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Ari Henning from Motorcyclist magazine dropped the 300s engine on his multi-trophy winning 250, and he's still winning. I asked him if its a direct swap or if it involved extensive mods and he said it was as good as swapping the old 250 engine for a new one. Needed a new crankshaft and crankcase.
Somebody here listed the overall changes the bike underwent from 250 to 286cc
CRF250 desplacement mods> CBR300? - Page 2 - CRF250 L/M - ThumperTalk
 

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Ari Henning from Motorcyclist magazine dropped the 300s engine on his multi-trophy winning 250, and he's still winning. I asked him if its a direct swap or if it involved extensive mods and he said it was as good as swapping the old 250 engine for a new one. Needed a new crankshaft and crankcase.
Somebody here listed the overall changes the bike underwent from 250 to 286cc
CRF250 desplacement mods> CBR300? - Page 2 - CRF250 L/M - ThumperTalk
That's the link I just posted a few posts ago. It doesn't say exactly what the guy changed when he did the swap besides the fact the crank and balancer drop right in. There is the supposed list posted of all the changes, but nothing confirmed. No idea what the changes to the crank case were, or what they could be without a clean-sheet redesign of the manufacturing. My thoughts were due to the 286cc displacement, that's as far as they could go in the EXISTING crankcase with stroking it (minimal parts redesign), so I have no idea what crankcase changes were implemented.

Would probably be easier and cheaper to just drop an entire engine in with the ECU and call it a day.
 

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Interesting. So, when mines go Kaboom I'll just drop the CBR300r/f Engine into the 250.
 

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Discussion Starter #25
Would probably be easier and cheaper to just drop an entire engine in with the ECU and call it a day.


My apologies in advance if this is covered in the link you posted (I perused it, but a bit more technical than I can easily parse) - Does the 300 engine fit if you drop it in wholesale (with ECU)? Also, it mentions the 300 engine, but with the big bore kit also - any explanation of what might have been meant?


Thanks again.
 

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... Also, it mentions the 300 engine, but with the big bore kit also - any explanation of what might have been meant?
...
Like the OEM 300 (286cc) kit would be a big bore for our 250's, there is a 349cc kit available for the CBR300R.
Will probably fit on the 250, too.

In case, you would need 'only' new cylinder, piston, crankshaft, ... but would not have a hole in the crankcase, you could look for the needed parts.

But better go for a complete engine, 250 or 300, up2you
 

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Discussion Starter #27
OK - I apologize if I'm being a bit pedantic, but the link is for a CRF250, and I want to make sure I'm not wasting money before buying a 300r engine.


Can someone confirm that a 300r engine will bolt right into the 250r frame? If so, I think I'm going to buy a 300r engine and ECU as the first step. After that I'll research the 349cc kit.


Again, can anyone confirm that the 300r stock engine (with ECU) will bolt right in to the cbr250r frame without modification?


Thanks in advance.
 

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The engines are the same physical size; not sure if the ECU has the same plug or not. Likely Honda changed it, but Shisoshin has a 300r and could probably confirm that for you.
 

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Discussion Starter #30
I don't know. Are the frames and the engine mount brackets the same part number? I'd check that first.


Here's a partial list of part numbers for the 2012 CBR250R and 2015 CBR300R. It looks good in general - the component numbers (first string of numbers) match (except for the frame, but that's probably because the 2015 only sells a Frame Set while the 2012 sells just the Frame). However, the Parts Classification Numbers (second string) are different in some cases. I'm not sure if that means the parts themselves are different, or if this is just an internal Honda classification for which bike they go on.


Any help? Thanks.




Part
CBR250
CBR300
Left Crankcase Cover
11341-KYJ-900
11341-KYJ-900
R Crankcase Assbly
11300-KYJ-901
11300-K33-001
PLATE, R. FR. ENGINE HANGER
50351-KYJ-900
50351-KYJ-900
PLATE, L. FR. ENGINE HANGER
50361-KYJ-900
50361-KYJ-900
STAY, R. COWL (LOWER)
64315-KYJ-900
64315-K33-D00
STAY, L. COWL (LOWER)
64325-KYJ-900
64325-K33-D00
FRAME
50100-KYJ-900
50010-K33-D00
 

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Discussion Starter #31
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
 

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Discussion Starter #32
Final update, unless you let me know you want more info...

I get my new, fully race built engine delivered on Monday. Mount it on the bike, install a new map to my pcv, change the sprockets to adjust for more power and low end torque and, hopefully, race at Cresson next weekend! Still looking to show the ninjettes and duke 390s that the little Honda can go fast.
 

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Nice one bro. Good luck on the track :)

Edit: Would you be willing to share your map data?
 

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Discussion Starter #34
Would you be willing to share your map data?
Do you mean the actual map, or the dyno results? The map I'm going to use this weekend is from Ari and I would need to ask his permission. Probably wouldn't be much help anyway, since its for a heavily modified engine. And I won't have dyno results until I get my own map, which I plan to do in the next month.
 

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No problem bro.

I was just interested to see the actual map. Mainly to see how different it is from my bike running with the Juicebox & full exhaust system. Mainly the deceleration values. If it is confidential. I totally understand. :) I still feel my map is a little lean on deceleration.
 
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