93 Octane??

Probably not, because its at best a "CYA" on the part of the manufacturer. Covering themselves "just in case" some high-octane fuel is found that still contains lead or manganese (kills the O2 sensors and the cat),or various non-lead additives that aggravate combustion chamber deposits, valve stem and intake port deposits, etc. With a good-quality (major name brand) modern unleaded, the only thing that's going to be harmed by running a fuel with a "too high" octane rating is your wallet.

MTBE is certainly an effective octane-booster. I don't know about under the current EPA gasoline rules, but for a number of years around my area, only 93-octane fuels contained MTBE at all, and it was there strictly to hit the 93 octane rating. The lower graded fuels were MTBE-free.

I'm not sure on that one, but I'd wager that if it does its an incidental side-effect of the additives that resist self-ignition. The ideal Otto cycle calls for a rapid (near explosion but still a controlled 'burn') iso-volume delivery of heat energy at TDC, just like the ideal Diesel cycle calls for a constant iso-pressure delivery of heat energy all the way from TDC to BDC. Both cycles run somewhere between the two in the real world, but deliberately slowing the flame front in an Otto cycle engine (especially a high-compression and therefore high-performance engine) is probably not desirable.

I know just enough about combustion dynamics to be dangerous (pun intended) :-) but my understanding is that a chemical's resistance to self-ignition can be changed without altering its burn rate or burn characteristics significantly or at all. In theory. In the real world, almost all parameters get "coupled" because you're changing fractional content of different chemicals every time you change a parameter. That's why the oil companies still need so many Ph.D chemists, after all these years. If they really did nothing but bribe governments, rape the environment, suppress competitive innovation, and ream the consumer the way some people believe, they certainly wouldn't need chemists :-)

And a lot of characteristics of fuels are very counter-intuitive. People think of gasoline as being more energetic than, say, motor oil or diesel because its so much more prone to combustion. But diesel, motor oil, and most oils have more energy per gallon than gasoline. Or to go to an extreme, think about diesel versus explosives. Diesel, gasoline, and candle wax all contain much more energy per pound than even high-powered high-tech explosives, but the WAY they burn (burn rather than a true supersonic detonation wave) makes them seem less energetic.

Just testing octane is expensive. With the changing formulations needed to meet performance, emissions and cost criteria it would be cost prohibitive to run the tests needed to confirm. But remember, higher octane fuel has other consequences other than retail cost, it takes more base gasoline to formulate so it wastes natural resources as well.

Richard.

I've never had a vehicle deliver poorer mileage on premium than on regular. I have had several that gave enough increase in mileage to more than cover the price spread. The price spread was a little lower back then than it is today in pennies per liter, but in percentage it was likely pretty close.

Not exactly true. Octane is a measure of the anti knock index. The fact that the volatility, or vapour pressure is SOMETIMES higher on hightest is only an artifact, not a cause. Adding propane (octane of 150 more or less) to gasoline will raise both octane and volatility. Adding TEL changes the octane, without affecting volatility.

The octane of gasoline can be modified in production by changing the ratio of hydrocarbon "fractions" in the distilation process. The old hightest white gas was lead free and had high octane, but was VERY expensive to produce at the time - hense the introduction of TEL.

Comparing mileage with different fuels is very difficult, because you have no control group for the experiment. Every auto trip is different, even along the same route you may encounter different conditions, traffic or get more red lights, etc. The difference in mileage due to gasoline can easily be less of a factor than such outside factors.

When you get 5-10% better mileage on one tank than you have gotten on ANY other tankful in the past, and all you have done is changed fuel, it is a pretty good bet the better fuel made the difference.

Also, on the Pontiac, OBD1, the scanner shows a significant reduction in knock signatures by switching from 87 to 89 octane, not much better going to 91.

But my point is that it is impossible to eliminate the non-fuel changes from the experiment because of the many variations of daily real world driving. If the engine was on a dyno in a controlled test facility, it would be a different story.

But - people pay money for fuel for trips made that have all these variables, they don't drive in the lab. So you got to make your testing real-world, not lab.

An accurate enough real world test would be to run about 3-4 tanks of regular gas through the vehicle, followed by 3-4 tanks of premium. You tell the driver what is being run. Then you follow this with another 3-4 tanks of regular and tell the driver it's premium, and then follow with 3-4 tanks of premium and tell the driver it's regular.

