What's the Deal with all of that Octane?


Okay folks, here's the skinny:

High octane fuel doesn't necessarily make any more horsepower than lower octane fuel, when put into the same motor.  In fact, the problem that was described that was supposedly "fixed" with a higher octane fuel was probably a case of inferior lower octane fuel that got put into the tank. Gas is stored and transported all over the place and there's no reason to believe that one is always going to get perfect gas whenever they fill up, regardless of the grade.

The condition that was described (sputtering and such) was probably due to old or moisture laden fuel and has nothing to do with the particular grade.  Octane ratings on fuel are a measure of "detonation resistance" and not of available "power".

When they distill gas from crude, they end up with an array of different hydrocarbons that range from a single carbon atom chain (methane) to an 8 carbon atom chain (octane).  Each of the different chain lengths of carbon atoms have a different ability to handle compression before the onset of compression induced spontaneous combustion, otherwise known as "detonation".

The described condition is caused by the detonation of the air-fuel mix within the combustion chamber either before, after, or coinciding with the spark of the spark plug, except in another part of the chamber.  When the fuel-air mix self-ignites, it is a more violent reaction than just setting it off with a spark plug and therefore creates the characteristic knock, or ping sound.  When this happens, the fuel-air is self igniting due to too much compression for the octane rating of the fuel.  Please note, this is a general characterization of detonation but detonation can be caused  by a number of other factors even with high-octane fuels, but, for the purposes of this discussion, we will limit it to combustion induced detonation only.

Since octane is a rather high-form of the resulting carbon chain groups (in other words, there may be less of the octane groups than of other lower carbon count groups, for a given amount of distilled crude), it is also more expensive.  Octane will then be blended with a lower form carbon chain (like heptane, which is 7 carbon atoms) to produce a lower cost fuel that may still meet the needs of most motors.  [Please note, other carbon chains may also be blended to yield the same octane rating]  The measure of octane shown at the pump is a relative number based on the detonation resistance of 100% octane (which would be a theoretical 100 octane fuel).  

You may have also noticed the formula (R+M)/2 on the little yellow sticker. This stands for research (R) + mechanical (M) divided by 2.  The end result is an octane rating that is the average of the research octane (theoretical based on blend) and the mechanical octane (actual test value).

In the old days, they would blend lead (actually tetraethyl lead which was cheap) to raise the octane rating of lower grade fuels.  Besides the marketing advantages of advertising a higher octane fuel, Detroit was making motors with rather high compression engines that needed the higher octane rating to prevent detonation.  The way you make more horsepower with higher octane fuel is to make engine modifications that the higher octane will support.

So, when you put high octane in your tank, you really aren't gaining anything unless you can make power producing engine modifications (compression, valve timing, ignition timing, etc), that can capitalize on higher octane rated fuel.

If the high-test solved the sputtering and spitting problem, I suspect it was a bad load of gas and had nothing to do with the grade.

Tracy Presnell