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Join Date: Jan 2011
Location: Smack in the middle of Illinois
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04 had a 10.5:1 compression piston - 05 and up had a 12:1 compression piston, so if it had the compression ratio lowered, it will rev a bit higher, but suffer just a tad on the low end grunt - you won't feel the difference, but it would show up on a dyno run.
Carb was rejetted for what? Altitude? Air box modification?
Difference in performance between the 400 and the 450 is huge, but the biggest difference in performance for the 450 was between 05 and 06 - 04 and 05 were the same - 06 through 09 were the same - more changes to the chassis than the engine - most of the changes in the chassis was BNG (Bold New Graphics). Biggest changes from 05 to 06 was decreasing the size of the carb from 42 to 40 for better throttle response and bumping up the compression. The hot ticket is an 06 or newer 450; better throttle response, more low end grunt and better handling.