Utinator

I have a question(s) about what AFR *should* be used for fueling. For this discussion, let's assume that we are all using pump gas. Pre-2005, gasoline was 100% gasoline. Now, it's more like 90% gasoline, at best. With 100% gasoline we should be using 14.7 AFR.

My question is, should we be using something less than 14.7 for today's 90% gasoline?

I noticed a table (Stoich AFR) in my HP Tuners program. It has AFR values ranging from about 14.7 to 8.7. It also has percent ranging from 0.0 to 100. How does the PCM use this table?

Hardtop Brian

I think mine uses 14.62/3 for stoich.

The Stoich AFR table sounds like a flex fuel table. If your setup doesn't have a flex fuel sensor, then I wouldn't worry about it.

​​I found this searching "Stoich AFR table':. https://forum.hptuners.com/showthrea...FR-table-to-me
Not sure if it's allowed to link other forums. Mods, please remove if it isn't.
isn't.

ChopperDoc

I don't think the 10-15% ethanol is going to cause the AFR to drop all the way down to 8.7. The table I think you are referring to is the Stoichiometry Table under the fueling tab. As stated above, this is based off of flex fuel (alcohol) percent present in your fuel. At 100% alcohol it's 8.97:1, 0% is 14.67. That is off of a 2014 Camaro, stock table. Here is the description of the table per HPT:

[ECM] 12300 - Stoich AFR: This table is used for vehicles with the dual fuel option (alcohol). The base stoichiometric fuel/air rate is determined by a returned value from this table. The position on the table is determined by the amount of alcohol in the fuel system as reported by the vehicles fuel composition sensor. The rate ranges from 0% to 100% alcohol.

Also, you can set this as desired for your application. If you want the engine to target something on the richer side, you can change the values (lower them slightly) to meet your needs. Something like 14.6, or 14.55 isn't uncommon really, especially for higher compression engines. This will just cause it to run a little on the rich side, and target that in closed loop. Even my WB thinks 14.7 is on the lean side since it goes into the yellow there lol. Of course this only *really* matters under high loads, i.e. WOT.

ChopperDoc

Also, what PCM are you running? If it's the 0411, then I don't know why you even have that table. I have a 98 with a 2002 0411 and I do not have that table. The 98 PCM did not either. That's a table on newer cars, not 98-02 f-bodies. I'm actually now scouring through other tunes to verify, but so far, never seen the flex fuel table on the older PCM's. And along with what was said earlier, unless you have the sensor, it's not going to work anyway. You can run alcohol, but you're gonna have to tune for it, same goes for any other fuel type like E85, etc. You don't have the nice little sensor to do this for you through a table. It has to be manually accounted for.

ChopperDoc

Forgot to add, that the 14.7 thing only applies to normal driving conditions. The general rule for Power Enrichment (PE) is 12.5:1 for N/A and 11.5:1 for boosted applications. That's also a table you should touch on, remove the delays, and on higher power setups, move up PE mode a bit since you will be adding much more stress with a modded engine than what your stock tune was intended to compensate for. The sooner you get into PE the safer you are from detonation and pre-ignition. You might be surprised at how long it takes for a stock tune to hit PE mode if you watch your commanded AFR in the scanner. You should be logging this anyway if you are trying to tune fuel.

Just a thought, but if you are making 550+ WHP and you get into the pedal 50% on the way to WOT, you are already basically putting down the same amount of power the stock tune was written for. Error to the safe side and get some more fuel in there sooner than later. It doesn't hurt anything other than the environment lol.

Utinator

Thanks Doc. I was actually looking at my parent's Surburban (2003) this past weekend. I also scanned the pastor's Yukon (2005, I think). Both vehicles have the 5.3L. Both engines have a random misfire. They are not setting off any DTCs, but I can see the mis-fires with the VMC Scanner. I'm wondering if it's possible for the 10% Ethanol to cause this in an engine not tuned for it. My engine has zero misfires, but it was tuned by Frost last year. He has the Stoich AFR set to 14.63. It doesn't have the flex-fuel table. The flex-fuel table was in the 2003 Surburban PCM. It's not a flex-fuel Surburban though.

Both of the surburbans I scanned are completely stock. So far, I have only played with the PE tables, and the idle on my parent's surburban. I'm not sure about the PCM #'s, but they are all original to the vehicles.

LilJayV10

iTrader: (39)

No that won't cause a misfire. There are millions of vehicles out there with a stoich of 14.7 that run just fine on E10. The fuel trims are going to take care of small difference.

ChopperDoc