When running a mix of ethanol and gasoline, you have to account for the difference in fuel makeup when calculating your global fuel adjustment. Some common numbers you'll need for this are provided below.
|Fuel||Stoich AFR||Gas ratio|
To calculate these values, use the following formula:
<% ethanol> * 9.0078 + <% gasoline> * 14.64
For example, for running pure E85, you would use:
0.85 * 9.0078 + 0.15 * 14.64 = 9.8526
The 14.64 figure above comes from the chemically correct stoichiometric ratio of gasoline. A lot more data can be found on the wikipedia E85 page.
You have to account for the difference listed above when calculating global fuel for a car running ethanol-based fuel. The simplest way to do this, IMO, is to first calculate a “corrected” injector size and then use the normal global fuel calculations everyone else uses.
For example, if you're running 100% E85 with 850 cc/min injectors, your injectors are flowing the equivalent of 570 cc/min injectors on gasoline:
850 * 0.670 = 570
The 0.670 figure comes from the gas ratio column of the table above.
So now you can calculate your new global fuel using the standard formulas and 570 cc/min injector size instead of 850.
450 / 570 - 1 = -21%
If you're running a mix (say 75% E85 and 25% gasoline), then you have to do a little more work. As an example, let's assume you're running 75% E85 and 25% gasoline. The new gas ratio in this case would be:
0.75 * 0.670 + 0.25 * 1 = 0.7525
With that new ratio calculated, you can calculate your effective injector flow rate the same as above (assuming 850 cc/min injectors again).
850 * 0.7525 = 640
So instead of 570 cc/min that results from 100% E85, your injectors will actually flow like 640 cc/min injectors with a 75% E85/25% gas mix. You can then calculate the global fuel same as always.
450 / 640 - 1 = -30%
As if it wasn't already complicated enough, the different fluid flow properties of ethanol compared to gasoline mean that your injectors are flowing a different volumetric flowrate! So in addition to the different stoichiometric ratio, you have to also account for a different flowrate too. This page has more details.
One of our ECMLink users, Craig Starnes, provided the following Excel worksheet for calculating various values related to running E85, particularly with a mix of gasoline. The spreadsheet allows you to enter the number of gallons of E85 and the number of gallons of gas, and also takes into account the actual E85 blend, percentage of ethanol in pump gas (usually 10% these days), and octane ratings. It provides corrected values for octane, ethanol percentage, stoich ratio and specific gravity.