Higher octane gas is merely the ability to resist pre-detonation. As a car ages, carbon builds up and you tend to get hot spots in chambers. Lower octane gas will detonate from this before the spark plug sets of the charge and when the two shock waves meet, you get engine knock. Higher compression engines will also require higher octane. It's basically a waste of money to use a higher octane gas if the engine doesn't need it as the BTU content of 87 or 93 gas is the same. Of course if you listen to all the marketing, some companies do end up putting more detergents to clean fuel injectors in their 93 than their 87, but their 87 has to have enough to do the job as mandated by the EPA.
