I have the 38 hp Kohler. It's my first chipper and a recent acquisition. Seems to me that I would not want an engine with less hp, especially if the knives were dull. I like my chipper very very much, btw, so I'm not knocking the 38hp kohler, but it does take a while to process limbs that are above 5" diameter. 4" and below they just chug right through. If the engine was less capable, I think you'd be looking at 2-3" going through without slowing down. I really think you ought to put the largest engine possible on it.