Most everyone i know that has a 400ex goes out and puts a mild cam in along with a high compression engine, and if you have cranked the compression up too much then you may notice the engine doesnt turn over to easily and if the motor doesnt fire right away or is cold and takes awhile before it stays running the battery may start to die on you, well an easy fix is, a new battery. but not any battery go out and buy a battery from a raptor 660, garentee you have never heard a 400ex engine turn over so fast, this helps to not only provid more cranking force but turning the engine over faster also helps to start it, kinda like pull starting it and popping the clutch.
to do it all you need is the raptor batter and somthing to cut the top of the batter box off with, because the raptor battery is alittle taller then the stock 400ex one, then you will need a new flat stock of metel that can be bent to form a new band to hold the batter in place or take the easy way out and use a tie down. the metal stock is much cleaner looking and professional but the tie down serves the same purpose and also makes for a faster removal of the battery if needed, now the fender support on the stock fenders do get in the way SLIGHTLY and can be left but the bar does make contact with the top of the batter, an easy fix is put a small peice of weather stripping on the top of the battery or just remove the bar eaither way works and has no ill effect