There is almost no limit to the wild claims about octane on both sides of this argument, and most of them are wrong. Too much fuss is made over octane ratings. Octane is a knock index, nothing more. It's all the same fuel. But people just can't resist turning into some kind of magical elixir with mysterious powers. As long as your engine does not ping, you can run any grade your wallet can tolerate. The only time you need to adjust your choice is when your engine pings on a particular octane.
Carbon buildup from high octane is one of the more common myths. On the surface, the theory actually has some merit, except for it to work out, you have to make some assumptions that are wrong, the primary one being the claim of differences in combustion chamber temperatures. Combustion chamber temps don't change with different octane. The same amount of energy is being consumed regardless of how fast the fuel burns. High combustion chamber temps are needed in order to maintain low emissions. If combustion chamber temps dropped with higher octane, emissions would go up, and it has already been proven that there is no difference in emissions with different octane fuel.
There are also some rather amusing contradictions in the claim too. It is said that a slower burning high octane fuel results in incomplete combustion, yet carbon is a byproduct of combustion. Why would there be more carbon from a fuel that is not being completely burned? Not only that, but if the fuel burn was incomplete, fuel mileage would have to drop, which doesn't happen. The argument doesn't stand up to scrutiny on any level.
The theory was debunked a long time ago, but unfortunately still hangs on. It will probably never die. Proponents are right about not using higher octane, but for all the wrong reasons.
Unfortunately, this whole debate is based on the fact that different octanes of fuel don't cost the same. The emotional correlation of higher cost being better is too strong for some. Price overwhelmingly drives our purchasing decisions. I put a lawnmower in my front yard once with a sign on it that said "Works great! FREE." Nobody touched it the whole weekend. Then, at the suggestion of a friend, I asked $25 for it, and it sold in the first hour. It doesn't get any dumber than that.
Last edited by Sparky57; 04-13-2017 at 06:38 AM.