Changing sprockets and chain

[frontman9000]

I have a 99' 400ex and it's time to change the sprockets and chain. Stock is 15/38 for the sprockets and 520x94 for the chain. If I go up to a 39 tooth rear sprocket, what do I drop the front one to? 14 or 13? And does that mean I use a different size chain or does it stay the same? I'm looking for a little more low end power. I just installed a white bros. e-series w/ curtis sparks header. Any suggestions?
My tires are 20" rear and 21" front. If that applies.

99 400ex
kenda klaw front & rear
houser stem
pro taper bars
ac racing nerf bars
white bros. exhaust w/sparks header
k&n powerlid
HydroDynamics front shocks w/+2 a-arms
g force axle

[nskukalek79]

i heard that 3 teeth on the rear sprocket is equal to 1 tooth on the front, but i don't know what the rear sprocket changes when geared up or down...but if you take your front sprocket down then you will have less top speed but more acceleration.

i would drop the front to a 14 and you shouldn't need a different sized chain. you should have enough travel to adjust the slop in the chain. but if not you can always grin the rivet and take out a link to make it fit.

another thing is if you really want lower end power you could get 18" or 19" rear tires and that should pick up the acceleration but they will wear faster than 20's