91 Octane: hard to find in US?

Yes. Sunoco's 94 octane uses ethanol as an additive to attain 94 octane. Petrocan does it in Vancouver and Montreal but they use an oxygenating agent so it contains more energy per liter.

I am well aware of that. However, the original meaning of "octane" was the percentage of octane blended with heptane to make the gasoline. Consequently, you could not exceed 100 octane or 100% octane. The AKI (or r+m)/2 is the resistance to detonation under compression and is an equivalent to octane rating up to 100. Higher octane ratings resist detonation even better than pure octane.

MTBE is one of the additives not legal in fuels in Canada, as is toluene because of its toxicity and corrosive effects. Because of that, the options for increasing octane are very few compared with what is allowed in the US. Ethanol and methanol are options, but as yet increase the cost of fuel significantly because alcohol doesn't mix well with gas so other agents have to be added to make it blend properly. To my knowledge, only the Sunoco refinery in Ontario yet does it.

The other options are oxygenation. Petrocan does that in selected markets where the cost is subsidized. Even then, 94 octane Petrocan is nearly 8 cents per liter more than 91; which is 16 cents per litre more than 87. That is nearly 50/gal in US dollars which would make it difficult to sell except to those people who are willing to pay for performance fuels. Hell, we have seen people wanting to use 87 or 89 in their WRXs on this board.

Yes, and it says 91 minimum. I have my owner's manual in front of me.

Probably because of carbon deposits. That can happen in older cars and it causes an increase in compression. When that happens, you'll get knocking on lower octane fuels. If the car is working well, higher octane does nothing for it.

Your assumption about higher octane fuel leading to more carbon deposits is incorrect with modern fuels. There was a time when higher octane fuels typically contained higher levels of olefins. The olefins also led to increased gum/varnish as well as engine deposits from incomplete burning. Modern reformulated fuel in all grades is far lower in olefins. Even back then, several higher octane fuel brands would use higher levels of detergents to compensate.

"With the advent of air pollution regulations, property specifications have been supplemented by some composition specifications. As noted in Chapter 2, the first gasoline-related air pollution regulation limited the amount of olefins in gasoline sold in Southern California by establishing a Bromine Number maximum specification. More recent regulations limit the amounts of both olefins and aromatics in reformulated gasolines."

BTW - apparently it's illegal for refiners to add manganese to gasoline in California, which would include MMT.

I didn't say that. I said that as an engine gets older (particularly if it is on the highway frequently after being used for city driving) it can build up hard carbon deposits. Doesn't matter what kind of fuel. Whne that happens, it increases compression in the cyclinder and higher octane fuel will reduce that. Hence, older cars frequently work better on higher octane fuel even if they were designed for regular.

Sure - higher octane fuel is a crutch for other problems such as heavy carbon deposits. However - the quality of the deposit control additives in current fuels is extremely high and the composition has changed with reformulated fuel. Carbon deposits shouldn't be the problem they were 15 years ago. And even if they do happen, some of the aggressive detergent treatments (Techron Concentrate, BG 44K, Regane) are known to work wonders.

Sorry - I had a little confusion - the original comment about turning down 91 octane was from "Moon Guy". My comment was really directed at him. There is no reason why he shouldn't have used it at the same price as regular given the composition of modern reformulated gasolines.

Definite agreement.

On a sad note, I'm noticing some engine ping recently, and I'm using 94 octane up here in Canada. :( Sucks to be me.

Detonation is supposed to be from a curve of pressure and/or temperature. Hard carbon deposits might result in a slightly higher compression as they displace some of the combustion chamber volume. That contribution to compression ratio increase is supposed to be minor compared to the residual heat that is trapped by the carbon, especially if the carbon is on the valves.

Agreed. I wouldn't spend the extra money since it won't make a car work better unless it needs it for some reason, but it won't hurt either. So, at the same price, I would take it too.

We once tested a "high compression" engine that had a designed compression ratio of 11.5:1 and the head pressure at TDC was supposed to be about 186 PSI at sea level. The head pressure was actually about 210 PSI and that worked out to almost 13:1; too high for regular gas. The car was 15 years old and had been an old person's car. It was bought by a kid and it was driven hard. That is pretty significant. However, the car worked better on

What kind of car? That sounds like an STi problem. I had it. Seemed to be most prominant when it was hot out. The dealer did a reflash of the ECU and the problem went away.

Sounds like "grandpa" didn't run the car hard enough to hear any knock.

However - I'm just trying to point out that the residual heat from moderate carbon deposits will affect the octane requirement before increased compression from heavy carbon deposits will. The case you cite seems to be rather extreme.

Yes, Canadian-spec STi, 2004 model. According to a few forums on NASIOC, there was a wide-spread annoyance with engine ping, and Subaru came out with a quick-fix ECU flash within.. what was it? A few days?

How long did it take them to reflash your ECU? There're reports of there being only a single reflashing unit in all the U.S. (or there was, for a while) so peoples' cars were being held for up to a week while the operation continued.

Did you notice any difference in power or response before and after?

They did it as part of scheduled maintenance and no hold-up at all. I noticed it being a bit less zippy at the bottom end, and the power came on more quickly near the top. However, when in the meat of power band, I didn't really notice much difference at all; except that there was much less pinging. It still pings a bit when it is really hot out, or if I have been sitting in traffic and the intercooler is hot. However, a shot of spray and it goes away.

Whatever. The point is that older engines, even when designed for regular, will frequently work much better on higher octane than on regular whatever the reasons for it because the higher octane resists combustion before the ignition.

You can find 91 all over the place in the US. East coast you get luckier with 93 and 94 found at the pumps

Yea.. right. Another non-performance car driver who thinks that because there's no difference in his car, there's no difference in anyone's car.