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Not sure if anyone has the good tuner stuff for 4.0l OHV(1990-2000) its 23 years since the last one was made/sold
And in Denver you have the issue of low air pressure and "thinner air", the 5,000ft elevation is Turbo country, lol, just to break even
Ranger 4.0l OHV came with Cold Air Intake and tuned exhaust in all years
Factory Exhaust is tuned for mid-range power, so 3rd party exhaust should be tuned for low-range power to get any benefit, but you do lose the mid-range, so more power "off the line", less power cruising
All gasoline engine power is based on one thing, AIR
A 5 liter engine has more power than a 4 liter engine because it can pull in 1 liter more AIR, simple as that
Bigger Cams allow in more AIR
Turbo or super chargers add more power because they can Force in more AIR
Nitrous Oxide releases more oxygen(AIR) when its used
So common theme here is AIR = power, more AIR = more power
If you just add more gasoline you get a flooded engine not more power
So of course Less AIR = less power, which is why higher elevations reduce engine power
Ford does a pretty good job in tuning the computers for the engine, but they tune for best performance, best MPG and best emissions
So a custom tune can forget best MPG and best emissions and focus on best power
But in the case of the 4.0l OHV you would be looking at less than 10 more HP, closer to 5HP and not much more torque, unless you went with a 93octane tune, might get above 10HP added, but not much above, and have to run Premium gas $$$