Read this on teamintegra.net and since I can not link them (pointless because you must be a member to view) so I ended up just quoteing it.
Originally posted by austin on Jul/09/03
Since there have been a rash of questions concerning using higher octane gas in the car...I've decided to round up a few dyno plots that I have of when I tested out running a higher octane mixture without having the car tuned to run specifically on the higher octane.
Why run higher octane?
1. higher octane slows down the combustion process giving you a slower, more controlled burn.
....A. advantage to this is that it allows you to increase your timing, and/or compression as you're less likely to have detonation.
....B. advantage is that it helps to prevent pre-ignition, preventing detonation from hot spots in the combustion chamber.
....C. advantage is that you can run run more advance, and/or higher boost levels on forced induction vehicles, the slower burn helps to prevent the combustion process from blowing up, rather allowing it to burn(when it blows up you have bent rods, burnt valves, holes in pistons...)
....D. advantage is that you can run nitrous oxide with more ingnition timing, and a smaller chance of pre-ignition(read C above)
....E. disadvantage is that if you don't have the car tuned to run specifically on the higher octane gas, you'll end up with less power since the combustion process is still attempting to burn the fuel when the exhaust valves open, dumping still burning fuel into your exhaust.
....F. disadvantage is that it's going to make your wallet considerably lighter.
Information on the plots:
The difference in air temperatures was ~4 degrees, the barametric pressure were fairly similar, this is an SAE corrected graph. The only changes between the two graphs is that I added in a few gallons of 100 octane gas, the net octane was ~96.X that I computed at the time.
full size, cut and paste: http://www.imagestation.com/picture/sraid6...d6.jpg
Have a good day...
Austin