just thinking about 400ex header

[xtullyx16]

Why did Honda make the 400ex header split into two?? why not make it like 450r when it came out?? When it splits into to does it make any performance changes??

[rooster300ex]

Correct me if i'm wrong. I'ts like that because the 400ex engine has 2 exhaust ports. Bulletproof motor in my opinion.

[GPracer2500]

Any engine with two exhaust valves also has two exhaust ports. The 450r also has two exhaust ports even though it only has one exhaust header. On the 450r the exhaust ports merge into one space as they leave the head.

All other things being equal, I believe having a seperate header for each exhaust port is more effecient. But wait--it's not that simple. The entire engine's design has to be taken into account. Things like valve spacing, valve size, combustion chamber design, port design, and yes--header design (along with all sorts of other things), have to be carefully considered. Change a 450r head just enough to make it have twin headpipes and that new configuration might be less efficient because of some other characteristic of the engine.

And aside from outright efficency there are packaging, weight, manufacturing, and cost considerations.

Remember, the venerable 400EX engine is really an XR400 engine. It was not a clean-sheet design. It's roots go all the way back to the XR dirt bikes of the late 70's. For packaging reasons alone twin headpipes make a lot of sense on single downtube frames like those found on XRs. How are you going to fit a single, larger, centrally located headpipe on this:

http://www.academyrentals.com/images/xr400.jpg

I guess my only point is that there are a myriad of reasons why an engine has some particular feature or another. Some reasons might be performance based, some not.