Disclaimer: This has not been thoroughly tested, and is not guaranteed safe. Take this info with a grain of salt. If you blow your car up, it's not my fault or my problem.
So awhile back i started logging the knock readings with the TQ App. On 87 Octane the car was constantly experiencing 2-3* of KR in literally every loaded situation (even light load), and running very low total timing. On 93, the car switches to the high octane timing maps, and even with the added timing KR was improved, but still had frequent, repeatable, consistent knock.
There is only one E85 station within 50 miles of me, and their prices are high because they are the only one around. It's very inconvenient for me to go there and is completely out of the way, so price aside i haven't had interest in it despite the amazing performance (proven equal to leaded 112 octane in high boost applications). Yesterday i started thinking about blending a low dose of E85 with 93 and running it on the stock tune. Winter blend is closer to E70, but we will disregard that for the purpose of this post.
Now, i'm going to get a bit technical here so bear with me.
The first thing you need to understand is that the ECU adjusts the fueling automatically based on the o2 sensors. If the fuel mix ethanol content increases, the car will run lean for a bit. The ECU will immediately compensate by adjusting short term fuel trims, or STFTs for short. After doing this for awhile, the ECU will then begin to adjust the long term fuel trims (LTFTs) in an attempt to regain homeostasis if you will, and zero out the STFTs.
Now, at full throttle the O2s aren't used, but the fuel trims are, which means while the O2 sensors are not providing information used in fueling, the information previously provided is used in the calculation.
What this means is, the car will automatically retune its fueling at both part and full throttle, for the changes in ethanol content...within reason.
If you just fill the tank with E85, the car will be well beyond the ECUs acceptable fuel trim parameters. I'm not sure what those parameters are, or what point will trigger the CEL.
I did some basic math at the gas station on my phone, and decided 10% E85 blended with 90% 93 Octane would be well within the ECUs limits, while still increasing the knock threshold by a good amount.
On the 15.6 Gallon tank, 1.5 gallons of a typical 93 octane fillup are ethanol. 1.5 Gallons of E85 is 1.275 Gal of ethanol, and .225 gal of gasoline...so adding 1.5 gallons of E85 makes it 2.675 Ethanol, to 12.8 Gallons of 93.
This comes out to E21, for a total of 21% ethanol.
E85 is 105 octane, but runs like 112 in a boosted engine thanks to its cooling properties, so 112 is the number to use.
So with 10% E85, the effective octane is just under 95 octane for the E21.
Now, E85 requires roughly 30% more fuel in most applications, but compared to the E10 we're all already burning, we're only adding 11% E85, which according to my road side cell phone math, would require 3.3% additional fuel, netting a 1.9% gain in "real" octane.
3.3% should be well within the safe margin the ECU can compensate for before it notices anything fishy. Once the LTFTs are adjusted +3.3%, the fueling at WOT will be adjusted accordingly.
Now, because E85 has less energy than gasoline, and i can't adjust the timing map to regain the lost power, i want to use as little E85 as possible to prevent knock, and no more than needed. This will allow the car to make more power, safer. Overkilling the E85 without advancing timing will cause the car to lose power, and run inefficiently.
So far i've driven about 40 miles. The first 10-15 were gentle to let the car start to adjust. Then i beat the **** out of it.
I've done dozens of full throttle pulls at any and every rpm.
Results: No CEL, and i can't make the car knock. I've tried like ****. I slowed down to 25mph, floored it in 6th gear, and didn't lift until i was well above the speed limit on the highway...not a single fleck of knock while generating as much load as possible. 6th gear brake boosting at WOT, 0 knock.
Car is on the high octane map (obviously) and I couldn't make the car knock if i had to. With 93 alone and the abuse i just gave it, i would have had consistent 1.5-2.5* of knock retard.
Next time i'll try a little less E85 and see how it runs. Like i said the goal is as little E85 as possible.
Time for Xmas dinner, best to all of you and your families this holiday season.