[Discovette]

Hello, I have a question about how the software works in regards to gasoline versus E85 AFR's. I've tuned my engine on gasoline and my cruise AFR's are close to stochiometric at 15 to 1. I had just filled up with E85 which put my alcohol content at 55 percent last night and I noticed my AFR's were reading virtually identical and the car ran perfectly fine. So my question is, does the computer adjust actual AFR's to make it compatible with the gasoline scale or am I over lean when I am showing approximately 15 to one on cruise and 12.8 under WOT with the E85?

[jonbill]

Really its reading lambda and mapping that to gasoline AFRs. I.e. reporting same AFR really means its the same lambda, which means its working fine.

[PSIG]

Lambda * fuel factor = AFR displayed
Lambda is 1.0 for stoichiomteric of any fuel, and is the scale the sensor reads. If whatever you're burning is at stoichiometric, it will read Lambda 1.0.

Gasoline stoich is 14.7:1, which is Lambda 1.0 for that fuel, so the factor is 14.7. The ECM or gauge optionally displays Lambda, or a factored value of Lambda. If burning a different fuel, and it is at stoich, it will read 14.7 if this factor is applied (in the calculation at top).

Most tuners are now using Lambda directly instead of a factored fuel AFR, as it always represents equivalent rich-stoich-lean for any fuel or blend. This example Lambda-to-AFR chart shows relative values. Methanol alcohol stoich is 6.4:1. But if your setup is factored for pure gasoline/petrol display, and you are burning pure methanol alcohol (or any other fuel), at stoich it will again read 14.7:1 If set to read lambda, it will read 1.0 at the same value:

Lambda to AFR.jpg (204.95 KiB) Viewed 3835 times

[Discovette]

Thank you PSIG, that clears things up a great deal. I think I will switch to display Lambda since its obviously a better scale to use than AFR.

[PSIG]

That's a personal choice of what you can be comfortable with, but is really no different to learning to use kPa and cc/min versus inches of mercury or lb/hr. Also keep in-mind when others say an AFR, and you are not yet used to it, you can divide the AFR by the gasoline factor for the Lambda, e.g., 12.5 AFR / 14.7 = 0.85Λ (Λ is Lambda, sometimes seen as λ or L). Likewise, Lambda can be used as a direct percentage, so if you want to lean 10% from stoich, 1 * 1.10 = 1.1Λ. Have fun!