400ex mods

[400exfan19]

i have a 400ex with a full fmf exhaust, jet kit, and k+n air filter. what do you guys recomend i buy to make it more powerful for as little money as possible?

[NY06400EX]

Cam - seems this is the mod that is most suggested at this point.

[ImplodedMindZ]

If you havent done so already remove your aibox lid and get an outerwear for your filter.

[GPracer2500]

The performance per dollar of a new camshaft is hard to beat. We can mostly thank Hotcams for this.

A one tooth smaller front sprocket can be an inexpensive way to boost acceleration. Obviously that makes all the gears shorter, so it's applicability depends on the terrain you ride. But a new front sprocket is so inexpensive it's worth at least trying. I think a PrimaryDrive front sprocket from Rocky Mountain ATV is less then $10.

Pricing on dynamometer time varies, but if there's a dyno near you consider having your carburetion tuned with it. In my experience, most poeple leave some power on the table because of jetting. $100 spent on a dyno can be fun, informative, and will usually make your engine faster.

[krt400ex]

all of what they said, and a higher comp piston too

[nightmare311]

cam stage 2
450r carb.
rev box

[newrider4002001]

i have all the mods you do plus cam i defiantly suggest the cam and if theres money left over ket a sprocket

[400exfan19]

is it ok to put a cam from a honda 400 dirtbike into my 400ex? they are almost the same motor

[krt400ex]

Originally posted by 400exfan19
is it ok to put a cam from a honda 400 dirtbike into my 400ex? they are almost the same motor

ya, it is fine. i think the hotcams cams r the same anyway...but don't quote me on that

[GPracer2500]

The XR400 cam might be thought of as a 0.5 stage cam. It's kind of in between the stock 400EX cam and a typical stage 1 grind.

[400exfan19]

it is a stage 1 xr400 cam. its not the stock. im just makin sure it wont screw anything up b4 i put it in

[GPracer2500]

Originally posted by 400exfan19
it is a stage 1 xr400 cam. its not the stock. im just makin sure it wont screw anything up b4 i put it in

Oh, I see. Yes, your good to go. Cam offerings for the XR400 and 400EX are identical cams.

The only caveat is make to make sure the XR cam is compatable with stock rockers. I don't know what specific cam you're looking at, but a hardwelded reground cam may require hardwelded rockers for reliable operation.

[supernatural]

how do you like the mods you've done? i am planning the same and was wondering the gains you got. also is the hmf unreasonably loud compared to stock? i ride in areas where noise could cause problems. which hmf did you go with? i am looking at the sport pipe

[supernatural]

read your post quick, thought it said hmf. sorry my fault. still would like to know how you like it. i tried a 14 front sproket it killed my top end but i liked it in tight woods. took it off after one ride. you pay shipping and its yours. im in massachusetts.

[Kaleigh]

the cheapest and easiest would be go to a higher octane fuel. revbox and a smaller front sprocket.. im just think about ways for you to get power with opening your engine.. you can change you a lighter oil too..

[newrider4002001]

Originally posted by supernatural
read your post quick, thought it said hmf. sorry my fault. still would like to know how you like it. i tried a 14 front sproket it killed my top end but i liked it in tight woods. took it off after one ride. you pay shipping and its yours. im in massachusetts.

i thought 14th sporcket gave you more top end:confused:

[400exstud]

Originally posted by newrider4002001
i thought 14th sporcket gave you more top end:confused:

Nope. Faster on the bottom - Slower on top.

[400exstud]

Originally posted by Kaleigh
the cheapest and easiest would be go to a higher octane fuel. revbox and a smaller front sprocket.. im just think about ways for you to get power with opening your engine.. you can change you a lighter oil too..

Octain rating will do nothing for power. As long as you are not suffering from pinging you are above you required octain rating. An oxygenated fuel could add a little bit of noticeable power but that is a whole different discussion.

Where are ya GPracer2500?

[GPracer2500]

Originally posted by 400exstud
Octain rating will do nothing for power. As long as you are not suffering from pinging you are above you required octain rating. An oxygenated fuel could add a little bit of noticeable power but that is a whole different discussion.

Where are ya GPracer2500?

I'm around. ;) But since you spoke up I don't have too. :)

[400exfan19]

what exactly do these revboxs do? what is the price range?

