My question is this: Was this slight increase really necessary for the marginal increase in the maximum speed of the later FEF classes, or was this really a status exercise by the Union Pacific to boast that their 4-8-4's had 80" drivers like their two main rivals, Santa Fe and Southern Pacific?
Stuart
Increase driver diameter, you get two benefits...
1) Horsepower peaks at a slightly faster speed, so you get more horsepower at those higher speed ranges.....you can move the same train over the road a little faster, or move a slightly heavier train over the road at the same speed.
2) At that high speed for the larger drivered engine, you get a small increase in fuel savings for the same length train, because the engine isn't working quite as hard as the earlier one would be--if the engine with smaller diameter drivers was operating at a speed beyond its horsepower peak.
Remember, horsepower (tractive effort at speed) is the game here. It increases to a point of maximum efficiency, then if you go beyond that speed, horsepower falls off. Sometimes falling off fast, sometimes more gradual depending on design of the locomotive. But it will fall off. Increase your driver diameter, that speed where you reach maximum efficiency increases. So you basically shift the curve to the right for more horsepower at higher speeds at the expense of slightly less horsepower in the lower speed ranges.
Note: you can get into the situation on a certain route with a certain train that you don't have the horsepower to get to that higher speed range with the larger diameter drivers, and the engine will perform "worse" than an engine with smaller drivers. Real world example: Milwaukee 261 vs. NKP 765 on the New River Train. 765 actually handled the train over the road faster than the 261 did.
So, back to the UP....they either wanted to power the same train at faster speeds, power a heavier train at the same speed, or save money moving the same length train at the same speed compared to what the "smaller" engine costs because the first engine was "maxed out". One way or the other, like most everything else, it probably came down to money. They decided an 80" drivered engine was better value for what they needed.
