350x / 400ex Header Swap

[MagicJames]

I finally got around to measuring up the headers and here is what I found.
(I will attach photos later, my camera is MIA)

The Good:

The 400ex header will bolt up directly to the 350x head no problem the OD of the 400ex header flange going into the 350x head ID is exactly the same, and the header will attach to the head no problems. The fittings and spacing are exactly the same

The Bad:

The end of the 400ex header will obviously need to be modified as it is too long and does not match up to the midpipe of the 350x exhaust (back half). I don't have a solution for that yet, as I haven't got that far yet.

I got to looking at it while i had my micrometer out and was measuring around, and the Stock 400ex header is EXACTLY the same OD as the 350x header in all the same areas, going into the head, the curve in the pipes, the merge of the pipes, and the pipe going back to the silencer. The only difference between the two is again the length, but also the length of the single pipes before they meet, the 350x header's single pipes meet up about 2 1/2 inches sooner than the 400ex header.

Summary:

Not sure what this does for performance, but now I'm scratching my head as to what to do. the 400ex header is shiny and clean, and the 350x header is old and rusty and been repaired twice, so I'll probably swap them out and give detailed pictures of that as I go, but I'm wondering if there will be any performance difference just by swapping headers.

[cattle-dog]

i will tell you this how long the single tubes are and where and when they merge will affect the engine. for better or worse you will find out when done, i am by no means a atv/atc engine guru but am a street racer from way back and know what diffrent sets of headers and pipe sizes do to a engine and its all basicly the same thing.
it will more than likely change the torque range a little but may not be noticeable at all, should be interesting forsure as i have a couple 350x`s myself.
good luck cattle-dog

[mrtwidster1]

James, I done this mod on my X that you rode at coal creek. You are correct it will bolt right up but you will have to modify the tail pipe. Ther pipe setup added bottom end. I also used an after market megga max muffler. This muffler has the oval disks on the end to adjust for low to top end. The lower # disks the more low
end, the more disks you add the mor top end. Hope this makes sense. My son fitted my pipe and mufler on my bike. So yes you will have to modify your pipe to match your muffler.