My owners manual says Volvo recommends 91 octane with a minumim of 87. Besides the idea that better gas will make my volvo perform better is there any other reason for their rec. of 91? will I be doing some damage to some obscure sensor (etc.) by running 87 on a regular basis? Thanks.

I've been using 87 on my '87 for a while. I haven't noticed any difference between 87 and 89, and I ran it up to 5200rpm just last week with no pinging.

The octane rating has to do with the flash point of the fuel. The higher the rating the harder it is to ignite. A spark has no trouble igniting the air/fuel mix but what we're trying to avoid is the pre-ignition of the mixture. When the piston compresses the mix in the cylinder it starts out at a certain volume and then gets crushed anywhere between 8 to 11 times as small(that's the compression ratio, eg 8.9 to 1). The air/fuel mix gets really hot just because it's getting squished(that's what gasses do) and if the temp reaches the flash point, BANG it explodes before the spark plug can do the job. The down side to this pre ignition is that the piston has still not completely finished the top of the stroke and now must do so in the face of an explosion. That means that the carefully timed explosion event which should occur when the piston has just started down(lots of energy) is early and the explosion energy that should be carried to the crankshaft via conecting rod is lost and even can contribute to slowing down the piston or in a bad scenario bend the rod or blow a hole in the piston. The flash point is reached easily in high compression engines, 9:1 to 11:1 and not so easily in low compression engines, 8:1 to 9:1. You need not worry if you run 91 as recomended but you should only run lower octane if you run in cooler weather. Conversely don't waste your money on the highest octane fuel either just because of the advertising hype. You should only buy as high a rated fuel as your car needs to run without pre ignition, any more is wasted if you have a lower compression motor. Turbo owners should go with the highest they can find because the air gets hot by being compressed by the turbo and then again in the combustion chamber.

Congratulations, Herman! What a well done explanation. Best regards.

I used to use ONLY 93 octane (the "good" stuff).

But when gas prices went apesh*t, I cheapened down to 87.

No difference. How about that. As long as you use a quality brand, you shouldn't have problems, unless you are getting pinging already, like Jim said.

I never noticed any difference, and i get excellent mileage on the 87 anyway.

Higher octane will only increase your car's performance if it pings on 87 so that the ignition is retarded. 87 should be adequate unless you're flogging the motor very hard.

87 octane won't damage anything unless you're car has other problems.

When I'm towing in the mountains in really hot weather, I sometimes use 89 in my '85 245.