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Reading AFR Gauges For Pump Gas VS E85

Understanding AFR

Discussion and questions related to the course Understand AFR

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After taking the Understanding AFR course, I still have a lot of confusion when it comes to interpreting an AFR gauge when switching fuels. For example, lets say my innovate gauge is programmed for pump gas and to display in AFR. I would target roughly 14.7 in idle and cruise, and roughly 12 at WOT (turbocharged engine). Without changing the programming of the gauge, how would I know what the proper AFR would be on E85?

Same thing if I switched the programming to convert for E85 Stoich? What would I be targeting and interpreting at the gauge?

Lastly, I believe some AEM gauges natively read in Lambda but have a set conversion to read in AFR? What does one do in this case when switching fuel? I'd assume in this case we can still follow the "same" method when on pump gas as the gauge is a direct translation from Lambda??

Sorry for possibly being all over the place, I'm just quite confused on the interpretation.

Thank you in advance.

P.S. I am about to begin the E85 & Flex Fuel tuning course, maybe it'll clear things up. I'd still like a personal response, it would help. Thanks again :).

Ok so first things first, every wideband sensor natively reads in lambda. The sensor doesn't know, or care, what fuel it's running on. Irrespective of the fuel you're running on, lambda 1.00 is always the stoichiometric AFR. From here the gauge/meter will multiply the lambda reading by whatever is programmed as the stoich setting in order to display AFR. This is why i recommend learning to tune in lambda.

So to be more specific, E85 and pump gas tend to work well at very similar lambda targets. For this reason you can (and most tuners do) just leave the wideband stoich setting at 14.7:1 (correct for pump gas), and tune for the same targets you'd use on pump gas. This is technically wrong, but it will get you the right result. If you want to be truly accurate then the proper approach would be to change the stoich setting to suit whatever blend you're running - 9.8:1 for E85 for example. If you do this then the target is different. For example let's say you will be targeting lambda 0.80 on both pump gas and E85. on pump gas 0.80 lambda = 0.80 * 14.7 = 11.76:1. On E85 this becomes 0.80 * 9.80 = 7.8:1.

Hopefully that clears up your confusion.

Thank you for your reply Andre!

I do prefer lambda ! Good to know all sensors natively read lambda. This fundamental on its own helped clear it up.

Essentially, no matter the AFR conversion setup on the gauge, stoich AFR is always 1.00 lambda, so we can still target same AFR values as long as we understand the conversion being used. Am I correct?

no need to target a 7.8 AFR (with a pump gas conversion) when running e85. This will only be correct If the gauge was converting 1.0 lambda to be 9.8?

Please correct me if I’m wrong, but I believe I understood correctly.

Lastly, I have never ran e85, hence taking the courses first haha.

If I went from pump gas at .8 lambda. And made zero tune changes and put e85. (No AFR table targets, solely inj. Pulse width.). The engine would run Lean? Just confirming. Sorry.

Thank you again.

Leaner, might still be chemically rich, as I can't recall the comparative fuelling ratio.

As Andre said, use lambda - there may be an option to change the guage readout - or forget trying to work out conversions and go by the pure petrol/gasoline numbers for relative changes.

Found this comparison chart when researching for a different, but similar, question -

Thank you very much Gord