Given what looks to be a change in the AMA rules that would push the 150R up a class, a few people have asked if it would be possible to sleeve a 150R down and make it a 125. I gave it some thought and came up with the following.
As it sits today the CRF150R has intake valve area that represents 31.04% of the bore area, by comparison the 250R has 31.59% valve area and the 450R has 28.13% area. The 150R has pretty major valve area from a japanese design standpoint. Considering it's making around 160hp/liter it kinda makes sense.
Sleeving it down to a 125 would forced you to drop the bore size from 66mm to about 60.4mm. This would bring the valve area up to 37.06% of the bore.
Once you get above around 33% valve area you run into some real issues with valve shrouding, and combustion chamber shape, not to mention that much valve area forces a very real bias towards high rpm power.
Dropping the bore size 4.5 mm would wreak havoc with the quench area built into the original cylinderhead design and really shroud the valves in a major way. I'm tempted to make up a 60.5mm cylinder flow bench adapter just to see how bad it would be.
Shortening the stroke the 6mm or so it would take to bring it down to 125cc and lengthening the rod would seem the be the better way to do it but it would require an upgrade in valve gear and a change in the cam design. If you assume you want to keep the same mean piston speed of 3600 ft/min at the power peak that Honda is using, then you'll be making peak power at 15,000 rpm instead of 12,500, and redline would be flirting with the 17,000rpm range. Titanium valves and GOOD springs would be a must.
It's always tricky when you try to out-engineer Honda, and this is no exception. :banghead:
As it sits today the CRF150R has intake valve area that represents 31.04% of the bore area, by comparison the 250R has 31.59% valve area and the 450R has 28.13% area. The 150R has pretty major valve area from a japanese design standpoint. Considering it's making around 160hp/liter it kinda makes sense.
Sleeving it down to a 125 would forced you to drop the bore size from 66mm to about 60.4mm. This would bring the valve area up to 37.06% of the bore.
Once you get above around 33% valve area you run into some real issues with valve shrouding, and combustion chamber shape, not to mention that much valve area forces a very real bias towards high rpm power.
Dropping the bore size 4.5 mm would wreak havoc with the quench area built into the original cylinderhead design and really shroud the valves in a major way. I'm tempted to make up a 60.5mm cylinder flow bench adapter just to see how bad it would be.
Shortening the stroke the 6mm or so it would take to bring it down to 125cc and lengthening the rod would seem the be the better way to do it but it would require an upgrade in valve gear and a change in the cam design. If you assume you want to keep the same mean piston speed of 3600 ft/min at the power peak that Honda is using, then you'll be making peak power at 15,000 rpm instead of 12,500, and redline would be flirting with the 17,000rpm range. Titanium valves and GOOD springs would be a must.
It's always tricky when you try to out-engineer Honda, and this is no exception. :banghead: