E85 has a 104 octanerating, compared to the usual 91 or 92 rating for premium gas.
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Nesbitt says the octanerating equivalent for natural gas is 130, so the engine required specially forged high-compression 12.5:1 pistons and high-lift, short-duration camshafts.
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OctaneRating: The octane rating of a fuel determines the propensity to explode rather than simply burn when mixed with air and ignited.