Contrary to what seems to be popular belief, using higher octane fuel in your car will NOT necessarily increase its performance. In fact, it will often decrease it and hurt your engine in the long run.

Here's why.

After fuel is injected into the intake tract and sucked into the cylinder, the intake valve closes and the piston moves up in the cylinder and compresses this charge of fuel and air. Here's where the octane rating comes in to play. If the engine compresses the fuel/air charge a lot (as in a sports car, which is typically 10:1 or more), the temperature will rise quite a bit. If you're using a low octane fuel, the chances that this temperature/pressure increase will spontaneously ignite the fuel charge are much greater. Since the mixture is supposed to be ignited by the spark plug, which does not fire until the piston reaches its highest position, the resulting explosion will be pushing AGAINST the piston. This phenomenon, called preignition or detonation, is what causes the "knock" or "ping" that people refer to.

The key difference between different fuels is that higher octane fuels are harder to ignite, and they generally burn cooler. That, in turn, lowers cylinder temperature and decreases the likelihood of preignition.

If you're talking about a high compression sports car engine, that's the reason for higher octane ratings. But if the engine in question is a typical sedan or van (or whatever else) engine, chances are it has relatively low compression. And if you're using high octane fuel in a low compression engine, not only will it be harder to ignite, but the chances are good that it won't burn as fully. Incomplete combustion will leave nasty carbon deposits in your engine. The thing about carbon deposits is that they retain heat much better than metal does and they decrease the combustion chamber area, thus gradually increasing the compression ratio. That means you've got a higher compression, higher temperature cylinder. And this, of course, will require higher octane fuel in order to prevent detonation.

So that's the deal. If you use high octane fuel in an low compression engine, you'll be ultimately increasing your octane requirement, and possibly causing carbon fouling of spark plugs, valves, and other important stuff.

All things considered, you'd be wise to just go with the recommendations in your owner's manual. If it says use "87 octane or higher", then use 87! This is one time that it pays to be cheap. If it says "91 or higher", then don't hesitate to use 91 or higher. Usually only 92 or 93 are for sale, though-- the available octane ratings vary depending where you live-- higher altitude areas have lower octane fuel due to the lower oxygen content of the air.