Octane 86 with an 87 performance tune??

[chito]

Hello,

i have a 2004 FX4 and had purchased a gryphon back in 2009 with an 87 performance tune due to the fact that i had installed 35" tires and put a leveling kit on it. The problem is, i used to live in New Mexico where the gasoline octane grade is 87 (regular), BUT, in 2010 i moved to el paso where there is no octane grade 87, only grade 86 for regular gas....

since 2010, i have been using grade 86 on my 87 performance tune, i havent had any problems, but should i order a new performance tune for the grade 86? is there even a performance tune for gasoline grade 86? or should i just leave it the way it is?? should i start using mid-grade 88?

My last question, i have 35" tires, and i was thinking of regearing from the stock 3.73 to 4.56. Can i keep the 87 performance tune i have on it now if i switch to the 4.56 gears, or would i have to get a new performance tune?

Your help is appreciated, thank you

[Jackpine]

The Ford Owner's Manual says you shouldn't use gas rated lower than 87 octane, but I have, when that is the "regular grade" and have experienced no problems. The regular grade in Colorado Springs and similar higher altitude locations is 85 octane.

If you operate the truck at high altitude or in an area of very high temperatures, the air is less dense. This has the effect of reducing the actual compression in the engine because even though the "ratio" is the same (9.85:1 as an example), the "1" part is less than atmospheric pressure at sea level (14.7 psi). So, multiplying that lower pressure by 9.85 gives a lower compressed value and less tendency for the air/fuel mixture to detonate (ping). As a result, you don't really need the increased anti-knock properties of the higher octane.

You don't need a new tune if you burn 86 octane where you drive. You see 86 octane gas there because of the elevation (3740 ft) and the generally warm temperatures.

Switching the gearing to 4.56 will not merit a new tune either. But, you will want to change the Gear Ratio to that value in your Custom Options when you tune your truck to make the speedometer and odometer read correctly and to have the truck shift at the appropriate points.

- Jack

[Longshot270]

Jack nailed it

[chito]

Thanks Jack!