You have it right. In general, the old 12V engines are best on mpg. Why?

First is emissions limitations. The "optimum" timing for mpg is much more advanced than the "optimum" for emissions, so guess which comes from the factory?

Second, is power output. The older 12V engines are simply a lot less powerful. Early models are only 160hp, with the later 12V maxing out at 215hp. Compare that to a common rail truck at 305-350--big difference.

Keep in mind that the 24V engines have twice as many valves, twice as many springs, and all that. IOW, the 12V should have less "turnover drag" or parasitic loss from the valvespring pressure and such.

IN addition, the 12V engines have more tq output at lower rpm. The smaller breathing area increases velocity at low and no boost conditions and you get more air in the cylinder at low rpm-- hence, you can burn more fuel at low RPM.

Finally, it takes a LOT of power to drive a common rail pump because of the high pressures. I've been told that the VP44 actually takes the least amount of power to drive, as the P-pump is pretty power hungry itself. So that one would seem to hurt 12V economy relative to later vp44 trucks.

Finally, as the RPM range of the trucks expanded, the engine becomes less optimized at any given RPM. In other words, the narrower the operating speed of the engine, the more I can optimize it for that engine speed. The Redline RPM of these engines has been raised higher and higher as the years passed-- and the peak HP gets higher and higher too. Early trucks made peak hp at lower rpm. Mine peaked at 2700rpm, factory. Newer trucks are at 2900 rpm. Up and up it goes, taking good mpg away with it.

Now, if the OEMs were free to apply the ultimate in HPCR technology to fuel economy alone, we'd have CTDs getting 28mpg regularly. But that would mean open exhaust and other things that are .


The final thing is keep in mind that the trucks keep getting bigger and heavier, laden with more features. This takes more fuel!

Given the handicap of a 4x4 that lacks CAD, I'm VERy happy with 16mpg bumming around town.

JH