I installed a White brothers e on my sons 99 400ex, it already had a 16 th sprocket,k&n dynojet needle,170 jet air box lid removed and the slip on didnt hardly help at all. The biggest thing that helped was the 16 tooth sprocket for drag racing and top end like we do. We run about a 1/4 mile and one with the 16 sprocket will put five bike lengths on a stock one or modified ex with stock sprockets(especailly you guys with a 14 front,these things already have TOO MUCH BOTTEM END NOW STOCK,BESIDES IF YOU ARE SPINNING OR WHEELIEING YOU HAVE LOST AND WITH A 14 FRONT SPROCKET DONT TELL ME YOU CAN EVEN USE 1ST GEAR OR KEEP THE FRONT END DOWN) and the 16 tooth front sprocket(or rear 36)evn better, hardly affects bottom end. From 3rd gear on its over with! and side by side on take off and through third. But the way our White Brothers went to performing was to remove the spark arrestor and it picked it up tremendously! im running all discs and spark arrestor removed. TRY IT! MY 2000 ex has the dyno jet kit,kn,supertrap(awesome pipe makes power) and REAR 36 tooth,another secret to going fast top end and not losing bottom. We were close until We removed the spark arrestor, now he can pas me top end. Also removed the choke and IT WORKS! MORE TOP END! Ive since ordered 175 jets from dynojet to compensate for more air flow. TRY IT, ITS FREE! and took 15 minutes tops! If you do these things ive mentioend you will be amazed at a drag race with your buddies stock banshee or 250r or soemone with a stock geared 400 with mods. You can have all the power in the world but if you cant hook it and be there on the big end you will lose!
I can let a stock 400 or piped one leave and run him down and pat the gas on him then shift to fifth and say bye bye!