Plain old gasoline needs a 14.7:1 air to fuel ratio (by mass) in our engines. Ethanol contains some oxygen and requires less air so the proper air to fuel ratio for ethanol is 9:1. If my math is correct, you'd need 6% more fuel to get to the proper air to fuel ratio with E10, and 9% for E15.
Modern car engines have closed loop fuel injection and will adjust the air to fuel ratio on-the-fly based on the oxygen content in the exhaust. Boats engines, even those with fuel injection (except the catalyst equipped models, as far as I know) don't have the ability to adjust air to fuel ratio automatically. They run open loop and simply calculate the ratio with a fixed table that injects a specifica mount of fuel for a specific amount of air. What this means is that on E10, we are running 6% lean and on E15 we'd be running 9% lean. This is going to affect HP for sure, and potentially engine longevity since lean conditions cause excessive heat.
Has anyone ever had a ECM re-programmed with a richer air to fuel mixture? I'm thinking I'd like to do this on my Mercruiser 7.4 MPI engine, if we get stuck with E15.
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