That would do several things, first it would eliminate a lot of the trip differences because they would average out over the 3-4 tanks, second it would show if the driver was subconsciously interfering with the test, third it would demonstrate that the benefit of the better fuel would go away when you switched back to the regular.

The big problem with Greg's method is that he as a driver is conscious that he's running premium. Thus if he expects better gas mileage out of premium, he's may unconsciously drive in a more fuel-efficent manner.

And of course, understand that once all that's done, the data is only good for the tested vehicle. Unless you did an experiment like this with dozens of same-model vehicles, you can't draw conclusions that are applicable to other vehicles.

Ted

The spread would have to be tiny for premuium to deliver enough better mileage to cover the additional cost. And if your vehicle does not have a knock sensor, and a PCM programmed to take advantage of higher octane gas, there is no reason to expect any increase in mileage with premium fuel, unless the car was knocking on regular. Out of curiosity, I tried premium in my '97 Expedition for many thousands of miles. The Expedition does have a knock sensor, and supposedly can take advantage of premium fuel. There was no significant change in fuel mileage when running premium. I also tried the same thing with my Mustang (which did not have a knock sensor) and, again there was not a detectable difference in fuel economy.

Years ago, when I was in college, I was taught that premium fuel often had a slightly lower energy content than regular becasue of the blend of componets used to achieve the higher octane. As far as I can tell, this is no longer the case and in fact there are numerous references that claim premim has a slightly higher energy contnet than regular. Persoanally, I don't think there is a greater difference in the energy content between regular and premium than the normal variation in different tankfuls of regular.

From

"Gasolines with higher heating values give better fuel economy. Differences can exist, but they will be small compared to the benefits to be derived from the maintenance and driving tips in the above answer.

"Traditionally, premium has had a slightly higher heating value than regular, and, thus, provided slightly better fuel economy. The difference ? less than 1% better ? is not large enough to offset premium's higher cost. The difference is likely to be less or nonexistent between grades of reformulated gasoline.

"There can be differences in heating value among batches of gasoline from the same refinery or among brands of gasoline from different refineries because of compositional differences. The differences are small and there is no practical way for the consumer to identify the gasoline with a higher heating value."

Other interesting references:

Ed

I have kept a mileage book on all my cars for years. I can show you examples of tankfuls that appeared to get almost 20% better mileage with no changes in the vehicle or typical driving pattern. You need to check mileage over several tankfuls (I recommend at least 5 full tankfuls) to get anything approaching a meaningful result. In my case I ran premium for 6,299 miles in a 1997 Expedition. The average fuel economy while running on premium was 14.1 mpg. The fuel economy for the 6,230 mile before I tried premium (running on regular) was 13.9 mpg. The average fuel economy for the 3,316 miles after I switched back to regular was

14.3 mpg. While using premium the best tankful average was 16 mpg and the worst was 11.3. With a five tank average the maximum was 14.6, the minimum 13.5. Overall, my best one tank average was 17.4 (running regular) and the worst was 9.6 (also running regular). Using a five tank average the best was 17.2, the worst 10.5. The Expedition had a knock sensor, and it was claimed that using premium would improve both power (3 to 5 hp) and fuel economy. I could never tell any difference in either, although I know other Expedition owner's that claimed their personal butt-o-meter could detect the extra power.

Ed

How did you know the mileage was higher? Often people compare one tank of regular to one tank of premium and think the difference they see is because of the gas.

Matt

Not at all. I always fill my tank when I get gas and I check the MPG on every tank and keep it in a log in each vehicle I won. 10% variation tank to tank isn't unusual at all. Unless you've checked at least five consecutive tanks on regular and then five on premium and averaged each of the five, you don't know anything about whether the gas made a difference.

That makes sense as higher octane is designed to knock less. That is what octane is a measure of. If your engine is knocking heavily on 87, then it is likely that the timing is being retared quite a bit causing a loss of power and possible mileage as well. In this case, the higher octane might well deliver slightly better mileage, but you'd have to check several tanks to have a statistically valid comparison.

Matt

But the problem with your method is that every trip is different, even when the gas stays the same. I've kept fuel records since my car came off the dealer's lot and the mileage varies considerably, even among trips when the car is just used for commuting on the same route and when fuel is purchased from the same station.