Thread: Octane and Oxygen

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Your altitude difference thing is a function of dynamic compression ratio. Maybe a definition of both static and dynamic will help those that are unfamiliar with the terms.

Static compression ratio is what we are most accustomed to talking about. In simple terms that's the ratio of the cylinder volume at bottom dead center vs top dead center. For the sake of discussion, if at bdc the volume is 10 times larger than at tdc then the static compression ratio would be 10 to 1.

Dynamic compression ratio takes into account the volume of air in the cylinder. The greater the air density (whether caused by altitude, temperature, supercharger, & so on) the higher the dynamic compression ratio. In your example the volume of air (not just oxygen but all components) is greater at sea level than on top of a mountain (and at varying degrees inbetween). If you watch drag racing on the tube you'll hear Dunn talking about calculated altitude and grains of water, these effect the dynamic compression ratio as well as cylinder temperature. Speaking of temperature, that's the basic culprit in detonation (not to be confused with preignition which is usually a mechanical caused ignition of the fuel prior to the plug sparking). As the charge air is comprssed it heats up, adding to the ignition process. As the flame front of burnt fuel expands and progresses across the combustion chamber it further compresses the unburnt charge. That heat of compression could contribute to sudden ignition of that remaining fuel charge (which results in the characteristic ping/knock) rather than a smooth completion of the burn. The higher octane rated fuel has more stable molecules of fuel that resist that sudden heat of compression ignition better than the lower octane rated components.