No premium has more BTU. That's why some people see an increase in gas mileage because the engine can get more power out of each stroke. I will get back with more detail soon.:thumbup:

Well I had my morning highway runs today and.......

:(

Drives like I've got it fully loaded with only 20psi in the tires... it kicked down on an uphill merge, seems to be running rich because it smells, and took longer to warm up AND wasn't running as hot on the highway, at about 1/4 gauge instead of 1/3 to 2/5.

I thought it would be quick and easily to google for:
gasoline octane btu
and get the answer, but it's not. Anyway, I'm pretty sure that BTU content doesn't vary by octane rating, except as a result of the gasoline being diluted by whatever was used to raise the octane.

The reason higher octane rated gasoline can produce more power (or, in certain conditions, can produce more mileage) is because it can be used at a higher compression without pre-detonating. That allows the existing BTU content to be extracted more efficiently.

Edit: Simultaneous posts....dude, the gas you got was defective or contaminated, or coincidentally you've had a malfunction. Ask them if anyone else has been complaining about the 91 that they got that day.

Yeah, me too, can find lots of people saying premium has less BTU, but can't find any real figures.

However, I do suspect that this is 10% ethanol, and when I've had 87 octane that I know has had a few percent ethanol in.. it didn't run great.

I'm thinking if it had water in, I should be hearing it stumble more, and I'm thinking if it had particulates in the fuel pump would sound loud (That's usually how I tell how to change the fuel filter)

Agreed though, extra power with high octane in engines designed for it comes from higher thermal efficiency of a higher compression ratio, it's nothing to do with the BTU content of the fuel really. Stock compression ratio of this motor is 8.9:1

Around here, all gasoline of any grade is 10% ethanol. There's a big sticker on the pump proudly proclaiming it...

Could be stale gas? Maybe that station doesn't sell premium very quickly.

What other kinds of contimants or defects could there be?

If there is ethanol in the gas and it happened that it was contaminated with water, the ethanol would allow the water, ethanol, and gasoline to mix evenly. This could prevent the stumbling of water and gas mixes, because the engine isn't getting occasional chugs of water, but a constant stream of a tiny amount.

I've heard the higher octanes also burn slower than the lower in addition to resisting pre-det better. So maybe you are getting an incomplete burn in the lower compression.

Most ,if not all, of the newer preformance engines can adjust for lower octane fuel. They'll put out less power, but my parents seem to be getting EPA with regular in their Acura without trying.

Water is only bad when it's not mixed, ethanol holds very little in solution in gas only a couple of percent, that's where the trouble comes, because it continues to be hygroscopic and draws in moisture. If it was running with a high proportion of water held in solution, I'd expect better performance due to higher cylinder pressure. It's apparently possibly to run 120 proof alcohol in most motors, as long as you drain every last drop of gas first, because it's only when ethanol and gasoline mix there's a water problem.

Do you mean 200 proof? I suspect that the other 40% of volume in most 120 proof alcohols is mostly water...

Ah, I was thinking in the British proof rating and aiming to mean about 70% alcohol by volume, yes, the remaining volume would be water, that'd be 140-150 proof I guess, US rating..... and it will stay in solution unless it's mixed with gasoline or similar, which is where the trouble is. As far as I remember, it goes something like this ethanol 97%, methanol 93% isopropyl 68% for the amount of water if can keep in solution while in solution with gasoline.

Well the numbers is in... see gaslog, doesn't seem to have dinged me... but I was h8ing on it all the way... filled it a little early, but my "canary" station I see off the highway had it's price up, so figured I'd get gas tonight anyway and save $2.

Actually, it did seem to have improved the last two or three trips, since I had to go WOT to merge onto the highway, due to some octogenerian in a cadillac in front of me going 25mph down the on ramp...

Maybe if this premium has those (mythical?) extra detergents it dislodged some flakes of gum which was goobering up an injector until that blew it out.