Originally Posted by 67mustang302

Higher octane fuels do burn slower. They don't simply have some arbitrary octane rating, they're all made from base fuel stocks, then specific components are added in the blending process to achieve the desired result. There's a whole array of chemicals that are added, each to achieve different effects. Some increase the fuel's flash point, so more energy is required to ignite it, some help to control burn rates for a more complete chemical reaction during combustion, some chemicals are added back in to try and increase burn rate to offset the side effect of flash point raising chemicals etc etc. Modern gasoline, and especially racing gasoline, are very complicated and sophisticated chemical blends....they're base gasoline blend with sometimes dozens of other chemicals added in(in highly controlled amounts) to alter the properties of the fuel.

They don't just "add octane" to higher octane gasoline, it doesn't work that way. "Octane rating" is actually a scale referencing iso-octane(100) and n-heptane(0), a fuel with an octane rating of 100 means that it has the same resistance to ignition/combustion/detonation as pure iso-octane, that is to say it has nearly identical combustion characteristics. But generally during the blending process the chemicals added to increase the octane rating result in a fuel that is harder to ignite and slower burning, sometimes the octane increasing chemicals are MUCH slower burning, so burn rate increasing chemicals are added in, but you have to be careful not to blend it back to a lower octane rating again while trying to speed burn rate back up. That's one reason why race fuels are so expensive, they use very expensive blending components to achieve higher octane ratings with acceptable burn rate(xylene, toluene, benzene, cyclohexane etc), and it's in very VERY controlled amounts.

Then end result is that a higher octane gasoline is not only harder to ignite(greater resistance to pre-ignition) but it also burns slower(greater resistance to flame front generated detonation). The thing is, in a high output engine that NEEDS a higher octane fuel to run maximum effort tune, the slowed combustion of the fuel is more than offset by the characteristics of the engine causing combustion rates to increase. That's why beyond the needed octane you end up losing power, since the fuel can't completely combust.

Whether a higher octane fuel is richer or leaner than a lower octane fuel is entirely dependent on the blending components. Some have higher specific gravities(more fuel molecules per cc) and as a result need less fuel volume due to increased density, so they run richer. Some have lower specific gravities and run leaner.

As for pump fuel, there's not a huge difference between 87 and 91/93, but it's enough to make a difference in some cars. And the blends between different manufacturers can be quite different.