my friend and i were aruging about cdi box on his 400ex.

i said it would add about 10-15mph to his quad along with his HMF pipe.

any no if im right.

also i have a 300ex and like to know if a cdi is worth buying. or just big bore 352.

if i go for the bog bore should i need a better cooling system.

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300ex for sale $2000 obo
e-mail me at --paintballkid_5732@hotmail.com-- if intrested.

 

minus the exhaust....if everything is stock and the cdi adds 1000 rpm to the max rev limiter u would gain about 8.32 miles per hr in 5th gear

thats assuming the stock rev limiter is set at 8250 and the new one is 9250

i took my info from http://home.earthlink.net/~gellett/400ex.htm

 

You might get 3mph, and even then you would be about ready to blow the engine.

Unless the pipe, cam, piston, and porting is setup to make hp past the limited revs then you aren't doing any good.

 

I agree with Maddog. You will only gain 1-3 mph from the cdi box. If you are looking for more top end you should change the front sprocket to a 16 tooth now that you have the extra 1000 rpms to turn the tires. You will probably only notice a difference from the rev box when coming out of turns and hammering the throttle or other situations where you accelerate quickly.

I would either go with a big bore kit or I would see if there was a higher compression piston available...there probably is. You probably won't need a better cooling system, probably will have to rejet the carb. JUST MAKE SURE TO CHECK THE OIL BEFORE EVERY RIDE. I just blew up my 400, after adding a rev box because I didn't check the oil before a ride. I checked it the weekend before without the rev box and it was fine, and I did not ride during the week. That is how fast it burned oil on mine. It probably didn't help that I held it wide open for 3-4 miles either. Now I am going with the 440 big bore kit (11:1 compression piston), stage II hotcam, dynojet jet kit with K&N airfilter. Hope to be on it in 2 weeks.

 

why a cdi box? are you always on the rev limiter? if not than you are waisting your money.