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The cheapest way to increase the compression ratio on the CBR300R engine is to replace it's piston with the one of the earlier CBR250R.
In order to maintain the same compression ratio of 10.7:1 on the CBR300R like on the CBR250R, despite strocking it by 8mm, Honda flattened the piston top of the new version. Simply by switching it with the piston from the earlier model will therefore do the trick. (same cylinder used but stamped 286cm3 instead of 249cm3)
 

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Oh yea, would you be able to provide any further data? Its not that I doubt its just I'd like to see what the befits of the parts and labor are...

Also I have to assume honda didn't flatten the top of the head just to take the piss, there was obviously a functional reason. Regardless, thanks for the post :D
 

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One reason that I can see for Honda wanting to maintain the same 10.7:1 compression ratio for the 286cc version of the motor, is so that regular unleaded gasoline can be used. For U.S. spec bikes this would be 87 PON gasoline (I believe in other countries the equivalent regular grade gasoline is 91 RON). If one were to change out the stock 286cc piston for a 250cc piston, it would follow that a higher octane fuel would be required to prevent detonation.
 

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A higher compression ratio doesn't result in increased fuel efficiency, rather it is a way of increasing power in an engine. Increased power comes at the cost of using more fuel, not less.
 

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Another way of increasing power in these bikes is to install an aftermarket fuel controller, which allows the fuel map to be reprogrammed to richen the fuel/air mixture. Of course a richer mixture results in higher fuel consumption as well.
 
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Another way of increasing power in these bikes is to install an aftermarket fuel controller, which allows the fuel map to be reprogrammed to richen the fuel/air mixture. Of course a richer mixture results in higher fuel consumption as well.
The increase of power should make the extra fuel consumption well worth it :D
 

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Interesting. If I wanted more power, I would just buy a bigger bike though. It seems like people who make adjustments to their bike to get more power wind up with more blown engines. Might be because they ride really hard too (above red line often) or they wouldn't bother to try and increase performance. I'm thinking specifically of the Grom riders who do the big bore kits, etc. Several of those have failed, but I haven't heard of a single stock Grom engine going south.


I'm trying to understand how the 2 pistons would be compatible. I'm not sure how that could be. The changes in the cylinder have to match up with changes in the piston in order for the valves and cams to continue to operate properly. It looks to me like if you replace the 300 piston with a 250 piston, your compression would go south.
 

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Hm, whid 8mm longer stroke, and the same cylinder, the piston will reach
OD. 4mm earlier, I belive the shaft is 4mm shorter. So why make an shange on the piston. Could be another reason.?
Are You sure about the cylinder ???? Any possibility to check the shaft.

If the cylinder is same, there should be a chanse for the big bore KIT !!!
 

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Sounds like a good way to void your warranty. The bikes a 300. It is what it is. If you want a faster bike they make those too.
 
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