Stoichometric / Fuel Table

[Willie]

I'm going to use representative numbers here to ask my question.

I have stoich in my tune set at 14.64 and my fuel table commanded lambda at load / rpm at 0.8 (or 11.7 AFR). I change my stoich to 14.08 (for 10 percent ethanol). So to maintain an 11.7 AFR, I would need to change the fuel table lambda to 0.83. I'd just like verification this is correct. Thanks all..!

 

[smashedheadcat]

You’re right, but why would you want to go back to 11.7? Going from .80 to .83 is leaning your car out by almost 4% no matter the afr number. Let it be.

 

[Willie]

You’re right, but why would you want to go back to 11.7? Going from .80 to .83 is leaning your car out by almost 4% no matter the afr number. Let it be.

This is why I asked. Follow me on these calcs:

Stoich = 14.64:
(14.64)(0.80) = 11.71

If I change the stoich and leave the commanded lambda as is:
(14.08)(0.80) = 11.26 (Too rich)

Stoich = 14.08:
(14.64)(0.83) = 11.68

So I need to change the commanded lambda to 0.83 to keep it at 11.7...

Willie

 

[Willie]

Double post...

 

[stang910]

Your going to melt a piston tuning in stoich. Once the stoich is set in the tune let the PCM do the math. Forget stoich AFR readings and tune in lambda. Running leaner than .82 lambda on a FI/pump gas setup probably won't end well. 0.80 lambda is a good number to be at.

So your aware, your AFR gauge reads in lambda and converts to whatever its set for. If you change your lambda to .83 your AFR gauge will read 12.2 and not the 11.7 you think it should read. You can switch to a 9.85 stoich in the tune, but your AFR gauge is still going to read the same numbers unless you change the stoich in the gauge as well.

 

[Willie]

Your going to melt a piston tuning in stoich. Once the stoich is set in the tune let the PCM do the math. Forget stoich AFR readings and tune in lambda.

What I'm trying to ask is not so much the actual values in the tune, but hypothetical. Rephrasing, if I change the blend of gas I use and it's stoichs are different, should I change it in the tune? A second question below.

Running leaner than .82 lambda on a FI/pump gas setup probably won't end well. 0.80 lambda is a good number to be at.

Basically what you are stating is lambda for power is 0.8 for gas and for 10 percent ethanol blended gas. My question boils down to this then. Is this true?

So your aware, your AFR gauge reads in lambda and converts to whatever its set for. If you change your lambda to .83 your AFR gauge will read 12.2 and not the 11.7 you think it should read. You can switch to a 9.85 stoich in the tune, but your AFR gauge is still going to read the same numbers unless you change the stoich in the gauge as well.

I do not have an OEM wideband. It's aftermarket and in the programming of it, you set the stoich to whatever it should be, so with this gauge, the AFR displayed if you choose to display this instead of lambda should be accurate.

Willie

 

[decipha]

if you set your wideband for AFR you have to update it everytime you change fuels. If you set it for Lambda it can never be wrong.

The ecu uses lambda it doesnt use AFR. So if you set enrichment fuel to .8 lambda at WOT your commanding .8 lambda. Stoich AFR in the ecu is a global fuel modifier. As long as it matches the actual stoich for the fuel your running then your fueling will be correct so long that your fuel is dialed in properly.

this is all covered in my fuel write up

best bet is to stop using that archaic method of AFR and upgrade to using lambda and you will be a much more proficient tuner.

 

[Willie]

Perfect. Thank you. I'm in the process of updating my wideband gauge since I found out yesterday what brand 100 octane I use (Sunoco), and to appropriately change my stoich value in my gauge.

Now the next question. Is the commanded lambda for best performance different with different fuels? 91/93, 100, 104? For educational purposes, I'd like to know what the lambda should be for methanol also, since its stoich is so much lower at 6.45.

My ultimate goal, once my tune is completely dialed in (getting very close, thanks Decipha), is to create different tunes for different octanes.

Willie

 

[Willie]

I've been doing a little reading on this and it seems every engine design likes different lambdas, and the only way to know exactly what "my" engine likes is on a dyno. I use Sunoco 100 octane exclusively with a stoich of 13.9. Once I program my wideband to this stoich AND its digital display to read lambda, I'm going to initially tune it to 0.8 lambda and go from there.

Willie

 

[03cobra#2]

All good reading. My thought is it's just way easier to read your af gauge in lambda. When I set up my tune for E85 pump I set the stoich in the tune for what E85 calls for and used the maf transfer function to finish dialing in things.

At first when I set up my guage to display lambda it took a little getting used to. Now that I am I don't have to worry about changing my gauge to show the correct af everytime I switch fuel.

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