There was an EPA study of gasoline at retail outlets several years ago showing a pretty large range of 107,000 to 116,000 BTU per gallon across the 50 States.

The automakers for years trucked in Chevron Techron 87 AKI to their Detroit labs for emissions tests because it was consistently high in BTU per gallon.

My fuzzy memory is that pure Octane is around 121,000 BTU per gallon.
It used to be a standard that pure Octane fuel was used in scientific engine tests.
While costly this was a good thing for consistency and "replicating" experiments.
It is not real science unless original test tesults are replicated.
That is always worth remembering.
Science is not theories or computer models.
Science at heart is solely successfully replicated real world tests.

I would bet that no one on Moparts has run as many BTU/calorie tests on the original Parr Calorimeter machine with its "simulated perfect insulator" water jacket system as I have, even though I have not done one since 1990.