what is the better motor and the pros and cons over each other?
Both are great motors. Your choice should depend on intended use, parts availability
and to a lesser extent, brand loyalty.
I did a lot of research before starting a similar project a couple of years ago. The
advantage I had is I raced 500s before the class went away. I knew what I was
getting into and I knew what modifications were needed to be able to survive a
moto.
To be honest, I didn't even consider the KX as I consider it more of a desert racer
than a MX motor. The CR is also easy to get parts for and there is a ton of support
available due to the popularity of the numerous CR based "AF" conversions out there.
With that said, In stock trim, the late model CR is not a good MX engine. It comes with
a wide ratio transmission and has a substantial hit that will stretch your arms and wear
you out in short order. It also vibrates pretty good. The one thing you don't want is a
mid 80s CR500 engine. Those engines were so powerful that even the factory riders had
them detuned. Throw in the corrosion problems from Mag water pump covers and the very
long list of NLA parts and you'll have a big mess on your hands.
Here is a partial list of the modifications I performed to correct a 2000 CR 500 shortcomings:
Close ratio tranny
Balanced and trued crank
Aluminum cylinder sleeve
Digital 3D ignition
Port and head work to lessen the hit
Aftermarket pipe and silencer
Aftermarket clutch
I opted for a 2008 Honda CRF250R frame over any of the newer models. The frame geometry
on the 06-09 CRF250 is very similar to the 93 CR250, which is one of my favorites. I also like
the slimmed down size and lighter weight over the 450F. I'm 6'1" and that frame fits me
perfectly.
What I have now is 60+ of more useable horsepower in a lightweight, modern chassis with state
of the art suspension. The bike is deceptively fast because you don't have to wring it out to make
power. All you have is the wind noise in your helmet and the corner approaching way quicker than
you expect - the first mods I made after I finished the bike was a pair of oversized wave rotors. If
I had this bike back in the 90s, I would have won a lot more races.
If you are a Kawasaki fan, some of the same mods would apply as well. Like I said, both are great
engines and both will make an awesome MXer in the right roller.
I would say you have a lot of research to do and some big decisions to make.
My two cents.
Good luck.
dogger