Nebraska Tractor test summaries used to be available on the web. You could try U. of Nebraska test site. Unfortunately, the last time I pulled this up, there was nothing more than the "rated h.p." The real Nebraska tests are mines of information. You get MAX h.p., which is all the tractor can do, and RATED H.P., which is what the manufacturer claims the tractor will do with a certain amount left over. This was a kind of insurance against overloading. Back in the 20s, 30s and 40s, if you bought a tractor rated at 10 db hp, for example, you knew it would pull a 2-bottom 14" plow at about 3 mph (I'm using numbers that were applicable back in the 20s, 30s and 40s; plows were designed to be used at about that speed). A tractor that would be rated at 10 hp would probably put out around 15 at MAX H.P. An interesting factor is the wasted power on steel wheels (probably 35% on average). This meant that the engine had to put out a lot more power just to get the rated h.p. AFTER subtracting the wasted power through the wheels and lugs. This is why early tractors always had two ratings (10-20, for example). When tractors began to be designed primarily for rubber tires, which waste a lot less power, the engines could be made smaller and the difference between engine/pto/belt h.p. and drawbar h.p. was a lot less. Compare a Farmall H with its displacement just over 150 c.i., with a 10-20, which displaced over 280 c.i., whose horsepower, at much lower rpms, was about the same as an H. When I was growing up, we used a rubber-tired F-12 (rated to pull 1 16" plow on steel) to pull 2 12" plows. Our steel-wheeled 10-20 would pull
2 14" plows, but not much more than that. The F-12, in other words, was doing nearly as much work as the 10-20, using less gas, and was a lot easier on the old backside (in my case, the young backside). Recent Nebraska tests show that rubber-tired tractors, properly weighted to reduce slip, put out almost as much horsepower on the drawbar as on the PTO, because the tires produce less "rolling resistance," as the engineers refer to it.
Some agricultural college or university libraries have the original Nebraska tests on their shelves. Unfortunately, I don't think Nebraska ever published the complete tests. CF Wendel published the tests, but did not reproduce the tables; occasionally, some of the results got overlooked and are missing. Still, it is one source of information.A note on horsepower versus torque. The old tractors I describe above usually had huge engines that ran at slow speeds. Their torque was tremendous, but because of the slow speed, not a lot of horsepower was developed. With the development of "high-speed" engines, displacements went down. A tractor like a Farmall H produces almost as much h.p. as a 10-20 or F-20, but by running a smaller engine much faster. When a 10-20 or F-20 is overloaded, it will slow down, but usually manage to keep on running. A small, high-speed engine (F-12, F-14, H, for example) will stall out much sooner when overloaded. I've spent hundreds of hours on 10-20, F-20, F-12 and H, and I've experienced this many times. The smaller engines pull fine when up to speed, but die out pretty quickly when overloaded. The H could actually be stalled out when heavily overloaded, but the F-20 and 10-20 would just keep on chugging. A C or SC compared to an H or SH would have the same problem--the horsepower might be there, but the torque wouldn't.
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