I have always liked running my C21, perhaps because it is somewhat of a compromise between the smaller C16 and the K series engines. The only problem was it always seemed to lack any guts and my C16 could pull more cars. I had always attributed this to it being a piston valve engine. This last Diamondhead I happenned to ask Dave about it and he told me that it had smaller cylinders than a C16 and that he had bored his out to 3/4" cylinders and it made a completely different engine out of it. After I got back home I sent Dave an email and asked him if he had the time would he fix mine like his. Well.......when I got this engine back I was amazed at the difference. I believe it will out pull my K27. The engine has amazing low end torque and will pull stumps out. The larger cylinders also make it run slower, but now it marches up any grade with ease. I put three videos out there on You Tube if you want to look at them. Dave says he will do this mod for anyone that wants it. The locomotive is never more than half throttle in these videos. I picked places where there is a slight grade so you can hear it dig in for the pull. The engine simply will not stall. Enjoy.