I've been told the 87 will make you bike run cooler because it takes less heat to burn the 87 in your bike, but I don't know.
Actually, you're a little turned around; the higher octane fuel REQUIRES higher heat to ignite. Therefore, one would use a higher octane fuel for a hotter-running engine, not to make it run cooler. Here's why:
The octane rating of a fuel is a measure of its resistance to autoignite -- before the spark plug fires -- from heat and/or pressure. Autoignition is more commonly called preignition -- that little rattling sound you sometime hear on certain engines under acceleration -- and detonation -- a much more destructive ignition malfunction.
The mixture is SUPPOSED to burn in controlled manner, from one side of the cylinder to the other. Many of the recent power and efficiency advances we enjoy in modern internal combustion engines result directly from clever changes to cylinder design which better control the ignition event. When the mixture ignites properly, the expanding gasses push down on the top of the piston at just the right moment, and for just the right amount of time, to convert gas into horsepower to turn the crankshaft, and subsequently the drive wheel(s).
Preignition occurs when a portion or portions of the fuel/air mixture in a cylinder ignite early, before the piston is in the proper position to accept the push of the expanding gases. This sometimes happens when something inside the cylinder (the piston top, a hot spot on the cylinder wall, etc) is hot enough to cause the mixture to ignite. This early ignition -- or "pre-" ignition -- pushes on the piston top while it's still traveling upward on the compression stroke. The energy of that burning fuel is wasted and the engine is actually working against itself.
Detonation is a much more serious problem. This occurs when the mixture simply explodes -- like a firecracker. When an engine experiences detonation the mixture ignites early and burns violently (compared to a normal ignition event). One of the primary causes of detonation is excessive heat. Spray a mixture of fuel/air into a cylinder where the internal heat is hot enough and it will explode -- all at once. That's detonation, and it's bad.
The max cylinder head temp on a typical four-cylinder Lycoming aircraft engine (according to the manufacturer) is 500 degrees F. Just about ANY fuel you can buy at your local filling station for use in your car will ignite at temps far below that number. Which is why these engines use 100 Low-Lead AvGas. (The "100" is the octane rating; "Low-Lead" is a reference to the reduced amount of lead additive compared to the 130 octane AvGas used in WWII era and later reciprocating [piston] aviation engines.) The higher octane rating (and the lead additive) means the fuel simply WILL NOT ignite in these engines until the spark plug fires. The "Old Wives' Tales" you hear about the extreme volatility of AvGas are bunk; you can throw a lit cigarette in a bucket of AvGas. High-octane fuel needs a hotter spark (more heat) to ignite.
Getting back to your statement above, a "fix" one might consider if excessive engine heat were causing preignition or detonation is higher octane fuel. It won't help your engine run cooler, but it might keep it from destroying itself...
Regards,
Mark
P.S. I don't recommend throwing a lit cigarette in a bucket of AvGas.
P.P.S. I'm not aware of anyone reporting any preignition or detonation problems with their Spyder.
Good explanation!
