Nice to see other people here that are interested in these same questions!

According to Kevin Cameron (there's no other motorcycle performance wizard and engine tuner I'd trust more than him) in a properly sized exhaust system the size of header tube should be determined based on exhaust valve-open area. This of course assumes that the valve-open area is properly chosen for the cylinder size and "engine tune".

There is "wiggle room" around the theoretical optimum header diameter to slightly push the engines characteristics one way or another, but there is optimum size that gives highest torque peak. (forget about the RPM of that torque peak for now).

Valve-open area is defines as circumference of the exhaust valve x valve lift x number of valves per cylinder.

So, at least for the 300R engine, the valve-open area can be calculated as;
25mm x Pi x lift x 2.
I don't know what the lift of the stock cam is but I'd be surprised if it's more than 0.3", so I'll call it 7mm.
Then valve open area becomes 25mm x Pi x 7mm x 2 = 1100mm^2

Mr. Cameron gives a rule of thumb estimate for the optimum header size as:
header cross section = 1.16 x valve-open area.

1100mm^2 x 1.16 = 1275mm^2

From that we get the ID = 40mm.

That is for 7mm valve lift and 25mm valve dia.

The persistent rumor out there is that KLX250S uses smaller valves than the 300R. If the valves were say 2mm smaller the optimum header for 23mm exhaust valves would be still 38mm.

--
Mikko

 

I'm guessing that was directed at me.

I stand corrected and apologize for posting incorrect information.

Ron

 

Not directed at you, Ron. I've seen that stated more than once on different threads and different forums, and I've been guilty of quoting it at least once myself. Hardly the end of the world, and I'm glad the correct info has come out. I used the term "enternot" purely in a joking manner, as I think quite highly of the internet overall.

 

Bill.
Are you still modifying heads for the KLX ?? Do you have a new email address ? I want to buy a ported head.
Thanks.

 

Bill Blue web site

 

Nice resurrection KLX678, I memorized Bill's "34 hp"post long ago but could never have found it.. This is equivalent power levels to a stock KX250F..

 

Thanking the wrong guy. Subseageorge asked.