I have a daily driver tune set for 93 octane that. When the truck has 93 in it, I see Lambda and AFR's right at .82 and Spark Advance of about 28.5-29 at WOT. When I run E85, I see both LAMBDA and AFR's of about .85-.86 (a little richer), and I see lower spark advance, usually right at 27.5. I'm just kind of curious as to why the tune would be set to run lower timing on E85? With either fuel in the tank the knock sensors are very happy and will go as far as -5 or so. Thanks for any replies.

 

E85 has a faster flame front and runs less timing than 93oct to make peak power, E85 is also more knock resistant and can run more timing than 93oct before knock occurs. It sounds like your setup isn't knock limited so you probably aren't gaining much by running E85. On my MPT tune my tune would normally run 18-22 degrees of spark advance at WOT past about 3500RPM when running 93oct, when running E85 I would see 22-25 degrees of spark advance so I was gaining power by running it. In your case your running less timing with E85 because you were already running the optimal amount of timing with 93oct.

 

Is there a power benefit being able to safely run a little leaner .86 compared to .82 at similar timing?

 

I can't really speak to what the ideal lambda is on different fuels for a coyote because it varies so much from one engine design to the next and I just haven't spent any time playing with this on our platform. Some engine designs make the most power running 13:1 others make the most power running 10.5:1 running the same fuel, but if I had to guess I'd say your tune is set to run a little leaner at 0.86 lambda on E85 vs 0.82 lambda on 93oct because of how knock resistant E85 is and because it has a greater cooling effect on the cylinder, both of which lead it to not require as much extra fuel to add an extra margin of safety. In this sense you may still be getting a slight gain from running E85 even if you were running the ideal amount of timing on 93oct.