SawTroll, you're right, a 22" bar may be a little much for 4hp, but in my part of the country, I rarely have enough "big" wood to fully bury the bar. Occasionally I might have a big white ash that will test the power, but I just use patience instead of power, or fire my 288XP. I never thought about the S and P designation as semi-pro and pro, even though the side decals are right there!! I have had many of these saws (small frame) apart, and I can't tell any difference in the bottom end. They all used the same stroke, and the top ends interchange. One S50 (55cc) I have does have a different looking piston style. All the porting looks very similar. Carbs do vary by model. Partner used marketing more than engineering changes to differentiate models and fill the line. The F (farmer) series was more "frugal", with minimal decals, spur-type clutches, and generally no anti-vibe handles. Some later models were switched to plastic recoil housings. But after owning a few Husky's now, where it seems like most parts are "just a little different" between models, Partner really had a good thing going on part standardization. In fact, my first Partner R19 was found sticking out of a snow bank while I was hunting some big paper company land. It looked as if it was there at least a couple years. I knew my neighbor had a blown S55, and after some parts exchange, I had a great runner. I guess I got bit by the bug right there, because now I've got over ten good runners, most of them built up the same way.