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Originally Posted by WagnerX5
WOW..if I could get 98 octane I would be excited....
Engines are designed around a specific octane level. Thus an engine that requires 98 octane will run worse and can possibly detonate early on a lower octane fuel. Higher octanes are essential for FI vehicles because of the increased air flow and heat.
You'll see with many Turbo cars that detonation is a HUGE problem (causing the piston to hit the valve).
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You can, just look at the RON value, which is what most of the countries use, the US uses R+M/2. All engines have detonation, that’s what makes them run. I think you mean pre-detonation and that’s when the fuel mixture ignites early. Detonation and pre-detonation have nothing to do with the piston hitting a valve, that's a different problem.
Octane has nothing to do with if an engine is FI or not.
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Originally Posted by MikeB
The purpose of octane is to slow down the rate the mixture ignites. The higher the octane, the slower the fuel mixture ignites. High compression engines require high octane because the heat and extreme pressure will cause the mixture to ignite prematurely. A high compression engine running low octane fuel will not produce as much power. A normal compression engine running high octane fuel will not run better. It is a waste of money. Only a high compression engine will benefit from high octane fuel. The petroleum companies explain this in the fine print.
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Octane requirements have nothing to do with compression. You can have a low compression engine and still require high octane, just because the head is inefficient. You can have an engine with a CR of 8.9:1 that takes regular, or an engine that has 11:1 that takes regular as well. Engine displacement, head design and compression ratio are just three factors in what octane will be required.