As far as I know, there is no 'octane sensor' on any modern EFI engine. It's just a KNOCK sensor(s) that adjust timing according the knock being sensed. So even though engine computer may be tuned to run 87 Octane, the higher octane will allow the computer to keep timing advanced within the parameters programmed in the computer, without pulling timing to keep engine knock under control. So the increased performance from running a higher octane is coming from the engine not pulling timing.

Also, Shell in Canada does not add any ethanol to their 91 octane fuel marketed as V-Power here in Canada. I've done the calculations, and the price difference is about 10% between 87 and 91 octane. The mileage difference is about 10% greater running 91 octance with no ethanol vs 87 with 10% ethanol.

 

Good point RickyBobby... I have noticed my mileage on E10 blends SUCKS compared to pure gas. Someone mentioned this earlier... since the PCM can adjust for E85, where is it getting the info that it's got E85 to burn? Is there a bouyancy difference? Is this sensed in the fuel tank? If that's the case E85 tune simply runs when ethanol is in the tank... so gasoline of all octanes will run on the same tune while E85 would have it's own factory tune. Or does it make the determination based on knock sensor data? If that's the case, you should see better performance on higher octane fuels.

All interesting stuff... I put a tank of E85 in mine about a month ago... talk about waking up the beast. It got so much better power, I couldn't stay off the throttle. Sure was fun to drive, but 11 MPG was not worth it in my opinion. When I turned off traction control, I could easily smoke the tires from a stop.

 

From info that I have seen, vehicles equipped to handle E85 don't just have different tunes or pcm and etc. They are also designed to safely handle more ethanol. Ethanol is very corrosive and will completely destroy certain metals and rubber.

I have noticed this in my older Honda lawn mower. I have had to replace the fuel line a number of times since ethanol has been blended with gas.

 

Yep, I had to replace my bay boat's fuel tank due to corrosion caused by E10. I learned the hard way to watch where I buy boat gas and not purchase gas with 10% ethanol.

 

In most FFV's, E85 is detected by electrical conductance/resistance. Regular gasoline is not electrically conductive at all, E85 is. There are sensors in the fuel system that measure the electrical resistance of the fuel. The ECU then adjusts the pulse width of the fuel injectors accordingly to provide more a rich mixture with E85, a leaner mixture with regular gasoline. In some vehicles the ECU may also adjust the ignition curve accordingly since E85 has an octane between about 100-105. Not all FFV's have the same abilities. I dont know if the F150 adjusts the ignition curve with E85.


Most of the added power comes from the ability of the E85 to run significantly richer. Unfortunately besides having less energy potential per a given volume contributing to poorer MPG, the richer mixture also contributes to less MPG.

 

I found a good response to these debates...

http://www.fordf150.net/forums/viewtopic.php?t=74426