<font color="red"> I'm a little confused by all the different PTO hp requirements different manufacturers list for their tillers. </font>
Slim, listen to the manufacturers recommendations, they are there for a reason. For example, Brand X may say they need 20pto hp to operate their tiller, but Brand Y needs 23pto hp to operate that brand . . . why? Well you need to look at the design.
-- For example a reverse spinning tiller requires more horsepower than a forward spinning tiller.
-- Gear drive is supposed to require less hp than chain drive.
-- The shape and number of the spades or tines will also make a difference. If the tines are simple bent tines, then they require less hp than spades.
-- The number of tines/spades will also make a difference.
-- And then don't forget about soil conditions, that can make a difference too.
I have clay on my property, but there are 2 different clays, on my upper ridgeline the clay is manilla colored with no real organic content, I turn a 50" tiller (LandPride RTA1550) with no problem at full depth on the first pass with a tractor that has 19pto hp. But when I go down into the valley area (60' elevation change) the clay is black and sticky and clumps even when it has not rained because it holds so much moisture. When it is dry as a bone, I can bury the tiller down there and churn it without out any problems, but if it is just a bit moist, the tiller will slow down and even at full throttle the machine won't quite hold 540 pto speed. My brother lives much closer to Lake Michigan, his soil is very sandy, running the tiller to build his yard was so easy it was like the tiller had no load at all! My friend lives 40 miles west of Chicago in a new suburb and his soil for his yard came in on by the truckload and was nice black topsoil, again, absolutely easy to churn it up and mix it with the hardpack that was under the new soil.
So consider the manufacturer's recommendation and consider your soil conditions.
