I have a Fatctory 4.1 on my 08, it made a big difference over stock. More bottom end pull. I do think it's funny seeing all the posts on here. These are 450's, do actually think you are using all of the power the bike produces? No. So a couple of horse power is not going to make or break your race. Even on starts, I used to race a KTM 200sx in 250B (when 250 B was 450's and 250 2-strokes) and get great starts. You can have all the power in the world but if you can't put it to the ground and get traction you will go nowhere. Oh yeah and momentum helps just a little. Thats just my opinion on 450's.