I thought this article really nicely explained what the octane rating of gas means.

http://www.straightdope.com/columns/041008.html

 

Also explains why running octane booster in a stock engine is a waste of money. Unless the motor has high compression and you're getting knock with premium I wouldn't waste my money. Mostly like running synthetic oil, doesn't run better or faster but makes the owner feel good.

 

Well it was an explanation, I don't know how good it was. There is info posted otherwise in a recent similar thread, feel free to try and refute it. While Cecil Adams never admits he's wrong, I believe this would be one of those situations where "it's more complicated than he first let on". Most after the fact octane enhancers are useless for most situations though.

 

The part that surprised me was this:

"Car engines nowadays contain knock sensors that detect detonation and automatically retard the spark to compensate. The delay means maximum gas expansion occurs when the piston is farther along in its downstroke and thus there's more room in the cylinder head. This reduces peak cylinder pressure, eliminating knock but also giving you less power and poorer mileage."

 

I'm feeling kinda stupid right now, so I didn't read the article. But www.howstuffworks.com has pretty good explaination of things. Even octane (I've read it before)

 

I know how what high-octane gas is for, I've read all about it, but I've had vehicles where I got better mileage with the premium gas. It would take a chemist to explain it, I know, but I don't care.

 

typically if you are getting better gas mileage and power in a older vehicle (without knock sensors) that still run fine on regular, it is because the slower flame front works better with the shape of your combustion chamber/pistoon top. I think even a lower compression hemi and semi-hemi engines run better on higher octane due to the way the flame front travels across the piston.