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Noticed the thread over on Speedzilla about the S4 headers onto an ST4s (http://speedzilla.com/forums/showthread.php?t=27559). Seems this boy got a little more grunt with that change. I've always been curious what the crossover is for on my M900 pipes. Seems that 'we' add slipons, open the airbox, add a K&N and PC3 to get it all inhaling and exhaling freer and feeding it more food. But then we get to the upper intestines and it seems rather constricted. I've noticed there is the diameter difference between the header and the newer slipons pipes diameter.

So - the two questions are - 1) what is the purpose of the "+" shaped crossover??? (Do sbks have a crossover??)

2) (if we've done all the other mods) why wouldn't it be good to enlarge the diameter of the header pipe to allow the gases to leave quicker?? Especially if we've done a bigbore kit and/or flowed the heads?? I've not read anything about doing the full exhaust except on SBK motored bikes. Why not ST's, MS' and Monsters??
 

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This is what I think/remember from reading exhaust threads on a bunch of different boards

1) it's to even out the pulses and reduce bad reflection in the system. Also helps with initial scavenging on exhaust stroke (i think)

2) There are full systems for monsters. header diameter determines pulse, scavenging, reflection issues. At least that's what I remember from the I4 croud. Although I don't think there's a way to get the valves to float on a monster unless something is going wrong. The pipes keep the gases moving. Too big a pipe, and there will be a time when things are just sitting still in the pipe, something or other about cylinder volume being equal to the CFM of the pipes...

so I don't really know, but that's what my tired brain is recalling.
 
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