I understand the D600 is DESIGNED for 3600 RPM. This should be where the best fuel-consumption/power ratio is.
The D600 was a favorite for electric-generators which MUST maintain EXACTLY 3600 RPM. (3600 divides down to generate the 60hertz frequency)
Lets not forget that 1000s of D600s were used in refrigeration units (on tractor-trailers) These things ran for months at a a time.
Personally, I like the D600 engine in my G5200 garden-tractor. The Hydrostatic xMission does not like when the engine is below 3600. When the engine is below the specified RPM, it forces the HST to work harder than it should. The manufacturer of the HST in my G5200 (Eaton) specifies 3600 as the proper input RPM for best power-transfer to the wheels.
