BannedCR500Riders.com

So for my 125 build I had to do quite a bit to a PC pipe to get it to fit. The major change is that I had to take 1 1/8" out of the head pipe right where it exits the exhaust flange. The other changes I made were minimal.

What are some of the theories on how that will effect the performance?

My speculation is that it will move the power up a little in the RPM range, shorter expansion chamber, the reverb returns quicker etc.

Judging from the test ride, that was not the case, but I had nothing to back to back compare, nor tach readings from before and after.

On TT for the YZ250 they go on and on about a 2mm spacer in the pipe where it connects to the exhaust flange, I doubt there would be any significant difference in reality but I am going close to 30mm.

theres a spacer for the 250 cr also, supposedly more tq

goes with my theroy spacer more torque, shorten it and higher revs

From what I have seen on sled motors, shortening the head pipe to match the porting program and ignition curve can net success when a target rpm is chosen. However, that is quite a bit to remove. But..... since there was no other ignition or port changes targeted, and the huge amount of torque you already have. I would lean in the direction that the change in pipe may not be an issue. Since our bikes are not running at peak power all the time, more on and off the throttle, I would not be concerned. Curious is good, it would be fun to do a back-to-back comparison though, put it on the dyno to see the actual effect....... Adam.....wink, wink.

I took a look at doing that for the dyno lastnight, i dont have pipe to rail clearence to shorten it there, would have to cut it out of the U bend

Installing a spacer in a CR250 does change the power, less high rpm pull, more low end. I removed mine, as she is at the stops quite a bit.

1 1/8 is a lot, I would think you would loose a crap load of low end grunt.