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compression or RPM? You are being very confusing now. But I doubt it would have a problem keeping up.

Raising compression actually requires the same amount of fueling but makes a stronger burn.
Both compression and redline need to be raised. And it just so happens that they both start at a similar looking number.

The stock compression ration of the CBR250 is 10.7 to 1.

The stock redline of the CBR250 is 10.5k rpm.

The compression on more serious bikes is around 12.5 to 1 or 13 to 1. This is the case with the CBR1000rr which shares the cylinder specs with the 250r.

The redline on more serious bikes is around 12.5k to 15k rpm. This is also the case with the CBR1000rr.

To make power on the 250, given that it shares the geometry and valve specs as the 1000rr, it is logical to match areas that are decreased to the 1000rr's stock specs, thus raising the compression and redline to the 1000rr's stock numbers.

Both an increase in redline as well as an increase in compression require more fuel, as does any other way of increasing power. An internal combustion engine is just an air-pump. That is what it boils down to, it pumps air expanding by an explosion. More power means more force, caused by a larger explosion, needing more air, and thus more fuel, keeping in a proper ratio. The same is the case with forced induction as it is with high compression, more air going in means more fuel required, and thus, a larger injector is required after a certain threshold.

And if you don't need the additional injector capacity, the larger injector won't hurt the engine as would be the case if you needed a larger one and had a smaller one instead. It is just a safety precaution.
 

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Increase in compression requires more fuel because the VE has just gone up. Plus, a little more fuel will keep the cylinder temperatures in check.
 

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Regardless, I am disappointed with the forum. This is the first time I've seen people actively discouraging others from doing radical modifications on an engine. Pity the small thinking.
 

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Regardless, I am disappointed with the forum. This is the first time I've seen people actively discouraging others from doing radical modifications on an engine. Pity the small thinking.
Dude..... goto town. Be the guinea pig.
I would think that most here want to keep the cbr as streetable as possible while they still owe money on it. Add to that, if they had 2 or 3 k to burn they would have bought something bigger.
I myself am interested in a racing spec cbr which means no internal motor mods.
 

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I am going to be the guinea pig, still waiting for delivery of my bike. Have a whole set of things planned, right from speccing the engine, porting the head, bumping up the compression, intake modifications, etc.

For some people, like those in my country, we don't have options unless we spend something like 8k more. And for me, it is a case of optimising whatever is available to the max. Sorry if it annoys some of the people here. I guess I'll find a different group to hang out with. If it bothers some of the people here so much, they should learn to stay away from those threads. Taking a **** in a thread when some people actually want to discuss is incredibly rude.
 

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but the shaving for CBR is 0.45mm. and you should be seeing a lot of difference and will enjoy it

first check this 0.45 and then u can do further..

Note.. forget the size and grind it like the on on the pic/// you see the extra space between the valve ring and mating surface.

I found this on another site
 

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Thank you! Are there any markings on the underside of the head to indicate different depths that can be milled?

First I'll need to cc the combustion chamber and piston before I can fool around with milling the head though lol. Do you mean valve seat instead of valve ring?

On our car heads, we have successfully milled big amounts before touching the valve seats, in effect giving us a nice quench and resisting detonation even with high compression ratios.
 

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Dude..... goto town. Be the guinea pig.
I would think that most here want to keep the cbr as streetable as possible while they still owe money on it. Add to that, if they had 2 or 3 k to burn they would have bought something bigger.
I myself am interested in a racing spec cbr which means no internal motor mods.
i live in israel cbr250R abs costs 12000$
kawasaki ninja250 costs 13700$
cbr600RR costs 29500$

plus insurance for cbr250 is 1000$ per year.
for the cbr600rr is 3500$ per year.


so if the mods in these thread will bump the HP i will be very happy because the mod will cost me 1500$ and i will pay the same "low cost" insurance

and thank you guy's for you'r hard work in gaining performance
 

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You want my advice? I would be patient. Im sure there will be a "big bore kit" for the cbr soon enough. That kit will undoubtedly come with a new piston that will increase not only compression but cc's as well. Why start milling the head right out of the gate? If the kit comes with a "dome" piston you might need the clearance that you just milled off the head. Then what do you do?? buy a new head and start all over?? It seems cost prohibitive at this point. On top of all that why cut the head?? wont that change the shape of the combustion chamber?? That might hurt more than it helps. Why not mill the block??
I know JE pistons will make a set of 4 pistons just about any way you want them for $800 for the ninja. You can get 12, 12.5 or 13 to 1 pistons with bores totaling up to 282 cc's for the ninja. Im sure they will get into making pistons for the cbr if there is enough interest.
Maybe a few emails and a call or two to JE might yield good results. Theres always a group buy option. 4 guys $200 each and your talking turkey. right?
Milling the head should be one of the last things you do. Not the first. IMO

Start with the external bolt-ons the exhaust will work for everything. How about a programable power commander. That will work for everything too. A few mods to the airbox wont ruin any future plans for more compression or bore. Then sit tight and enjoy the 20-30% increase in power you will get over stock. When a "kit" comes out for more compression and bore then plan your next move. Otherwise you are wasting your money and hoping that what you have added will work with what is now avalible.
 

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My advice is keep it as it is. I'm not even gonna put an exhaust on mine. I like it quiet and reliable. I will just buy another bike to go fast on.
 

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My advice is keep it as it is. I'm not even gonna put an exhaust on mine. I like it quiet and reliable. I will just buy another bike to go fast on.
External bolt-ons wont hurt. Nor will they void the warrany. You will lose mpg for more power. Thats the only trade off.
 

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You want my advice? I would be patient. Im sure there will be a "big bore kit" for the cbr soon enough. That kit will undoubtedly come with a new piston that will increase not only compression but cc's as well. Why start milling the head right out of the gate? If the kit comes with a "dome" piston you might need the clearance that you just milled off the head. Then what do you do?? buy a new head and start all over?? It seems cost prohibitive at this point. On top of all that why cut the head?? wont that change the shape of the combustion chamber?? That might hurt more than it helps. Why not mill the block??
I know JE pistons will make a set of 4 pistons just about any way you want them for $800 for the ninja. You can get 12, 12.5 or 13 to 1 pistons with bores totaling up to 282 cc's for the ninja. Im sure they will get into making pistons for the cbr if there is enough interest.
Maybe a few emails and a call or two to JE might yield good results. Theres always a group buy option. 4 guys $200 each and your talking turkey. right?
Milling the head should be one of the last things you do. Not the first. IMO

Start with the external bolt-ons the exhaust will work for everything. How about a programable power commander. That will work for everything too. A few mods to the airbox wont ruin any future plans for more compression or bore. Then sit tight and enjoy the 20-30% increase in power you will get over stock. When a "kit" comes out for more compression and bore then plan your next move. Otherwise you are wasting your money and hoping that what you have added will work with what is now avalible.
Thank you for a cohoerent encouraging response.

The kits you are talking about, I have plans to manufacture my own. Regardless, swapping pistons for a small compression upgrade is pretty expensive, and something which will usually require an overbore (unless we are talking about a brand new block with a fresh hone from the factory). Secondly, a dome top is a pretty unsuitable shape for a piston. Not to take away the fact that most high-compression motors have domed pistons; but if given the choice, I would always choose a smaller combustion chamber over a domed piston. Simple reason being that a domed piston is going to push quite a bit of the combustion force towards the liner, rather than downwards. This causes a loss of efficiency.

I do not believe in piggybacks like Power commander. This is precisely why I have created a seperate thread in this very forum to ask about ECU options. With a piggyback, you do not really get access to long and short term fuel trims, injector scaling, and hundreds of other parameters. They are nothing more than band-aids.

Thank you for a useful reply. Appreciate your inputs.
 

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Thank you! Are there any markings on the underside of the head to indicate different depths that can be milled?

First I'll need to cc the combustion chamber and piston before I can fool around with milling the head though lol. Do you mean valve seat instead of valve ring?

On our car heads, we have successfully milled big amounts before touching the valve seats, in effect giving us a nice quench and resisting detonation even with high compression ratios.
I found that on another site. I'm gunna pull the head, measure the squish to the piston, measure the valve clearance to the piston and go from their.
 

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Great. Will you be sharing any of that info? Can understand if you don't want to, just curious.

Once I have the bike, I'll be in contact with ARP and have them design a set of head studs and rod bolts. The plan is to replicate the engine in my simulator and work from there. Saves a bunch of money that would be spent on prototyping and trying out parts that would then be junked. I saw your pipe, looked nice. Do you have any sound clips of the open exhaust?
 

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I will share the info and will take pics of the process. I also plan on putting the bike on the rollers once I'm done and will post my dyno sheet. As far as the gp pipe I originally made I did not do a sound clip. To sum it up it sounded like a tractor. I have a Vid of the current pipe I made if u want to hear it.

 

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I would like to see what you come up with.

I recently pulled my cat/muffler and reset my ECU, and removed the intake cover. It is much smoother at higher rpm ranges and pulls much harder up top, but it is very loud, like loud enough I have to wear plugs so my ears don't ring when I ride it. I may look at an M4 short muffler some point in the near future to give it a slightly quieter note. I don't think I will have problems with local LEOs but the neighbors might not like me so much when I fire it up at 6:45 every morning to go to work.
 

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You all got me thinking.
I wouldn't be much surprised if in a year or two, a couple speed shops (and cyanide too I hope) begin to market kits. New head, piston, cylinder, etc...

I can think of a couple shops that offer such things for HD Sportsters and Buells.
It's just a matter of time so long as there is a demand for it.

But be forewarned, As the saying goes:
Speed costs, how fast do you want to go? :D

Though I have no interest in spending mass amounts of $$ to make this bike faster, I can definitely see the appeal. Especially for those who want to use this bike on a track. Whether it be racing or simply to hone one's skills.

Personally, I plan on winning the Lotto and buying a Buell 1190RS for my high speed shenanigans. (Highly improbable but not entirely impossible. Just mostly :))
 

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External bolt-ons wont hurt. Nor will they void the warrany. You will lose mpg for more power. Thats the only trade off.
Who said anything about that? I'm not going to do it because a 250's not worth doing it to. I bought it for mileage that's it.
 

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08-11 1000RR piston is a drop in 12.7:1 compression. :) you can get wiseco 13.5:1 pistons in singles for around $150.

look. more free R&D courtesy of Davis Rod And Cycle.
 
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