The 400 is a sweet engine... bullet proof... it has all the same bottom end as the 600... you can add a pair of CR33's and a predator exhaust... both fairly simple to fit.... you'll get a bit more oomph and it'll sound good, i think the 400 revs more freely than the 600, and although down on power, is still happy galloping along at 75 - 80 all day... and with the front sprocket taken out to the largest possible (14 t i think), it becomes a very nice bike.