I ran premium in my 300-6 for two gas changes, and saw my mileage drop 1mpg compared to 89, and when I ran 87 my mileage was up by .3mpg, though I only did one tank of 87 so I can't confirm it for sure. This was all in the exact same driving. Course...a 300 isn't a performance motor, so the modmotors and any other motor are probably different.

 

Octain is a detonation inhibiter. Its used to raise the flash point of fuel higher to prevent pinging. The down side is that higher octain has lower BTU value. The lower the Octain, the better.

My 98 Jeep TJ will struggle and pop off road and at low RPM on anything higher than 87 octain, were as my MG need 923 to keep from pinging at idle.

 

91 is the highest available here and even my 10:1 compression BMW does just fine with it. Unless you have bad carbon buildup and/or are running way too much spark advance, I cant imagine you need 93 plus booster.

Jim

 

First, you've got to understand octane numbers are a (sort of) rating of how well something resists pre-ignition under adverse conditions. Adverse conditions can be something like hot temperatures, high boost, more spark advance, higher compression ratios, and things like that.

Octane numbers do not say anything about how much power something generates, or how good a fuel it is. How much power something generates is related to how many BTU's per gallon it has, and things like that. You might be interested in the fact that #2 diesel has more BTU's per gallon than gasoline. That's one of the reasons diesel engines go farther per gallon of fuel than gasoline engines. It certainly doesn't mean that you want to put #2 diesel in your gas tank.

Alcohols will raise the octane level of a fuel mixture, they resist pre-ignition. BUT, alcohols have considerably less BTUs per gallon than gasoline. So if you mix alcohol with your gasoline, you WILL raise the octane rating of the mixture - and you will reduce the BTUs per gallon of the mixture. Which means worse fuel mileage and everything else being equal you get less power.

Remember, some race cars used straight alcohol like the old time Indy 500 racers. But, to make it work they had to run VERY rich mixtures. So while there was fewer BTUs per gallon of fuel they were using more gallons of fuel. Plus those engines were supercharged so they could put in as much air as it took to burn the greater amount of fuel. They needed the higher octane for the boost pressures they were running.

Pure old fashioned gasoline has 114,100 BTU's per gallon. With additives like 10% MTBE it falls to 112,000. That's why fuel economy falls off when winter mixes are put into the tank.

Diesel fuel has 129,000 per gallon which is why diesel engines go farther on a gallon of fuel.

For tdister: I won't argue with anyone. I have learned a long time ago some people just don't want to be confused with the facts. Empirical research doesn't cut it with me.

 

Too many people reading articles that are dumbed down for the general population. Higher octane does NOT necessarily mean lower BTU's. Nor are BTU's (though a very imp. factor) the loan factor in fuel economy or power.

If oxygenates are used to raise the octane, yes the BTU's are lowered. However, if Benzene, Toluene, Xylene, Methyl Benzene are used, then you get an increase in BTU's.

As I stated before, not all vehicles will benefit. My case might even be fairly rare, I don't know, but it is not scientifically impossible. My 4.6 F150 has consistently shown a >2mpg increase on the HWY using premium gasoline (21-22 mpg). My freinds civic in one of the exp. showed no change (less than 1 mpg increase). It all depends on the specific engines needs which, I admit, I have no fool-proof formula to help determine. I'm going to leave it at that however, if you have more evidence, I would be more than happy to discuss it.

 

I had a '99 Jetta, and got about 3mpg better with 93 than the called-for 87 octane. I haven't tried it in my truck yet, still saving up for it.

 

Sorry, I remembered MTBE incorrectly. It is in fact an oxygenate and will lower btu's. It does however add more O2 into the mix, which CAN/MAY (again, in the correct application/circumstances) provide for more power production. I know it makes some cars run poorly, but that's a whole other subject.