michigan

Member
Mar 9, 2001
424
0
I've been reading about how suzuki's have weak low-end due to the case reed design, and now the mags say the 02 cr250 has weak bottom end because of the switch to a case-reed design. Aren't KTM's case-reed? They have good low-end torque. Is it true that the case-reed design produces poor low-end, and if so, how can KTM use this design with good results?
Thanks in advance!
 

bwalker

LIFETIME SPONSOR
Jan 10, 2000
839
0
I've been reading
Typical bs from the mags. Consider the fact that most snowmobile engines are a case reed design and they are tuned for much more torque than a bike. I would wager that crankcase volume and porting influence torque properties much more than the position of the reed block.
 

spanky250

Mod Ban
Dec 10, 2000
1,490
1
Rotax rotary-valve engines were case-induction engines, and they weren't devoid of torque by any means. There are many variables that determine the torque curve of an engine. Port area/timing, compression, displacement, carb venturi size, intake tract length, exhaust pressure-wave tuning, ignition curve, etc etc...
 

michigan

Member
Mar 9, 2001
424
0
Thats what I get for reading.
Maybe if the '02 cr250 came with a larger rear sprocket the mags would proclaim, "excellent low-end torque, but poor over-rev."

Thanks for the prompt replies, guys!
 

spanky250

Mod Ban
Dec 10, 2000
1,490
1
Originally posted by michigan
Thats what I get for reading.
Maybe if the '02 cr250 came with a larger rear sprocket the mags would proclaim, "excellent low-end torque, but poor over-rev."

Thanks for the prompt replies, guys!
If you put blue plastic on a CR and took off the Honda stickers, MXA would proclaim it to be "the best, most advanced bike on the planet".
 

marcusgunby

LIFETIME SPONSOR
Jan 9, 2000
6,450
2
Pete Payne has analysed a 02 Cr250 and it has very advanced port timing and this i think has more of an affect than the reed design.
 

EricGorr

Super Power AssClown
Aug 24, 2000
708
1
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
 
Top Bottom