambda, doesnt mean its not commanding or targeting a certain air fuel ratio.
There is still a reference AFR that all the fuel calculations in the ECU use. A flex fuel tune expands this based on error that shows up in fuel trims. With out a flex tune, what you will see, when using the wrong stoich that the calibration has, is fuel trim error.

The stock calibration is setup for E0-E15 stoich as indicated on the fuel cap. This is 14.7 stoich down to 13.8, any stoich point lower than that you should consider a tune for the fuel as the fuel trims will start to be too far off for optimal performance. A flex tune wouldn't just cover ethanol content of E0- E85, but any fuel with the proper octane rating in that stoich range.

 

[chops44]

See, I was thinking wrong, thanks for correcting me. I am so used to thinking in afr’s, I only assumed the factory tune was afr based. Glance at the dash and see an afr gauge lol. I now know that gauge is a direct reflection of the current lambda value and is not stoich based off a certain fuel. I understand the lambda tuning approach and it really makes more sense with the varying quality of fuels. Also if one adds a race fuel with a big stoich difference than their car is used to runnning, they would need to give the tune time to adjust so the stft aren’t trying to make huge corrections under wot. I would assume the calibration was written for a ~14.1 fuel, that’s why a retune would be optimal.

 

[Jay-rod427]

See, I was thinking wrong, thanks for correcting me. I am so used to thinking in afr’s, I only assumed the factory tune was afr based. Glance at the dash and see an afr gauge lol. I now know that gauge is a direct reflection of the current lambda value and is not stoich based off a certain fuel. I understand the lambda tuning approach and it really makes more sense with the varying quality of fuels. Also if one adds a race fuel with a big stoich difference than their car is used to runnning, they would need to give the tune time to adjust so the stft aren’t trying to make huge corrections under wot. I would assume the calibration was written for a ~14.1 fuel, that’s why a retune would be optimal.

Not quite. It gets it's afr reading as a calculation from lambda AND what it's been told is the stoich. For example put in E85 with a flex fuel tune. Lamda wot is still .8 or so, but afr will be like 9.5.

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