[n4mwd]

Burns faster or slower:

In a gasoline engine, the spark happens just before the piston is at TDC - aka spark advance. This is because it takes gas a certain amount of time to burn. If the gas burns at the right speed, the piston reaches TDC before any significant force is generated and the total fuel charge ignites more thoroughly. BUT If the gas burns too fast, you have a situation where the piston is being forced downward before its time and actually tries to turn the crank the opposite way.

Advancing the spark gives the engine more HP and makes it run cleaner and more efficient. There is a formula somewhere that tells exactly how much of an advance is optimal and is based on octane, speed and vacuum. Vacuum is an indicator of power drawn from the engine. Modern bikes/cars all use spark advance.

But it really shouldn't be confused with compressive pre-detonation. That is where the gas charge ignites on its own without a spark. If the fuel is volatile enough to cause compressive pre-detonation, then the engine isn't going to run at all. The compression ratio wont change under load so it will ether run or it wont.

Its not the same as a diesel. A diesel doesn't even have a fuel charge when it compresses so there can never be compressive pre-detonation in a diesel. Diesel fuel does burn significantly slower than gasoline so if it had an octane rating, it would be way higher than 87.

Older cars would sometimes keep running after the engine was turned off. This was called "dieseling" because there was no spark and yet the engine was still running. On first thought, it might seem that the gas charge was igniting based on compression alone, but the fact is that it actually ignites because of hot carbon/lead buildups on the spark plugs. So therefore, a car would never "diesel" when it was cold.

Ethanol:

My motto -> "If 0.08% is too much for the rider, then it ought to be too much for the bike."

But then again, as much as I hate alcohol in gasoline, it may be the lesser of two evils. Before they started using ethanol, they used to put this horrible stuff that smelled funny and gave you cancer in it. I forget what its called now, but I recall that it was a bunch of letters. I think the purpose was that it made the gas burn with fewer emissions. I'm pretty sure that ethanol does the same thing which is why they don't use it anymore.