I am reposting this here as this is a very often asked question.
Remember octane is "resistance to ignition".
Octane is the enemy and is a friend. If you run too low of an octane the engine can auto-ignite the fuel like a diesel.
If you run too high of an octane, the fuel will pass through the engine unburned or will burn on its way out the exhaust and cause issues on that end. Requiring more timing to get the fuel to burn "in the chamber" and give you the wrong crank angle when this maximum pressure is supposed to occur.
Choosing octane is very tough, so you have to look for signs.
Too high of an octane is the toughest condition to diagnose.
If the vehicle gets lean when its cold out, BUT adding jet makes it rich down track as the engine gets heat in it. Drop your octane.
If you are required to run more ignition timing than you think you should or you have to run a hotter plug to get heat on the strap than you think you should. Drop your octane.
Too low of an octane is easier to diagnose, the engine detonates and it hates timing. So you keep adding jet to stave off detonation and now it just rolls black smoke immediately when put under a load. Increase your octane pull the excess fuel back out.
Although there is more info you can add to this (we have a tuning DVD with a big section on this), these are the highlights of what you need to know for proper selection.
Now as we said in another post, with too high of an octane the engine can interpret the mixture as being lean as it has an unburnable mixture displacing a location where burnable fuel should be and there are left over available oxygen molecules looking for a companion. These molecules are what makes the mixture lean.
Yes the car can lean pop and backfire just like it needs more fuel because the engine does not know the high octane fuel is there as that part of the mixture is not burnable anyway.
But if you add the fuel, you are adding good light ends that will burn, but you are adding some unwanted heavies that will soot and load up ring packs and give you some serious grief.
Too high of an octane will also compound as it gets cooler out as you go even further away from the "auto ignition" that you were trying to escape with the high octane.
Now the mixture as the engine sees it will be even leaner and more of a problem (cooler fuel..harder to light..cooler air more available oxygen).
So then you really get into trouble.
Watch for these signs and adjust accordingly.
Hope this helps.
