I'm thinking of running a gallon or two of E85 mixed with 5-6 gallons of 91 oct. My friend runs a gallon or two of E85 in his MR2 and claims his car runs a bit smoother/faster. I don't plan to mix often.

 

It really depends on your tune. If you have a rich tune, adding a little E85 will help lean it out and as an added bonus increase your octane. If your tune is already lean, then it may lean your tune out a bit too much. If you have a way to log A/F ratios and knock, you could try adding a little E85 (maybe a gallon to your next fill up) and monitor the cars response. If everything looks ok, you could go to as much as 2 gallons per fill-up. I would not recommend going beyond that without retuning for the fuel.

If you want to stay safe and conservative, I would recommend no more than 1 gallon per full tank. Adding that one gallon to 11 of 91 will increase your fuel's octane from 91 to ~92.2. Adding 2 gallons to 10 of 91 will increase your fuel's octane from 91 to ~93.3. E85 requires ~30% more fuel than gasoline, so if you are tuned for 11:1 AFR, with 1 gallon added you will lean out to ~11.2:1, 2 gallons added will lean you out to ~11.4:1. If you are tuned more aggressively than that, your A/F ratios will be even leaner.

The other concern is your closed loop fueling. If you add too much E85, you may end up with a check engine light for "system too lean". Because of the additional fuel volume required with E85, the closed loop fuel trims may go out of range trying to bring you back to stoichiometric (14.7:1 gasoline equivalent A/F ratio).

Well, this should be enough information for you to make an educated decision on what to do.

-Paul

 

In my opinion you would not want to mix the two together, because of inconsistencies. It would be to difficult to get the same octane rating each time you mixed the two.

For example, the simple change from E10 (10 percent) to normal 93 pump made a change on my wideband. Almost half a point in a/f ratios (gas calibrated).