Honda CBR 300 Forum banner

Pistons and cylinders

43K views 112 replies 18 participants last post by  motorboy 
#1 ·
So I been doing lots of research between the cbr300, cbr250 and crf250l.

The cbr/crf250 motor wise is basically the same except:
1) The throttle body of cbr is 2mm larger then the crf.
2) ECU is different cause of mapping.
3) Header pipe on the crf is smaller.
4) CRF has judder spring on the clutch.
5) Transmission gearing is the same but the crf gears are wider by about 1mm
6) Final drive ratio is taller on the cbr

The cbr300 differs from the cbr250:
1) cams
2) piston, the 300 crown/dish is shorter to maintain the same compression ratio
3) head, the 300 has the O2 on the header while the 250s is on the head.
4) cylinder is actually the same except the color and the CC stamp
5) ECU, mapping difference, plug is the same with all three bikes
6) connecting rod/crank, obviously 300 has the longer stroke
7) counter balance/weight difference cause of the extra stroke
8) Tranny ratios is shorter on the 300
9) Final drive ratio is taller on the 300
10) Clutch is stronger, the 300 shares some parts with the cbr500

So with this info one can use parts for the cbr/crf250 parts on the 300. Normally folks would use parts from the 300 to make their 250 perform better but since this is 300 forum I'll focus on using the 250 parts to improve performance on the 300.

Piston, using the 250 piston will increase the compression ratio to about 12.3:1, but not sure if the taller 250 piston will clear the valves...I think it will. But the 300 piston is shorter/lighter so weight has to be added to the crank/counter balance to keep the shakes down, how much I don't know....

There are aftermarket BB kits for the 250 that will work if using the 283 kit then the 300 will increase to 324cc and the 305 kit will increase the 300 to 349cc. The 305 kit on the 250 will increase compression to 12:1 while the 300 will be somthing like 13.5:1. Don't know how high the 283 kit compression ratio is on the 250 or 300. Not sure how high compression ratio can a (R+M)/2 octane of 91 (premium) can get too..also dependent on ignition timing. I had a cb600f with 12:1 that honda recommended 87octane but timing was conservative. Again there is piston weight difference between the 300 and the kits...You can either remove some material on the kit piston or add weight on the bottom end. But looks likely to have to do both just to get the compression ratio down some on the kits. Other thing with the BB kits is that you have to run some kind of fuel controller other wise it will be too lean and burn up the motor.

There are several kits on the market the only one that I thinks works is the B&B kit the other kit from Takakawa won't work with the stroker crank from honda casuse of an oil hole on the sleeve that helps cools down the piston/sleeve will have oil ending up in the combustion chamber.

I had ordered the B&B 305 kit but canceled it the next day cause I didn't want to add a fuel controller and the reliability/durability of the 349cc build is not yet proven. There are folks that have tried it on their crf250l and many of them had problems. What I think I'm gonna do is get a cbr250 piston and pop that in. This is the easiest thing to do and don't have to mess with anything else really. I think the main thing is that I have to run higher octane gas but might not have to cause I think the timing on the 300 is conservative. I'll start with 91octane first.
 
See less See more
#2 · (Edited)
Good work on the research there DaBinChe.
Some good solid facts without the usual speculation. :D

You touched on something in points 8. & 9. That I looked into myself when I first got my 300R. I couldn't work out why with the taller final drive ratio of the 300 the gearing was the same as my old 250R. It was because they shortened the transmission ratios as you state.
So the question is why did they do that and end up with a bike that is IMO under geared and requires a sprocket change for sustained high speed running.

Anyways, good luck with the fitting of the 250R piston and keep us posted on your findings.
 
#3 ·
#4 ·
No 300cc kit for the 300, only 324cc or 349cc

You'll need a fuel controller or you will lean out and burn up the motor as I mentioned

Yeah there will be more vibes as I said but lots of it can be mitigated with the proper weight add/minus where it needs to be.

Other thing I didn't mentioned is that once up to 349cc the stock injectors (crf/cbr250 and cbr300 use same injector) can't output enough so you'll have to use a crf450 injector. The crf450 injector is plug and play, on the controllers you'll have to minus fuel. If using stock ecu with it it will be too rich. I'm not sure of the 324cc if the stock injector are okay or needs the 450 injector. As a 305cc kit the stock injector is fine with a fuel controller, it will add fuel.
 
#6 ·
I would be very interested to hear the results of putting a stock 250 piston in the 300.
An accurate measurement of the difference in weight between them would also be interesting.
 
#8 ·
You guys are using the term Injector incorrectly. I think what you mean to say is that a larger sized Throttle Body is what would be needed to feed a larger displacement engine.

