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Old 03-14-2013, 08:24 PM   #36
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Originally Posted by thejohncarlson View Post
All the charts that I see say that the higher the octane, the lower the BTU/gallon. Also, due to the fact that they burn slower, the latent heat of vaporization is lower as well.

I will give you this, the differences between 87 and 93 are so negligible it probably does not make a difference but, when you get up into 100+ octane gas, it burns considerably colder.
I'm not sure which charts you are looking at, meaning who made them, but you have to realize that they typically have to adjust the timing and compression ratio to make the charts come out correct unless they are extrapolated using a formula. Either way there are too many changing variables to rely solely on the chart. In theory you are correct that 100+ octane would burn cooler, but you have to see it relative to the amount of compression it was tested at as most engines dont typically like being outside the common range usually 87 to 96 Octane.

As for the amount of BTUs per gallon I think what you have is backwards but I'd have to check at work tomorrow, the longer hydrocarbon chains would tell me that there are more per Gallon, thus making it more dense with hydrocarbons. I know that sounds backwards, but I am pretty sure it is correct as it is a more pure fuel.

You are correct about the differences between 87 and 93 octane being negligible in the amount of BTUs but when looking at the required fuel in regards to the compression ratio they really are night and day. The reason why I recommend utilizing a higher octane than recommended every so often is based solely on the mechanical reasons that I am sure we all agree with. Ie. an engine running and tuned for 87 octane has a certain degree of timing for ignition. By delaying that ignition using a higher octane fuel you are delaying the burning within the cylinder thus getting more piston travel before detonation, this is why people say that higher octane burns hotter. You roughly have the same amount of BTUs but in less space.

If I messed up any of this let me know, I just quickly typed it up as I'm doing other things!

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Old 03-14-2013, 08:55 PM   #37
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I see what you are saying and the information I am talking about is not related to any application. I am talking about the raw characteristics of the fuel so, it does not take compression into account.

I am by no means an expert regarding what is happening inside the cylinder. Based on my limited knowledge, I would think that the compression ratio in the cylinder stays basically the same no matter what fuel is being burned. I can't imagine there is enough of a difference to dramatically effect the temperature.

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