I turn 6000 RPM’s, but this is not necessary, except on the big end on the track where you grind metal as we call it. I have pulled in the mid 5000 range under power, and it still pulls on the accelerometer, meaning its still making power. I pay attention to detail, on the valve train, and put an acceptable set of springs on the motor. The valve train on a Cummins has good geometry, and this make the job easer. The valve springs I run are about 30 % heaver then what the other guy sells, I run a titanium retainer, and tool steel bead lock keepers.
Hi rpms are very necessary in making usable hp on a Cummins; if you lug the motor it will defiantly spit parts at you. I balance the motor with in a half gram, and use balance the crank, all this is necessary to keep harmonics out of the gear train on the front of the motor , and saves a lot of grief in this area. I know every one will say don’t rev it , and use thew torque it will make , well hp is the only thing I am after , I never get with in a 1000 RPM’s of peak torque , but that’s just my way of running the little truck, to every one that says that is wrong , “ OH WELL “ if you want to come out and run , run it your way ,
The new motor’s rotation assembly is about 2000+ grams lighter, and that is each hole , so I am expecting a motor that will make power right up to the 6000 to 6500 area