I saw your quote about the 21hp diesel vs. a high hp gas engine. I think the 21hp diesel would be enough hp for your needs, but it would not be bad to have a 23 or 24hp diesel either. Usually, diesel has higher torque than a gas with the same hp value. For example, take a Kohler Command engine at 22hp and a Kubota diesel engine at 22hp.
The Kohler engine will reach its peak 22hp at 3600 RPM but its only able to creat 32 ft lbs of torque at that speed. When put under a light load, such as heavier grass, the engine is unable to maintain its speed and will progressively slow down and loose power. The Kohler engine torque will only peak at 37 ft/lbs @ 2400 RPM where it generates 17Hp and slipping much lower will cause this engine to stall.
The Kubota diesel engine will give different results when put under load. Just like the Kohler gas engine, the Kubota diesel has less torque when running at full throttle, however the results under heavier grass loads are not nearly as dramatic. At a full 3600 RPM the Kubota engine has about ~37 ft lbs of torque which is still more than the Kohler can create. When the Kubota engine is pushed harder and slows to 2400 RPM, the engine will be at 19.5 Hp and 41 ft lbs of torque. Given the torque difference between the Kubota diesel engine and the Kohler Command engine, the Kubota diesel engine will take much more strain and even more strain to bring it to its knees.
Another issue that is largely ignored is the way in which these engines are rated. These companies realize that consumers are easily swayed by horsepower ratings and so they do their best to always stay ahead of their competition. Kohler bench tests using the "SAE J1340" standard which has no air cleaner, exhaust, charging, cooling or fuel pump. Once the tractor manufacturer (Scag, Exmark, Gravely and etc.) adds on the rest of the components a substantial drop in performance occurs that skews the gas engine specs even further. By using Kohler's bench test instead of real world performance the tractor company can make it appear as if they have more power than they really do. This is a very common practice and can be easily spotted on spec sheets where "As rated by the manufacturer" is in fine print. Kubota along with Yanmar (John Deere's engine), Shibaura (Case/New Holland's engine) and etc., used the "SAE J1349" standard which includes standard accessories needed for real world use. Going through engine performance data from Kubota would be what you find in a production tractor (such as there BX and L series compact tractors). You will also find that Kubota's 22hp and higher hp engines are actually capable of producing higher hp if its RPM's are further increased. The key to remember is that if you have higher demanding tasks to do with your tractor, such as cutting thicker grass, high/wet grass and etc., high torque is more important than high horsepower.
This is just my opinion.
Last edited by eagle107; 05-02-2010 at 06:02 PM.