rev box?

[trx400exat05]

I was wondering what a revbox would do to my 400ex that has k&n, power core 4 and is jetted.... will it do much and what kind should i get?? i was lookin for gtthunder but i live in ohio and i dont know if they have a website or their phone number and where their located..... also i was looing at whitebrothers but i heard on here that they were hard to start up when u put them on...... and last i was wondering if they are hard on ur engine if it was stock with no engine work... thanks

[hondajoe05]

i've heard it opens the 400's right up jus like the cams but i dunno if it will hurt it thats what i was thinking cuz from 8250rpms -9000rpms thats a quiet a jump

I got a wite bros for my 400 back in the day when all it had was a filter. thare was definetly a diffrence! way longer gears. It will take advantage of the top end of things.

I think the wite bros lets it rev to like 11,000 or somthing:huh . seems like an awaful lot of revs to me but I use every one of them.

just hang on to your stock rev box for the day that you have heavy heavy mods and you dont want to rev the piss out of it.

[deathman53]

all the white bros. rev box is, is a 96-04 xr250r cdi box. On the one my friend had, you can even see part of a honda part number, and the first few numbers matched a xr250r cdi, others have said this in the past too. the reason why they are harder to start, especailly in the cold is because of the timing differences, kicking a motor put more power into it and increases rpm, the kick start cdi's are set to spark later than electric start cdi's(well atleast the xr250r, xr400, verses a trx400ex). I was also told that a 86-88 trx250r cdi, as well as a 350x cdi would work on them, not sure on what they would do.

ya, I heard that they are just a 250r cdi ( rev box) maby find one on ebay for cheep;) .

[deathman53]

beware, it may increase the campacity too much, and also beware 350x's cdi's don't have a rev limiter

don't waste your money...unless you are running wide open throttle constantly, you don't even notice that extra 1000 rpms...they only come on at the very end of the throttle, plus...

The CDI box comes from an XR250 kick start bike. It's equipped with a circuit
designed to help keep the engine from "kicking back" on the rider.

What it essentially does is not fire the spark plug if the engine doesn't crank
above a certain RPM. This preset RPM level is just barely under the normal
crank speed of a 400EX... So, throw in some thick oil & cold temps, or simply
a low battery, and it's just enough to cause the engine not to start.

To verify that this is your problem you can do two things. You can put the
stock CDI box back on and see if it fires right up (it probably will), or you
can roll start it.

When I had this CDI box on mine, it was obvious. My quad normally fires off
extremely easily, but after installing the box, it wouldn't fire off when cold &
first start of the day - wouldn't even hit. If I went out and roll started it,
it would pop off cleanly like nothing was wrong.

After doing that a couple times over the course of a few days, the next time it
wouldn't start, I simply switched CDI boxes (back to the stock unit). After
cranking for what seemed like forever with the XR unit (without even a hit), the
stock CDI popped off instantly.

What it all boils down to is unless you have a highly modified engine with a
wild cam and high compression, the CDI box is not doing anything but taking a
safety net away from your engine & causing it to be difficult to start.

If you have the mods to support it, there are a few engine builders out there
that have figured out how to eliminate the rev-limiter out of the stock CDI,
which avoids using the XR250 CDI and it's anti-kickback circuit


i killed 2 batteries before i found that out...and honestly the rev kit made very little differnce to me...i am never buried against the rev limit for very long on the trails, just sometimes when we hop out on a road...and my friend has one in his 2002 400ex and when we take off down the road...i am faster than him from start to wide open throttle...our bikes are very similar...both stock motors, same filters...only difference is he has a complete wb e-series pipe and i have complete big gun race exhaust...my pipe is supposed to add less top end than the way he has his wb set up...but i still got him by a good 3-6 mph when against the rev limiter..and since he is supposedly getting 1000 more rpms than me...i am content not running one

I noticed a diffrence. and run one.

[cals400ex]

the gt thunder aggresive curve box does make a throttle response difference, but it idles a bit irratic. the bike will still start in winter, unlike those xr250 boxes. it is a bit harder to start, but it will start. i would not get a rev box unless you have a cam that revs high and you are hitting the rev limiter when racing.

[2muchquad]

yeah the rev boxes are just more wear and tear on your motor.the 400ex has good torque,thats where a good rider will shift to stay in the meat of the power.dont get the idea that a rev box will help the 400ex stay with the high rev 450s,it wont.they have much lighter valve train and a piston with almost no skirt,they love rpm.:)

I still disagree. when I am on my 416 I shift it wayy late in the rpm range. I have done prolly 50 drags against and equal quad.(same speed) and if I dont absolutly rev the piss out if my quad in each gear I loose. w/o my rev box I would loose.

just a note, my quad doesent like to start.... lol. when it stalls from beeing under a load (hit the rear brake to hard or climbing hill w/o dounshift) it sometimes takes up to 15 secconds to start back up. I always thot it was carb issues but may have somthing to do with the "anti kick back" think someone was talking about.

[trx400exat05]

so u guys ar saying that i should get the rev box with the motor i have unless i get a higher rev cam.... also i do hit my rev limiter cause i like to drag race my brother and the kid up the road in this 1/2 mile pasture....that is why i wanted one but where do i get a gt thunder and are the white brothers any good?? Also the 250r stock rev boxes is what im buying ur saying?

[cals400ex]

the white brothers are junk. look at www.gtthunder.com

you will have to call. i don't believe they have these boxes listed on their website.

[whiteflash]

i've had my white brothers for about half a year now and i definantly noticed a difference. But i have no trouble starting mine in the summer or winter....i also got mine for 45 dollars so i thought i'd try it......

u prolly dont see a diffrence in starting because you have stock compression..

just thot of this,,, my enigne will barley crank over some days. the batery is 4 months old. this prolly has a lot to do with my starting issues also.

your engine prolly zings over at half a million.

[skull2002]

i have an 04 400ex, with w/b rev box. it starts with absolutly no problems. and i can run next to an 01 660 raptor. top end he gets me by a couple lengths, he gps at 86 mph, i gps at 73 mph. i am jetted dead nuts on,and 3rd gear wheelies are just a throttle blip. 74-101 is part number. get one, you will like it.

let me put it this way...after i took the one out of my 2000 400ex and my buudy still had his in his 2002 400ex, we ran down the road. we are pretty much equal in skill and weight. he had a wb e-series pipe, header, and jett, i was running a big gun full race exhaust. we had all the same stuff on, except i was running a-arm guads and he was not. other wise the bike were identicle.

i out ran him and i was revving longer than he was...when we took off, we pulled from his driveway in single file and as we accelerated, i gained on him and passed him witih in a mile or 2. it probably would have been sooner, but the road had some big arse rocks that he tossed back at me...i should have had a chest protector on, but...

[GPracer2500]

I also have a WB and have never had a starting problem. The higher rev limit makes a difference to me in the dunes....trails not so much, but in the dunes I bang the limiter when I run my stock CDI.

I run a 416, a stage 2 cam, one down on the front sprocket.

[strapped9]

I grabbed a procom box. junk! the add says +5 hp gain which is bs in itself. but Its hard to notice the little extra rpms. I'd guess maybe 800 rpms out of this box.