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I have seen in many tuner forums and recently read a document by Delphi in which various fuels were testes on NA Engine subjects various compression ratios.
Base on the results running anything higher than e50 yields no benefits or insignificant gains if any, now I personally seen no difference running anything higher than pump E70 but I have also tested different blends like FTW E85 and VP C85 and there is a considerable gain especially on turbo engines just by switching fuels.
Are these blends allowed more power to be made due to its higher octane levels 115-120 to 103 on pump e85? Since the anti-knock index difference is almost identical.
That doesn't surprise me on a naturally aspirated platform. There will always be an earlier point of diminishing returns by increasing static compression ratios versus dynamic compression ratios. Add to that the fact that ethanol has far less specific energy than gasoline and you'll reach a nice happy medium. Ethanol isn't some magical power adder, though it does have its place in a properly engineered engine control strategy.
My experience has been that the majority of the anti knock improvement comes as you move from E0 through to about E40-E50. Beyond this point I don't tend to see much change which fits with the testing results. There may be gains in cooling from the higher ethanol content on a turbo engine and this may not be so apparent on a N/A engine. Some of the specific race blended ethanol fuels may have other additives that offer their improved performance over pump E85.