Is high octane gas really worth it? It is my understanding that the octane scale is based on comparing any fuel to Octane and Heptane, with zero being equal to pure heptane and 100 pure octane. I also understand that the characteristic of octane which is desirable is its ability to be compressed without combusting prematurely. Now, at my local gas station, Regular unleaded is rated at 87 octane and premium(or “ultimate”)is rated at 93. Therefore the difference between them is only the equivalent of 6 percent octane(or heptane). Does something that performs under pressure like it is 87% octane actually prematurely combust significantly more often then something that performs as if it were 93% octane? It seems like, for the amount they charge for premium, there should be a bigger difference in rating? Am I missing something?
The following was taken from a question on How Stuff Works http://www.howstuffworks.com/question90.htm
"The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like “regular” 87-octane gasoline) can handle the least amount of compression before igniting.
"The compression ratio of your engine determines the octane rating of the gas you must use in the car. One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. So a “high-performance engine” has a higher compression ratio and requires higher-octane fuel.
“The advantage of a high compression ratio is that it gives your engine a higher horsepower rating for a given engine weight – that is what makes the engine “high performance.” The disadvantage is that the gasoline for your engine costs more.”
I hope this helps.
Tomorrow I’m going for a ride in my little MX-5 on a mountain batttlefield/road trip and I expect I’ll have to fill up about halfway. It’s going to be as many hills and curves as I can find for as much of the way as is possible. I’ve only put 87 in it since I got it, so perhaps I can give a valuable subjective opinion.
If there is such a thing.
If anyone has any recommendations as to what I should look for, I’ll be happy to observe and record. It ain’t science, but since ignorance is partly a matter of opinion, I might be just the fool to run the errand.
If we get enough fools on hand at once, we might have the blastula of a study.
Acceleration, Sofa King. I tried 87 octane in my car and noticed a loss of performance under hard acceleration all the way up to red line. Just as the owner’s manual stated. No pinging or anything, the computer takes care of that, but I lost at least a second or so 0-80.
No biggie, I guess, if that’s not important to you.
Peace,
mangeorge
I don’t think the octane rating is simply a percentage of octane (or heptane, or anything else) because I’ve seen gasoline with an octane rating of over 100. I’m really reaching here, but I think 110 octane fuel is commonly used in aircraft.
I suspect that this just muddies the issue even more, and for that I apologize. 
Well, I forgot to mention that my engine’s compression ratio is 10.1:1. The car’s an Audi A4, 3 litre.
Which reminds me of another question, probably best addressed in a New Thread. See “Are new cars de-tuned” if interested.
Peace,
mangeorge
Mangeorge , the loss of performance in your engine is from the engine’s computer sensors detecting knocking and pinging, and retarding the ignition.
Switching from regular to plus ought to keep the ignition timing at optimum performance under load.
I’m much more interested in “cat got a cup of hot tea spilt on 'im” performance, but I don’t think there’s much hope of that.
I know. I just experimented with half a tank. Surprising how much difference it makes. Been back to 91 for a while now. I read about requirements and effects in the manual and at a couple Audi related websites. Actually, the computer compensates before the knocks and pings. Magic, huh.
Thanks, though.
Peace,
mangeorge
The manual recommends using higher octane gas in an MX-5. I think it says “use premium fuel” on the gas cap, which is either 91, 93 or 89 octane gas. We used 87 in our MX-5 and experienced some pinging.
I think the “hot tea” in your case might be some higher octane gas.
Let us know the results after you try it. You should be able to tell just by jumping onto the freeway. Or by kicking it in the butt on the open road somewhere. I’ll meet you on HWY 1, on the PCH, Okay? Should be no problem for you. After all, my car’s an automatic.
Peace,
mangeorge.
What about the alcohol formulation for octane, is it the same, do they add less for higher octane? Cause I noticed on gas
pumps they say they add alcohol for oxygenation.
The octane rating is based on the compression ratio that can be used with the fuel in question. Originally it was assumed that pure octane was as good as you can get. The compression ratio that can be used with various mixtures of octane and heptane was measured using a variable compression ratio combustion chamber. Then various aromatic compounds based on ring molecules like benzene instead of chain molecules like octane were found to allow even higher compression ratios than straight octane. Some aviation fuels reached octane ratings of 130.
Yes it is possible for a fuel to be over 100 octane. I didn’t mean to give the impression that the rating meant that a given fuel actually was 87(or91,0r 93) percent octane. Gasoline is a mixture of more then just two fuels, and the rating simply uses octane and heptane as benchmarks. Oh and thank you everyone for the replies!
So, whatever happened to Ethyl? Remember her? She was too dirty 
We used to call gasoline either ethyl (high octane) or regular (low octane).
Union oil co once had purple (premium) gas. Purple oil, too. Royal Triton. I think it was.
Life was much simpler, and more boring, back then.
All right, here’s some results.
First of all I checked the manual again to make sure that 87 is okay. Sure enough, 87 is A-OK, even advised. It’s a '92, so perhaps things have changed since.
I drove hell out of the thing up and down South Mountain then put 9.2 gallons of 93 in it at a Sheetz (“100% Guaranteed”) in Charles Town. Then I took it up Rte 9 through Keyes’ gap, which is a pretty fun road when there are no doofuses in my way.
As luck would have it, the occasional valve-clatter at idle that I sometimes experience was happening when I pulled into the gas station. That disappeared after I filled up. One point for the good gas.
As far as performance goes, I tested it on the mountain, on some flat but curvy roads on the other side, and on the highways going back into DC.
I couldn’t tell much of a difference, with two possible exceptions. Above 4000 RPMs I thought I felt a very slight improvement in acceleration. But the difference was almost indiscernable, if it was there at all. I didn’t go above 5500 RPMs, because I don’t want to kill the hamsters. Testing would have been irrelevant anyway, because I’ve never driven the car at all at stratospheric RPMs. It would have been all new to me.
Probably more interesting, I kept getting the impression that I lost a little bit of low-end torque (and believe me, I don’t have much to lose). Again, the difference before fill-up and after was barely discernable, but I was paying close attention.
Sorry I can’t be more specific than that. There is one further test which may be of interest–fuel efficiency–but I don’t drive this thing much and it could be weeks before I fill up again. I did reset the trip meter as I always do when I fill up. The mileage per tank has hovered within a couple of decimals of 30 MPG since I owned it.
We need a mechanical engineer, or someone who’s a specialist in fuels. It is my distinct feeling that octane rating has to do only with prevention of premature ignition upon compression. I think that 92 octane fuel does not necessarily have any more BTU’s per pound, or calories per gram, than 87 does. The only way that 92 would improve performance is if the engine is designed for the higher octane and you can’t run it at full output on 87. Furthermore, I don’t think octane rating, by itself, will clean any previously deposited gunk out of an engine. If your engine is designed to develop its full output on 87 octane then paying extra for 92 is just a waste of money, assuming both fuels are the same quality as far as contamination is concerned.
OK, now have at me.
All qouted from the sire I listed earlier.
In fact, the higher the octane the lower the energy available from the fuel, as you have to break more bonds to get the gas to ignite. This allows you to use higher compression in high performance engines, but really what it is doing is allowing you to burn more fuel in the same amount of time.
Pretty much all gasolines have the exact same detergents. This wasn’t the case when the detergents were first introduced but it is now.
As long as your car doesn’t ping, high octane gas is just wasting money and possibly damaging your engine. If your car pings under normal load, you should get it tuned properly. If it pings under very hard acceleration, moving up a grade might make sense.