Originally Posted by oxycodone
That's quite possible in a high compression motor like the Crossfire's. If you run low octane fuel in your xfire, you'll probably experience pre-ignition, which means the fuel ignites and starts burning before it enters the combustion chamber. Therefore, you're not getting all the possible power out of the fuel that's being burned.
In a lower compression motor (read: normal everyday car whose manual says to use at least 87 octane fuel), higher octane fuel is going to have no effect whatsoever - except on your wallet.
What it amounts to is: put the kind of fuel in it that the manual says to, no matter what vehicle you're driving.
Other notes to those wishing to improve mileage: Check your tire pressure frequently. Under inflated tires can have a serious effect on mileage. Also, check your air filters and clean/replace them when they're dirty - Air/fuel mixture being off can have an effect on mileage.
You are right, if our motor was a carbureted Small block chevy from the early 80's and had 10:1 CR. then yes it would most likely have pinging issues with 87 octane. Now the later smallblock chevy's that came in the police impalas had LT1 350 motors with 10:1 CR, yet GM stated that the cops could run 87 octane in those engines for well into 300,000 miles, now why is that?
It's mostly due to the reverse flow cooling heads which pulls the heat out of the heads first instead of the block, and also the ECU's ability to sense detonation by taking out ignition timing.
So I wouldn't worry about running cheap gas, in your XF, it just wont have as much power because of the retarded timing, otherwise your good to go.