e85 Stoich value equations

[Joe6pt0]

I've tuned several cars including my own on e85. When I first started, I couldn't find an agreed upon stoich value for E85. I've seen everything from 9.7 to 9.765 and up spread around the internet. I used a little easy math to calculate it and have used 9.85 ever since.
Assuming 100% gas is 14.7 or 14.68 (depending how detailed you want to get) and assuming 100% Ethanol is 9.0, the following is true:




And for those that have had E85 changed to "winter blend" of e70, the 10.7AFR that has spread around the internet seems true. I purchased a Kent Moore professional fuel compensation tester and as of November 09, the E85 in central indiana had not changed from 85%. I parked my GTO for the winter so I didn't test any deep winter fuel.

[KLUG'S SS]

Up here in Michgan right now the E85 is right at 70%, I tested it.


I am running a stoichiometric value of 10.7 as well and everything lined up pretty well as far as fuel trims go.

[LazMan]

I use 9.832520 when I run E85. I used GM's stock table and interpolated the value between what would be E81.25 (which stock is 10.046875) and what would be E87.50 (which stock is 9.689941) on my 2009 GXP.

The computer isn't set up for FlexFuel, but the values for this table are there, and accurate.

I also use 14.108398 for E10, which I extrapolated from E6.25 and E12.50 in the same way.

-Laz
PS: Basically mine is just ever so leaner richer for E85, probably insignificant, and just about the same value for E10. Certainly 9.7 is WAY off.

[Joe6pt0]

I use 9.832520 when I run E85. I used GM's stock table and interpolated the value between what would be E81.25 (which stock is 10.046875) and what would be E87.50 (which stock is 9.689941) on my 2009 GXP.

The computer isn't set up for FlexFuel, but the values for this table are there, and accurate.

I also use 14.108398 for E10, which I extrapolated from E6.25 and E12.50 in the same way.

-Laz
PS: Basically mine is just ever so leaner richer for E85, probably insignificant, and just about the same value for E10. Certainly 9.7 is WAY off.

That's awesome because that's EXACTLY how I did it the first time too I used a factory Tahoe flex-fuel cal to interpolate to 9.8333. After doing the simple math above (and realizing it was close enough) I decided GM might have been compensating for something like they do in the early LS1 ECUs (14.62 afr) so I went with the above table. Plus I tuned a guy's car one time and the fuel tested e80 when I tuned it so I had to compensate a little more using the table above.

Great to see some people think alike

[Bob H]

I'm curious why you don't use lambda? It completely eliminates the small variances in blends - because your E85 might be E83 or E87 in actuality? Stoich = 1.0 regardless of fuel.
I ask because I went through a long set of calculations for a friend running meth with his turbo - and found out AFTER that I could switch the system to read lambda.... Of course, meth injection is a whole different animal anyways, but it was where Greg B. set me straight finally.

[Joe6pt0]

I'm curious why you don't use lambda? It completely eliminates the small variances in blends - because your E85 might be E83 or E87 in actuality? Stoich = 1.0 regardless of fuel.
I ask because I went through a long set of calculations for a friend running meth with his turbo - and found out AFTER that I could switch the system to read lambda.... Of course, meth injection is a whole different animal anyways, but it was where Greg B. set me straight finally.

Think about this for a second....target stoich in your cal file IS lamba = 1.0. That is the entire point of this discussion. We are not talking about tuning WOT.

[Bob H]

Think about this for a second....target stoich in your cal file IS lamba = 1.0. That is the entire point of this discussion. We are not talking about tuning WOT.

I guess I could have asked it better - I understood you were not talking about WOT,(and noone tunes to 1.0/Stoich at WOT anyways), and now that I've thought about it a little I may have figured out why you were trying to get the AFR for that particular blend. I haven't tuned a Flex Fuel car or for a particular blend with HPT yet, hence my question. The Meth calc I was doing was on a stand-alone.
Given enough time, the computer will adjust for the new blend. But I think that is why you are trying to figure out the AFR - so you know where to start with the fuel maps.
Is that why you need them?

[LazMan]

I think that is why you are trying to figure out the AFR - so you know where to start with the fuel maps.
Is that why you need them?

Thats exactly right Bob. A non-flex-fuel computer won't compensate for 60%+ fuel trims (from 14.7 to 9.85 for example) before throwing up a code.

Even for say E10, its nice to have the starting point for the computer's math be the actual stoich value for the fuel you are using. Keeps the fuel trims nice and small, and accurate, both at 1.0 (closed loop) and for example 1.15 PE (0.87 lambda)

-Laz

PS: Joe, I tune for E10 now whenever I touch a car, ever since Florida went to E10. Better to trim down (say if the fuel is E05 or whatever) than to start off lean imo.