</font><font color="blue" class="small">( SkyPup, you've brought up a topic that I guess I should post a new thread for, but here goes seeing that you are so very knowlegdeable in so many areas. Why doesn't our extremely well made diesel engines have hydraulic lifters. Many a fine diesel automobile have them. Volkswagen, and Mercedes come to mind. My NH TC48DA shop manual says to check valve clearance at 600 hours. I was a bit flabbergassed, assuming that the engine would have come equiped with hydraulic lifters. Just thoughts....... )</font>
I am not certain about that, but the VW & Mercedes have overhead cams with much more sophisticated fuel injection equipment (common rail, pump duse unit injectors, peizoelectric injectors, all at much high injection pressures, etc.) for much higher rpm operation.
I would guess that it has something to due with the cam grind and how steep the lobes are, how stiff the springs are, duration of the cam and total amount of lift.
Diesel tractor engines are low speed low horsepower diesels comparatively speaking to high output modern passenger car diesels, so I would imagine the air flow characteristics of the tractor engine combustion chambers are totally aimed at a different mode of output, ie. milder cam profile so not max lift for max flow at high rpms but probably longer duration to get more air into the combustion chamber prior to the end of injection and ignition before top dead center.