How to Determine Your Optimal Octane Rating.

First: if you are running any pistons manufactured by Yamaha and you haven't modified the deck height or head surface, you can run regular gasoline without problems. The highest compression you're likely to achieve by piston and head mix-and-match is going to be less than ten to one. Probably substantially less. Stock compression ratios run from 7.9 to 9.0 to one. Low enough to warrant regular. Carbs and pipes are likely to actually lower this requirement. The only thing you can do to make stock pistons and heads need premium is to 1) run long cam durations with little to no over-lap (ie, extremely wide lobe centers) this is very unlikely, as most cam grinders simply are not set up to make such cams. 2) Use some sort of forced aspiration. Neither of these are wise without changing to forged pistons and/or modifying the heads and valves extensively.

Technical manuals will start their explanations of octane by saying that it is simply the ratio of heptane and nonane multiplied by one hundred. Most folks don't find this particularly useful. In its most basic terms, the quality that we refer to as octane is that substance's' (presumably some sort of fuel) resistance to ignite spontaniously. So fuels such as methanol and nitromethane can be spoken of as having an octane rating although they containe neither heptane nor nonane - both of which are constituents of petroleum distillation.

Let me pause here to reinforce an idea into the reader: If you increase your known compression to over 10:1 - regardless of the cam(s) you run - always run the highest octane fuel that is readily available. This will usually be 91. 93 if you're lucky. I remember back on the farm, the smaller tractors were flathead gasoline powered units and they never showed the least tendency to ping or detonate, even though the fuel's octane was sometimes as low as forty. High performance for these was 6:1. If your cold cranking pressure is over 180lb/in^2, find a station that sells high performance fuels - just in case. From where I am typing there are two such staions selling 100 octane fuel within ten miles. One of these even sells 110 octane. I recommend against going to a small airport and talking your way into fuel there. First, it has no "road tax" applied to it. It is unlikely that you would ever get in trouble, but if you do it is a federal violation to run it on the street. Secondly, aircraft operate at altitudes higher than any roads you are likely to encounter. Light aircraft must run quite a bit of compression to get any performance at all. And this compression - combined with a lot of ignition advance - requires fuel with octanes around (and above) 120. Unless you run under boost all the time, the fuel simply burns too slowly for your engine.

That being said, if you plan on running 12:1 or higher or on boost a lot of the time, run at least half 100(+) octane fuel. It might be wasted money, but better twenty dollars too much than four thousand.

More to come . . .

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