3 more topic questions...

1)What is a decompressor and how does one yank it off? If I buy this xr400 cam, what can I expect with only rejetting and maybe airbox mods? More Hp?, more Hp at higher rpm?, etc...Is there a better way to get a little more power without a pipe purchase?

2) Your pipe info was very informative but it seemed to me that you were discussing the header part. Are there any formulas or anything to determine pipe performance? I an replicating the factory pipe out of aluminum. Why is it the way it is, chamber size, internal tube lengths, etc?
Here is what I found: ehhaust exits header and goes into 1.25" pipe that is 12.5" long. Exits this pipe into a chamber. Enters a 1" pipe that is 8.5" long and exits this second pipe into second chamber that is 5" deep by pipe exterior diameter. Then finally exits out of end plate pipe assembly. Why???

3) is it cheaper or easier to get the end plate assembly from a 99 400ex as opposed to getting the one you mentioned or can I just take out the whole center part and just leave the chrome outer part (would look like the Procircuit then)?