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Canadian Daves JustKDX
93 octane.....pros vs cons?
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[QUOTE="Rich Rohrich, post: 1338735, member: 16241"] You are working under a fundamental misconception here. The volatility of a fuel is determined by it's distillation curve and the Reid Vapor Pressure. You can have two fuels with the same distillation curve (volatility) yet wildly different octane ratings, and they will make the same power. On the flip side, you can have two fuels with the exact same octane spec yet very different distillation curves and volatility characteristics, and they can potentially make different power and have much different throttle response. The distillation curve of a fuel is critical to performance and good tuners match the fuel's curve to the engine spec and more importantly the engine application. Two fuels with the exact same pump octane number can have very different knock characteristics in a running engine if the distillation curve is modified certain ways. This is especially true in two-strokes where a change in the tail end of the distillation curve can be used to provide piston crown cooling and help control some of the precursors of detonation. Octane rating is just a very small and mostly over-rated part of a the fuel picture. A fuel's 10-50% temps on the distillation curve and the shape of the curve will have a huge influence on throttle response, and the 90% to end point temps will have an influence on piston cooling in two-strokes and high rpm (over 6000 rpm) power in all engines. In the case of the original poster's question. The only consistent advantage of running 87 octane is cost. The obvious downside being an engine that works well on 87 octane today may see a variance in fuel week to week, or a change in operating conditions that makes 87 octane inadequate. Once that happens two-strokes get destroyed pretty quickly. Seems to me like a foolish way to save money. The only downside I can see to running premium (93/91) other than cost is the chance that in you area premium will have a higher percentage of Ethanol. This poses a problem for a couple of reasons. Ethanol has a fixed boiling point, so large quantities have a major influence on the fuel's distillation curve, and the blenders have to account for this in other ways. For high speed engines (like motorcycles) that see major swings in rpm and throttle openings, this is a bad thing. Closed loop EFI cars (the intended market for pump fuel), have long very hot intake tracts and run much lower rpm, with much smaller, less frequent throttle transitions, so it's not a problem. Ethanol also has an impact on the octane distribution within the fuel's distillation curve. This can cause knock issues in some cases. The moral of the story? If you have any sense you'll run something other than that swill they pump at the local gas station. :) Seriously, if you are running pump fuel run the highest octane you can get and give yourself the safety margin it provides. It's cheap insurance. [/QUOTE]
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Canadian Daves JustKDX
93 octane.....pros vs cons?
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