Something I found in another forum.
Even those of you running standard pump gasoline are probably getting a mild alcohol blend. By law, most gas stations today are now allowed to sell blends of as much as 10% ethanol as "gasoline". The ethanol is being used to displace the MTBE (methyl tert-butyl ether), which has its own environmental and health concerns. It used to be that stations were required to post a sticker on the pump declaring the alcohol content, but that's no longer happening. Some regions were quicker to adopt the maximum allowable ethanol content in the pump gasoline. In my travels and discussions with shop owners and calibrators across the country, I've found that almost every station can be assumed to be selling some alcohol blend unless stated otherwise. In most cases, this means assuming a blend of 10% ethanol. Any addition of alcohol blends to the gasoline will shift the stoichiometric balance point. The more oxygen carried by the fuel, the richer the stoichiometric point will be. Where E85 has a stoichiometric ratio of 9.85:1, E10 (most pump gas blends) will balance at about 14.2:1. This means that tuning to an assumed pure gasoline composition with a stoichiometric point of 14.64:1 may include a 3% error before the engine is ever started. The wideband oxygen sensor will still typically display "14.64:1" at lambda=1.00, even though this is really an actual ratio of 14.2:1 with a 10% ethanol blend. Three percent may not sound lick much, and it really isn't, but the idea behind engine calibration is to get all variables as close to optimal as possible. Leaving a 3% error in the fuel's stoichiometric point just makes tuning the volumetric efficiency table, startup fuel, and transient fueling that much less accurate. It's a good idea to just adjust at the beginning before baking that error into every other calculation later on.
Well I'm finally getting a chance to drop back in here and I had a request to chime in on this thread a while ago. It seems that in the mean time, you guys have more or less cleared it up, but let me restate a little differently for those who may still have some confusion.
The Stoich point is primarily there for OPEN LOOP operation in order to define the balance that is lambda=1.000 for a given fuel. Changing the stoich point will skew the desired fuel mass prior to the EEC calculating the pulse width necessary to deliver this target fuel mass. If you have a known air mass (MAF reading), then the stoich balance determines the desired fuel mass that will go with it. Obviously, the chemistry of the fuel drives the balance point.
In closed loop, the EEC just looks to the narrowband HEGO to see which side of the balance it's on. It will push the fueling one direction or the other until it sees a switch in voltage that indicates crossing the real stoich point. Changes in fuel blends will just require more or less adjustment from the reference point (stoich point in the Open Loop cal) to toggle around the real chemical balance. If you define the calibration's stoich point equal to the real chemistry of the fuel, there's not much learning needed as long as your MAF and injector cals are good.