Join Date: Aug 2013
Location: Wimberley, TX
Thanked 28 Times in 24 Posts
A couple of updates on my dealings with Ari. First, he's incredibly knowledgeable and helpful.
Second, the 300 engine does bolt directly to the 250 frame. But the 300 engine doesn't have a place for the O2 sensor, as that screws into the header pipe on the 300 (not into the header as does the 250). Most people who would do this change don't use the O2 sensor anyway, because they've replaced it with a PCV.
The 250 ECU will work with the 300 engine (although you'll need a PCV and a custom map).
Jett Tuning's custom PCV is the same for the 250 and the 300. So if you use Jett Tuning's PCV on your 250, it will work on the 300. I highly recommend it.
I don't have a complete list of parts for the 250/300 that are interchangeable, but it seems that most are identical, other than cams (different base lobe diameters), balance shaft, clutch hub and transmission.
Millenium Technologies in Wisconsin makes a bore kit for the 300. 78mm Wiseco Hi-comp piston. Increases the 300 to 300.8 cc (from the stock 286 cc). However, the 78mm Wiseco slug is made for the 250, so it has a much higher dome (and thus compression ratio) than is acceptable for the 300. You have to remove a lot of material from the piston dome to lower compression to a usable 13:1 in a 300 engine.
Ari has convinced me to stick with the CBR250 for racing. Even with a lot of work, it will never have as much top end speed or HP as a Ninja 300, R3 or RC390, but it handles better than any of them and with Ari's help I think I'll be able to get a new engine that is reliable (again, don't get the Takagawa) and can keep me close enough in the straights to pass again in the twistys
2014 R6 - Track Bike
2012 CBR250R - Track Bike
2088 VFR800 - Daily Commuter