Effects of 94 octane?
 

Coming back from Whistler I filled her up with 94 octane and felt that the car ran much better. It was smoother and didn't feel as labored up hills, etc. Maybe it was just whishful thinking, but should one tank of higher octane make a difference? How fast will it adjust to the higher octane gas? Usually she gets whatever we get here in Seattle, 91/92...

Sadly, spending several days in freezing temperatures further highlighted the need for some new door lock actuators. :(

91 vs 94 shouldn't make any difference. Two things could make a difference, though:

1) I assume you purchased the 94 octane in BC, likely the Lower Mainland/Greater Vancouver. If so, the fuel is often higher quality than locations south of the border.

2) If you normally drive in the city, and you got some highway miles in coming up the I5, the vehicle may perform better due to having burned out deposits that build up over time.

Apart from that, you may just be on a natural high from the skiing/boarding ;)

Our "premium" in my area is routinely 93 octane. On rare occasions I have been forced to put something lower in it, and really can't tell much difference.

Lower octane is a bad idea...you stand a better chance of detonating under load, which can lead to a blown head gasket. I REALLY doubt it would ever happen, but why chance it. I run 93 from Chevron and NOTHING else. I also try to run some high rpms once a month to keep the engine clean.

Also, doesn't the manual say Premium fuel only? With higher compression motors you need to run high octane.

it says minimum 91 on the fuel door....were i live we only have 87/89/93...so always 93....i have only seen a few Sunoco's with 94....when i was visiting Europe I saw Vpower racing which was I believe 99 octane...I wonder if that does nething?

The European octane rating is RON, not pump octane or anti-knock index (AKI) as used in North America. Figure about 5-6 points difference in the two measures, although it isn't a direct conversion. That means the 99 you saw in Europe is for all intents the same as 93 in North America.

Putting in a fuel with a higher octane rating, or anti-knock index, does absolutely nothing on a vehicle that can't take advantage of it. The X5 can't benefit from that extra knock resistance. A heavily modified engine (different pistons, higher compression ratio, etc) or one using forced induction, can often take advantage of a higher anti-knock rating. All others are simply throwing money away, unless the higher octane rating is cheaper :) It doesn't go faster, develop more power, get better mileage, or thank you for the treat. It can be harder to start in cold weather.

JCL I normally run nothing less than 93 octane in my 4.6 but every now and again I will put a few gallons of Sunoco 100 octane in and unless I have had out of body experiences it runs night and day different with the 100 octane race fuel...more responsive, less sluggish up hills, etc.

I think +1 octane will not make a huge difference to justify the amount you pay extra. ARound here its 87, 89, and 93. Some off-brand stations will have a 92, not that I go there. Call it the placebo effect :)

Maybe if you put in substantially higher octane fuel like a race fuel, there might be a difference. But seriously, 94 versus 93 should be negligible.

I believe you, I just don't think you can attribute all of that difference to the octane/AKI rating (unless you have significant carbon deposits that are causing pre-ignition on the specified fuel).

The differences could be attributed to the oxygen content of the fuel, ethanol blend, additive package, or quality (most common). Apart from those items, some fuels just work better than others in specific applications. I do agree that you should find a fuel that works well in your vehicle. I would be interested in whether you have you tried a variety of 93 fuels, and whether there is any difference around town or on the highway, and in hot weather or cold.

True 'dat. Exxon is no good in mine. I guess too much ETOH?