Originally posted by EricGorr
I've been talking with Pete Payne and Wes Gilbert on the CR250. I think that the transfers are too high (advanced timing, long duration) and the exhaust port is too low (long effective stroke but retarded timing and not enough duration).
Wes has some interesting observations on the exhaust valves and how they fit up into the port. He's specced out the blowdown timing (distance from when the exhaust opening to transfer opening) and found it to be suspect at low rpm. Pete and I talked about adjusting the front cable so the exhaust valve doesn't close as far.
Lets remember that this engine has roots as a dual sport bike in Japan and Europe. In recent years it has had a motorized exhaust valve meaning that it goes through an open and closed cycle every revolution, in order to facilitate activated radical combustion.
Having an exhaust valve close very far like this could influence emmisions and noise too.
I've tried some different porting so far. The first iteration was to turndown the cylinder base 1mm and raise the exhaust port 1.5mm, and recut the head accordingly. The rider, a DRN member in Colorado said that made a difference but the jetting was where he found the most beneficial gains for the high altitude.
I'm also looking into some billet parts with AJ from Service Honda, in particular the pulley diameter and the butterfly shape and orientation on the exhaust valve servo motor.
One things for sure, its all new and the polishing kings are going to be in trouble :p
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