the two are pretty much the same, but the electronics are different from one to the other, the 05 changed to a simpler design, more compact than the 04, the 04 is a fair bit more to replace the ecm ect. than the 05, they also use a different idle strategy which really doesn't matter to anyone, the 04 once warmed up only idles on one cylinder only, the 05 idles on one as well but switches from front cylinder to the rear (stratified idle i think they called it?), you would likely never notice the difference. The exhaust is a little different as well, the pipe coming off the tuned pipe is different, I have seen the 04 style crack near the flange where it mounts to the tuned pipe, I have seen one where it actually damaged the tuned pipe as well. I am pretty sure the 04 originally came with the kart seat, and in 05 you had to buy it as an option. either one a lot of fun I think, especially with the Shoq attachment!