which fuel grade - 93 or 89 or 87?

[sunshine]

I'm wondering which fuel grade I should use for my 2004 XC70. I know that according to the manual, 91 or higher octane is recommended and the minimum required is 87. I've been mostly using 89 for the last two year (since I bought it), and I think it has been fine. I'm thinking about switching it to 87 and am wondering if it can do any harm. Does anyone know about this?

[KILOFINAL]

I would go with the highest octane rating you can afford.I put nothing but either 91 or 94 octane. In fact the difference in price between 91 and 94 is sometimes 1-2 cents per litre. In the grand scheme of things what would you save each year by using 87 octane? Not more than perhaps $150 and you would get a car that should perform and run better. Regards

[Phrog]

Discussions over which octane to buy are like those over oil change
intervals - lots of strong opinions, not much statistically-valid evidence,
in the end it may not matter all that much!

That said, my opinion is that you should run the lowest octane that
the car seems happy with. Higher octane is not necessarily "better" -
octane is simply a measure of resistance to knock. Higher octane
means fuel ignites less easily. For high-compression engines, this is
important, but less so for engines like the Volvo 5 cylinder. Oil companies
market higher octane fuels as somehow better for your car, but that's because their profit margins are much higher on those fuels.

[Pennhaven]

Higher octane
means fuel ignites less easily. For high-compression engines, this is
important, but less so for engines like the Volvo 5 cylinder.

Semi-true. A turbocharged engine like the Volvo T5, is a "high compression" engine, as the turbo charger raises the effective compression ratio.

[Phrog]

Good point re the turbo. However with an intercooler (which I think the
XC70 has), air temp is reduced. It's high air temperature, not pressure or
compression, that in the end causes knock (premature ignition).

Interestingly, here in Colorado we're at ~6000 feet and thus use lower
octane fuels. PV=nRT...less P means lower T, all else equal.

Any more engineers want to jump in here?

[SYS]

Good point re the turbo. However with an intercooler (which I think the
XC70 has), air temp is reduced. It's high air temperature, not pressure or
compression, that in the end causes knock (premature ignition).

Interestingly, here in Colorado we're at ~6000 feet and thus use lower
octane fuels. PV=nRT...less P means lower T, all else equal.

Any more engineers want to jump in here?

I also live in Colorado (Colorado Springs). When my car was delivered to me by the salesman, one of the things he had told me was that I should use a lower octane during the winter season. Can you or anyone on this thread tell me whether I should indeed take his advise or simply stick to the same octane regardless of the season? Thanks in advance.

[Pennhaven]

Anything greater than 91 octane is very likely a waste of money, as 91 is what the engine is designed to run optimally on.

If you alternate fill ups between 93 octane and 89 octane, the blend you achieve in the tank will average out at 91. i.e., Fill your tank with 93 octane, when it gets down to about 1/2 refill with 89, run it down to 1/2 and fill with 93, etc., etc. This strategy would get you the optimal performance for the least amount of money.

Personally I use 89 octane nearly all the time. I only blend in some 93 when the weather is very hot, which is when the engine is most susceptible to "knock", and most likely to suffer reduced performance as the engine managment system dials back the ignition timing to prevent the knock. This is when having higher octane fuel is most helpful.