The title is probably a little misleading. I'm not saying I found Premium for less per gallon than Regular. What I mean by that is that it seems that I am actually saving money by running premium rather than regular.

87 octane is the only grade fuel that my current engine has ever drank. but a couple weeks ago I notices that it has a knock/diesel noise at WOT or heavy acceleration. I thought maybe I just got a bad tank of fuel since it's never done that before and I hadn't made any changes. Next tank it still made the noise so I decided to take a look at the timing. It was around 12-14 initial timing, which is probably a tad high, so I dropped it to 10. It seemed to run fine after that, and just to make sure I thought I'd fill the tank with 89 just for the hail of it. During that tank of gas I spent quite a bit of it racing some friend on back roads and also took it to a dragstrip for the first time and made several runs. even with all that I still managed 15.78 MPG with this tank as apposed to about 14.4mpg with 87 and driving like a grandma. After crunching some numbers I found out that it was actually Cheaper per mile running the 89 than 87. About 18 cents per mile for 89 as apposed to about 19 cents per mile for 87. I figured surely even I stepped up to 93 and kept my foot out of it like I normally do, then I will actually be saving money because of the added gas milage.

One question I do have though. Is it normal for the carb to need to be adjusted to performance it's best when moving from one octane to another or does it make a difference?

I was pleasantly surprised when I came up with those numbers. Has anyone else had similar results doing this??

 

Yes, a carb should be tuned to the fuel. Once this is done properly you shouldn't see any increase in performance or mileage by switching to a higher octane although you probably will see a decrease in mileage and/or performance by switching to a lower octane. It sounds like your carburetor was tuned more for the higher octane all along. Without knowing the history of your motor/carb/tune, one explanation is the continuing degradation of fuels to meet higher emissions standards. Fine for computer controlled fuel injected cars but not so good for carbureted engines.

 

I live accross the border from Dubuque, IA where 89 octane IS cheaper than 87. The reason is because they use an ethanol blend for 89. It's typically 5 cents less per gallon.

 

You changed timing AND fuel, then noticed better gas mileage. You changed two things at once, which is a big no no, for checking the affects of something new. So you can't possibly conclude with any degree of confidence that the fuel alone, accounted for the increase. There is no difference in the energy available between 87 and 89 octane fuel. The fuel is the same except for the 89's better ability to resist detonation, that's it. If you only test 87 against 89, with NO other changes, under identical conditions, you will see no difference in performance or mileage. The higher octane can allow more timing, which may, depending on your setup, result in higher performance. But that higher octane fuel, all by itself is not capable of producing more power or mileage. Some new car owner's manuals state that you can run lower octane than called for, but with reduced performance. That is because the computer reduces the timing to keep you out of detonation, which in turn, reduces the performance

 

Well stated.

 

Additionally, if the engine calls for 87 octane (and is equipped with OBD-I) the O2 Sensor will detect more fuel coming out the tailpipe and lean out the mixture, saving you fuel $$.

 

A couple of years ago, I changed my timing from 35* to 36* and my mileage went up 30%!! (Oh, I also changed my transmission to an overdrive).

 

Testing rule #1: never make more than one change at a time, or results cannot be scientifically quantified. Nevertheless, there remains the possibility that the combined effect of two changes could yield a greater result than is attributable to the singular changes themselves.

BTW, my daily driver (STS Northstar) has somehow always managed both better fuel economy and performance on premium. I also see significantly better mileage (15%+ improved city) during non-summer fuel formulation seasons.