Oft Stage 2 e85 switch over question

[bdtbdtbdt]

When I switch from 93 gas to e85, there is always 1 or 2 gallons of gas left in the tank mixed with the e85. Should I re tune the car after a tankfull or 2 of e85 is run through because the tank is full of "pure" e85. Or does the OFT constantly re learn, re adjust?

[Gloonge]

Originally Posted by bdtbdtbdt

When I switch from 93 gas to e85, there is always 1 or 2 gallons of gas left in the tank mixed with the e85. Should I re tune the car after a tankfull or 2 of e85 is run through because the tank is full of "pure" e85. Or does the OFT constantly re learn, re adjust?

You don't have to re learn. The e85 tune is expecting certain results and when it doesn't get those results it adds a little more fuel to compensate. Once your tank gets more and more pure e85 it will dial that back accordingly.

[BigTuna]

Short answer: No, you do not need to re-tune. The ECU make fuel trim adjustments as well as it can based on A/F ratios. It will run better once the mix is flushed out, but the OFT doesn't tune based on ethanol content.

Longer answer: An OFT E85 tune is kind of like putting the cart before the horse in a fueling sense. A proper flex fuel system changes the map to always produce max power based on the ethanol content measured on it's way to the engine. On the other hand, an OFT tune is written to expect a certain ethanol content, and has to adjust fueling based on the A/F ratios as measured by the A/F sensor (pre-cat "O2 sensor"). Therefore you will only have max power when the ethanol content is exactly what it is expecting. The ECU can adjust to a certain degree, and this is why it works, but it's not the "best" way to run E85.

Also, if anyone knows better than me, please feel free to correct/educate me. This is just how I understand it.

[DarkSunrise]

Originally Posted by BigTuna

On the other hand, an OFT tune is written to expect a certain ethanol content, and has to adjust fueling based on the A/F ratios as measured by the A/F sensor (pre-cat "O2 sensor"). Therefore you will only have max power when the ethanol content is exactly what it is expecting. The ECU can adjust to a certain degree, and this is why it works, but it's not the "best" way to run E85.

Shiv mentioned that MBT is achieved at ~E60 on the FA20, so as long as the ethanol content at your E85 pump is between E60-E85, you should have enough detonation resistance to max out your ignition timing and hit MBT. Ignition timing aside, within that E60-E85 range, the tune should adjust for ethanol content using fuel trims as you mentioned.

As long as you're filling up solely on E85, I don't see much of a reason for a flex-fuel setup unless you don't trust the ethanol content of your local pump to be above E60.

[churchx]

I'd reduce trust in winter. I suspect that ethanol content might get reduced for easier starts on E85.
And isn't it other way around? As in, if more ethanol content then e60 doesn't net any improvements on FA20, then shouldn't it be 0-60, at which to adjust trims depending on content?

[DarkSunrise]

Originally Posted by churchx

I'd reduce trust in winter. I suspect that ethanol content might get reduced for easier starts on E85.
And isn't it other way around? As in, if more ethanol content then e60 doesn't net any improvements on FA20, then shouldn't it be 0-60, at which to adjust trims depending on content?

I've heard of reduced E content in the winter, but haven't had a chance to test. All of the pumps I've seen here are labeled as being minimum 70% ethanol, so even in the winter the range should theoretically be acceptable (E70-85).

Above E60 won't net any more ignition timing, but that's different than the fuel trims. Fuel trims will attempt to compensate for ethanol levels, even in the E60-E85 range (and of course below that too). There are limits to the fuel trims though and too much will eventually result in throwing error codes.

Long story short, the OFT E85 tune should be fine anywhere from E60-E85.