2000 400ex + End pipe, any of the good models = WB, Big Gun, Pro-C, FMF, etc. + K&N + jetting + WB-rev box. From what I can tell these are the first 4 things you can do to raise HP and perf. on the 400ex and not get out of control on money. So what is the HP gain? 3hp, 5hp, or 7hp?

After these 4 things what would the next best thing to do be??

I know they have done some tests on GT thunder, but not with only these 4 things done. Is the pipe, jet, K&N, and rev. box the first things you would do? Then what?

it would be around 10hp +/- depending on the brand of pipe and the type of airbox mod you did... a header is a must also

Do you think a header would really make that much diffrence? The 400ex header is thin, light, large, and turns well. What do after market headers have over the head pipe? My warrior it would have a lot, that ceramic coated, thick, heavy pipe. But I'm not seeing the 400 diff??

Drop a tooth on the front sprocket to 14 tooth, no horse power increase but it makes use of the horsepower better. Especially for woods and trails riding.

There is nothing wrong with the stock headers on the 400EX. I called TC Racing and Pro-Fab atv today and they said the stock headers work great. Just get a good slip on and you will be fine.

DSAPP... I would be very interested in a pipe that would gain 10+ hp on a 400ex. The dyno sheet that came with my WB R4 claimed just under a 2 hp gain. I noticed no gain. I'm not to sure that I lost some. Yes it was properly jetted with a nice cinnamon brown plug reading.

Highjumper if I was you I'd leave it stock unless you plan on major motor mods. Don't let all the noise fool you.

Decide what it is you really want to do with your bike and how much money you want to spend on more horsepower. If you want real horsepower... save your money and install a high compression piston, high perf cam, and high flow exhaust. That is the only way you can add ten horses or more. If that is too much money, then just buy a slip-on and have your bike PROPERLY re-jetted. Go to the GT Thunder website and look at some of the 400EX dyno runs and see for yourself what really makes horsepower on these bikes. Later.
http://www.geocities.com/~gtthunder/gt1.html

From what I've seen, you'd gain 2-3hp from a slip-on silencer (an additional 1-2 with a header pipe), maybe 1-1.5hp from a well modified airbox, and maybe 1hp from a K&N (although I'm not sure that it even helps at all). The rev box won't add any power, it'll just allow it to rev past 9100rpm (the rev limiter), so I wouldn't really recommend it. The EX makes peak power around 7500rpm anyways, unless you do a different cam and/or piston, there's no need to rev past 9K. So total, with correct jetting, you'll probably see a 4-5hp gain. My EX (mods listed below) only pulls a little lead on by brother's EX (which is stock except for the same gearing as mine), it's not a huge difference in peak power, but I think the low and midrange power are improved noticeably.

The 15T sprocket must be stock. Does the 14T lower top end much? I have raced some modified 400ex's at Little Sahara Sand Dunes and they only beat me by 1 or 2 bike lengths, I don't think it is worth all the $$$ for that. Do you think a 14T would help out in the drags? In 5th gear I have hit the rev. limiter once, but as you said I don't think peak HP is a 9000 rpms. The guys I raced had a pipe, K$N, jetted, and rev. box. I'm not sure about the 14T but I don't think so. I could not get the Raptors to race me, they were all "breaking them in" thats cool, no reason to hurt your new ride I said.