How Much Octane Is Enough for My Classic?

Which nozzle should you grab when filling up? When in doubt, read the manual. Assuming we’re talking about a stock, production street vehicle, the owner’s manual will explain which fuel to use–but, of course, questions might still remain.

Are all octane numbers equal?

In the U.S. and Canada today, gasolines are rated on the Anti Knock Index (AKI). That’s the figure found on our pumps and inside the owner’s manual.

The worldwide standard, however, is the Research Octane Rating (RON) octane rating, so you might also find this one listed in the owner’s manual, particularly for an older foreign car. The manual for our U.S.-spec 1984 Porsche 911 Carrera, for example, lists both minimums: 91 RON or 87 AKI.

What if the local stations don’t offer enough octane?

At higher elevations, you might notice octane minimums that are lower than the national norms. “At high-elevation areas, it’s common to see the octane lower,” explains Zachary J. Santner, technical specialist at Sunoco Race Fuels. “In the mountains, less barometric pressure means that you don’t need all the octane.”

“It’s not a bad thing,” he adds. “It’s simply science because of the less dense air.”

What about using too much octane?

Some of us have heard the rumors: Running too much octane will actually decrease performance. According to Santner, the science doesn’t point towards octane rating as the culprit. “Many fuel properties play a part in the perfect fuel/engine combination. The evaporation rate is a usual culprit for a poor fuel/engine combination, not octane,” he explains. “It’s like having extra insurance.”

High-octane fuel is also a more refined product, he continues, meaning it will age better and leave less deposits than a lower-grade product–big benefits for a car that might sit for a bit. “Vehicle mpg is impacted by a wide variety of conditions,” he says. “Some modern cars can adjust to the higher octane of premium and provide slightly better mpg.” The recipes used for pump fuel can vary depending on season and location. (Sunoco Race Fuels, he notes, are always blended to the same formula.)

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Comments

wspohn SuperDork
3/9/21 12:14 p.m.

There is so much folk lore about octane - all the people that go fo premium because they feel it is babying their classic when it doesn't need it and they are just wasting money.

Old British cars usually run just fine on 87 regular unless they have carbon deposits in the combustion chambers (fairly common) and/or hot spots in which case 89 or more may be needed, although the real solution is to pull the head and attend to any potential hot points in the chamber.

I have run up to 10:1 engines on 91 octane and up to 10.7 on 94 with no problems. That is all old engines using carbs. Modern engine management is a whole new ballgame - I run one engine on 94 with 11.5 compression and another with 9.5...but 25 psi boost.

jharry3 Dork
8/26/21 8:07 a.m.

I remember using 98 octane Amaco pump gas in my Mustang in the '70's. 11.5:1 compression ratio and a lot of timing advance.

Is that old 98 octane the same as modern day 93 octane? I've read that the rating formula changed since then.

Colin Wood Associate Editor
8/26/21 9:15 a.m.

In reply to jharry3 :

That's a really good question. Let me run it up the flagpole and see what I can find out for you.

gsarahs New Reader
7/3/24 7:52 p.m.

The really important point that needs to be emphasized is not so much the octane but whether the fuel contains ethanol or not. I run my 2008 Volvo C70 on Regular (which locally has ethanol) without issues for 15 years, while my friend with the same model of Volvo insists on using 91 octane premium. No problems with either due to the fact that they are modern vehicles. My opinion is that my friend is wasting her money, but its her money!

However, with my Jaguar E-Type, Austin-Healey, Triumph TR250, MGA Coupe, and Volvo 1800ES, I only fill with non-ethanol 94 octane from Chevron, since they are the only company that guarantees that there is no ethanol in its fuel. What is important to remember is that ethanol doesn't do good things to older cars' rubber seals and gaskets, and that is the primary reason for me making sure my fuel does not contain ethanol. I need to find out whether mid-rang 91 octane gas contains ethanol before I will use it or not. And I gather that this can vary depending upon the region you live in. I live in the Vancouver area of British Columbia.

As far as quality of fuel, 2 or 3 members of our local Healey club paid the price of filling up in rural out of the way locations when down in Washington State, since they were not able to get back home without bad fuel issues.

wspohn UltraDork
7/4/24 11:57 a.m.

All of those old British cars run fine on regular modern gas with no ETOH in it, except the XKE which should have 91. Having said that, any of the others could experience ping if they have carbon deposits and ping and a step up in octane can minimize that The main issue os to avoid gasohol if at all possible as that seems to give our older cars some issues.