I must have been bored yesterday (hey, I was at work) so I actually read the owner's manual for my 2008 F250. Nothing earthshaking there, but in the recommended fuel section it states that Ford recommends only fuel with a minimum octane of 87 - furthermore, it goes on to say not to use unleaded with an octane of 85 sold as Regular in high altitude markets - Now I live in a an area where regular grade is 87, but I travel in Utah and Colorado and the fuel there is sold as regular at 85. I found this on Wikipedia:

"In the Rocky Mountain (high altitude) states, 85 octane is the minimum octane and 91 is the maximum octane available in fuel. The reason for this is that in higher-altitude areas, a typical combustion engine draws in less air per cycle due to the reduced density of the atmosphere. This directly translates to reduced absolute compression in the cylinder, therefore deterring knock. It is safe to fill up a car with a carburetor that normally takes 87 AKI fuel at sea level with 85 AKI fuel in the mountains, but at sea level the fuel may cause damage to the engine."

My question is this - if I use the 85 octane in my truck, am I going to have more possibility of knocking, or does the computer alter the ignition timing to compensate for the lower air pressure/octane? Do I need to spend the extra money to buy the mid-grade, or is Ford playing the CYA game?

 

I don't have any personal experiance to back this up, but I do believe from the sources that I have read, that you can program the truck to accept lesser fuel ratings. Although the ones that I have read doing this were usually going from premium to regular, but if it happens in that case I don't see why it couldn't apply in yours as well. I would imagine it's mainly a timing issue and most custom tuners(people) have access to change timing on their tunes. If it's really bothering you, might want to look into that.

 

Depending on what engine you have, although I believe all the Superduty gassers have it, you SHOULD have a knock sensor.

That means if it pings, the PCM will retard the timing to compensate. I don't think it's a good idea to run it for LONG like that, especially pulling a heavy load, but it's not going to hurt to run a tank if you HAVE TO.

 

I've been through Utah and Colorado and never noticed the octane on the gas. Probably because I was driving Dad's 7.3L at the times. But I would wonder, here in Oregon we have regular (87 I think), premium (which is 91 or 92), and usually a mid-grade which is somewhere in between. I wonder if you can't just buy the mid-grade while in Colorado and Utah and get the Octane you need.

 

I'd buy the recommended octane for your truck, even if it cost a few cents more.

 

Your motor is equiped with a Knock Sensor. The computer will sense that and retard the timing to eliminate the "ping". I run 87 for my around town day to day with the truck and will run higher grade fuel for towing.

 

thats why they make the mid grade for this prob

 

If you do fill up with 85, to be safe, just be sure you burn it all "at altitude". Minimum elevation in CO is ~3500' and if you go over certain passes you'll be at 10,000'+. I've run 85 in my gas vehicles with zero problem, and it's fine because of the exactly the reason stated in Wikipedia. That said, on my way out of Utah/Colorado I'd be sure to fill up with mid-grade and maybe some octane booster, just to make sure you were at or above the minimum. You don't see locals that drive gassers in the shop for blown engines constantly. If that was a cause, don't you think some lawyer would be foaming at the mouth to sue the big oil companies?

85 is fine while you are at elevation.

Go back to 87+ when at lower elevation.

 

My own experience confirms what Wikipedia and papadelogan said. The old carb'd tow van was very sensitive to octane, and without a knock sensor, she let me know when she was unhappy. She was OK with a couple points lower octane at 5k+ feet.