Like Chris said, the Series I machines were non-turbo. They were rated at 117hp. The Series II had an updated engine with the turbo, and it was rated at 127hp. I've seen Series II machines turned up to almost 170hp, and I'm sure more is possible in the stock setup. I really like the D21s, too, and I hope to own one at some point!

From a show last summer:

Glenn; thanks for the kind (and very true) words on the AC D-21 SII. I know for a fact that a STOCK, factory D-21 3500 diesel engine can crank
out 275 PTO horsepower; because I was the mechanic
operating the PTO dynamometer;shortly therafter the D-21 twisted the PTO connecting shaft off the dyno; took me 3 months to rebuild it.I was going TOWARDS FULL throttle on the D-21 when the PTO C/shaft sheared, I was at 2100 RPM when the dyno read 275 hosepower. It takes more than just increasing the d/fuel to get these engine to make this kind of HP; most old, tired stockers will just come apart at the bottom end; I did it one time years ago; when I was younger and more stupid. I trashed the D-21 engine in the process because the crank broke and came out the side of the block sideways; good thing I was up in the tractor seat; crankshaft on a Seies II weighs 320 lbs w/flywheel!!!!