[Kaleigh]

it changes your power curve and gives you more rpm..the have cheap ones for like $50.. but remember you get what you pay for.. I got a White Brothers one.. before anyone says anything.. yes I got what I paid for...lol... its nothing special.. but does help enough..

[Kaleigh]

would it hurt if I use octane boost??

[GPracer2500]

Originally posted by Kaleigh
would it hurt if I use octane boost??

Most octane boosting products aren't likely to harm your engine. Most of the commonly found ones are pretty much junk though. And more importantly, don't use an octane boosting product unless your engine requires a higher octane rating than your current fuel provides. It won't improve overall performance and might actually hurt some aspects of performance to use auto-parts store type octane boosting products.

Octane rating is a measure of detonation resistance. In case you don't know, detonation is an abnormal combustion phenomenon. Deflagration is the good kind of combustion. Detonation is a bad kind. Deto is commonly called "spark knock", "knocking", or "pinging". Octane rating has NOTHING to do with how much "power" or "energy" a fuel provides or the manner in which a fuel deflagrates. The only thing octane rating tells us is how easily a fuel will succumb to conditions that cause detonation. As long as your engine does not detonate with whatever fuel you're currently using, then a higher octane rating won't do a darn thing. It's when you're running higher than stock compression and/or lots of ignition advance and/or a few other things, premium pump fuel starts to not provide adaquate detonation protection. Only then can additional octane rating can be utilized by the engine.

Don't do something to your engine or use some product unless you know why you are doing it. Some vague notion that high octane fuel is more powerful could lead a person down the wrong road.

[krt400ex]

Originally posted by supernatural
how do you like the mods you've done? i am planning the same and was wondering the gains you got. also is the hmf unreasonably loud compared to stock? i ride in areas where noise could cause problems. which hmf did you go with? i am looking at the sport pipe


hmf is very loud. even witht the quietcore it is 97dec. and without it it is 102 dec. most places reguire u to be at 94-96 dec. i got this info about the hmf pipe by calling them to find out, so my facts r correct. get a yosh pipe. they r 93 dec. with the quietcore. u will be legal just about anywhere

[Kaleigh]

Originally posted by GPracer2500
Most octane boosting products aren't likely to harm your engine. Most of the commonly found ones are pretty much junk though. And more importantly, don't use an octane boosting product unless your engine requires a higher octane rating than your current fuel provides. It won't improve overall performance and might actually hurt some aspects of performance to use auto-parts store type octane boosting products.

Octane rating is a measure of detonation resistance. In case you don't know, detonation is an abnormal combustion phenomenon. Deflagration is the good kind of combustion. Detonation is a bad kind. Deto is commonly called "spark knock", "knocking", or "pinging". Octane rating has NOTHING to do with how much "power" or "energy" a fuel provides or the manner in which a fuel deflagrates. The only thing octane rating tells us is how easily a fuel will succumb to conditions that cause detonation. As long as your engine does not detonate with whatever fuel you're currently using, then a higher octane rating won't do a darn thing. It's when you're running higher than stock compression and/or lots of ignition advance and/or a few other things, premium pump fuel starts to not provide adaquate detonation protection. Only then can additional octane rating can be utilized by the engine.

Don't do something to your engine or use some product unless you know why you are doing it. Some vague notion that high octane fuel is more powerful could lead a person down the wrong road.


thats awesome.. great information.. thanks so much.. i got 11:1..

ok this guy I know got a 440 kit and a 12.5:1 piston.. he says he runs pump 93 octane, but I heard you can only run race fuel cause its 12.5:1.. if it was 11:1 then you can run pumpgas?

[krt400ex]

Originally posted by Kaleigh
thats awesome.. great information.. thanks so much.. i got 11:1..

ok this guy I know got a 440 kit and a 12.5:1 piston.. he says he runs pump 93 octane, but I heard you can only run race fuel cause its 12.5:1.. if it was 11:1 then you can run pumpgas?

u need to run at least a 50/50 mix of race fuel and pump gas with a 12.5:1 comp in the 400ex/440ex. u can get away with 12.5:1 in a liquid cooled motor, but that is pushing it. if he has been running pump gas, then there is most likely some kind of engine damage....but u might be lcky and find nothing, u need to tear the motor down and check everything befor eu buy it though