knock

[jayjack]

can someone explain in detail how higher octaine reduces knock?

[curtis73]

Uh-oh... you said "detail." That gives me free reign to make a long-winded post

As the engine goes through its compression stroke, it heats the air in there through a process called adiabatic heating. Basically its the opposite process of when you spray an aerosol can and it feels cold. That is high pressure going to low which drops temps. Low pressure going to high adds temps. How much it rises depends on many factors including compression ratio, cooling efficiency, what type of material the heads are, size of the valves, humidity in the ambient air, and even cam timing events which determine how much air gets in to start with.

Any time you increase the temperature by changing one of those factors you need to increase octane. Octane is simply more resistant to self-ignition. It requires more of what is called Ea or Activation Energy. So, if you have a hot spot, a weak cross-over spark discharge, or other factor like that, the higher octane prevents the fuel from igniting before you want it to; at the spark.

[jayjack]

thx its simpler than i thought, and ps i like most long winded posts