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.

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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
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.

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