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Limits of E85

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E85 tuners... what's the leanest ratio on a boosted platform you've seen/tuned which hasn't melted down?

I've had a few cars come through pushing ~1.5 bar boost or more, with ratios at or above lambda 0.9... but was a little taken aback by a high compression car this week which hit lambda 1.0 toward redline. No doubt this wasn't ideal for power, but I'm impressed that it still contained pistons instead of aluminium slushes.

I continue to be surprised just how forgiving E85 can be.

Stories of melting pistons on E85 also welcome, let me know the conditions.. boost level? power per litre?

Well the air fuel ratio is only one aspect to consider and interestingly a very lean air fuel ratio isn't necessarily as dangerous as you may think. If you consider how the EGT (which is a function of combustion temp) changes as you vary the AFR, you'll find that the max EGT is reached near peak power (specifically a little on the lean side of peak power) and as you richen or lean out the AFR from this point you'll see EGT drop on both extremes. For this reason if you were running as lean as lambda 1.00 you'd find the combustion temperature would actually be lower than if you were at lambda 0.90 for example. As you've alluded to, you'd also be making less power.

The usual reason for failure isn't actually due to the lean mixture on its own but more to the point its the higher combustion temps making the engine more prone to knock and the failure is actually related to knock. With E85 this is less of a concern however E85 is not immune to knock as many think so some care is still required.