I don't think the octane rating of a fuel has anything to do with energy density. Ethanol fuels have much higher octane rating than that of petroleum based fuels, yet are much less "energy dense", I.e. for a given mass of both fuels, ethanol yield less energy output.vlad01 wrote:Yes it does burn cooler for a given CR, but the idea is its more energy dense and requires higher thermal input to react it with oxygen completely. So its required to have a higher dynamic CR to be able to release its higher energy content.TdracerTd wrote:Just something that I don't think has been mentioned. Higher octane fuels burn cooler and require more energy to ignite. So, by running a higher octane fuel your combustion temp for a given set of conditions will be lower, reducing the heat in the heads/combustion chamber, added to that, the fuel requires more heat/energy to actually combust. That's where the knock resistance comes in.
That is the reason it yields better economy in engines that have a high enough CR, if the CR is sufficient enough better power is yielded too. At those scenarios 91 fuel is virtually out of the questions due to its low knock threshold.
I have noticed 98 fuel always burns blacker in the combustion chamber and exhaust in stock V6 with 8.5:1 compared to 91 with the same AFRs, that means the combustion can't be fully utilized on 8.5, not even 9:1.
On ecotec with 9.4, that would be coming into usable range.
But with the quality of 91, I'd be using 98 regardless.
The reason that ethanol fuels have a higher octane rating is that it is harder to ignite and burns cooler.