The 550xp mkII Is 4.0 hp and MK II 545 3.6hp. And I have the 550 the 545 I get wed/thursday. But both of the mkIIs no comp release and 11.7lbsKinda wish I didn't buy. But the 550 runs like a dream.The other 550 I said I have is the 2253 jonsered.Not husq but I love it too.550 was 3.75 hp 545 3.3 but I have seen guys run a 2252 w 18in x3/8