A Fuel Injector doesn't need to be bigger from one engine displacement to the next… a Fuel Injector is really nothing more than a precision valve which squirts a precisely measured amount of fuel (which is determined and controlled by the ECM) into the Throttle Body where that fuel is mixed with the intake air.
 
  • Like
Reactions: shisoshin
#9 ·
Larger displacement requires more fuel. If the stock injector cannot meet these requirements a larger injector is needed.


A larger throttle body would definitely help with 305 plus cc. I wonder if the throttle position sensor etc parts match up from the crf450 could be a good upgrade to buy the whole assembly when going a Big bore kit.


Often as not there will not be much in the way of gains with a throttle body if the plenum chamber, runners and ports haven't been port matched as well.
 
#10 ·
Yeah I was under the impression that a fuel injector is designed for a specific range of fuel pressure. When more power is needed (up to a point) a stock injector will do but will only flow so much. I have read in the past the Mustang 5.0's (302 cu in V8) stock injectors are only good for so much more power. Any drastic change (Camshaft/displacement/ etc) would negate a change to a larger (higher flow capacity) injector and throttle body.
 
#11 ·
Yeah I was under the impression that a fuel injector is designed for a specific range of fuel pressure. When more power is needed (up to a point) a stock injector will do but will only flow so much. I have read in the past the Mustang 5.0's (302 cu in V8) stock injectors are only good for so much more power. Any drastic change (Camshaft/displacement/ etc) would negate a change to a larger (higher flow capacity) injector and throttle body.
The term negate doesn't make sense as used in the context of that sentence… replace it with require, and then it makes sense.
 
#12 ·
So far the only guy that I know that has made his crf250 into a crf350 lives here in CA, others have tried but failed. He has his running for a year now. The stock injector were not able to spray enough fuel. He has this documented and he is the one that stepped it up to the crf450 injector. What happened is that the stock injector was passing 80% duty cycle and got wonky and leaned out. The 450 injector dropped it down to under 50% duty cycle and works okay. He explained that when injectors get up to 80% duty cycle they become unreliable/unstable and better to keep them under 50% duty cycle. Got dyno runs to show before and after so I'll take his word for it. It wasn't a problem with pressure, but volume, the stock injector "jets" are tiny compared to the crf450 injector "jets".
 
#18 ·
BB = Big bore.
 
#20 ·
So I got the 250 piston, rings, head/base gaskets, cir clips and cylinder in....just now realized I forgot the chain tensioner gasket, hopefully the old one is in reusable shape. I'll be doing this in a few weeks when I have time. Actually I should of check for piston/valve clearance first before ordering all the parts. Hopefully the bore cam can see the clearance if not some solder wire should do it otherwise all the parts will have to be installed and spun by hand...
 
#22 ·
Using an open exhaust removes some of the back pressure, which might help with a bbk, to not have it ping (due to residual burned air/fuel (aka smoke) in the cylinder on the next stroke, causing higher psi in the cylinder.

It also runs cooler that way.

That way you may not even need to go hi octane, despite a higher compression ratio, overall psi could be similar.
 
#27 ·
This is the affect of the exhaust on a 2 stroke and is not true for a four stroke. On a four stroke the back preassure causes a little exhaust to stay in the cylinder which causes the temps to be reduced because the exhaust gases is already burnt. This can make the engine more emissions friendly and is the whole principle behind the EGR (exhaust gas recirculation ) valve which redirects exhaust back into the intake.
This is why some engines register an increase in cylinder temps when it is blocked off.
 
  • Like
Reactions: shisoshin
#23 ·
Got the piston in. Had a hellof a time. The new piston circlip grooves were machined a tad shallow compared to the old. Clips sits half way in groove while old one sat in 3/4 way in. So had to sand down piston pin to fit. Took off 0.026". Now the motor is all buttoned up just need to put all the lines, tank, etc back. Should have it running this weekend.
 
#29 ·
that 10% mix you referenced has already been burnt and therefore cannot be burnt again. this is what drops the overall combustion temperature. Lower combustion temperature means lower overall temperature.
 
#30 ·
Yes, but it also is remnant in the cylinder for an additional 360 degrees rotation giving off it's temperature to the engine block and incoming fresh air/fuel; causing that to expand and having a greater PSI.

The combustion is a short flash (of less than 180 degrees). Exhaust in the cylinder gives an additional 360 degrees of heat transfer.

I don't know.. I think for most use, an open exhaust runs cooler than one with back pressure, except if you use the bike beyond the limits you could do with an exhaust with back pressure.
 
#35 ·
As far as your film career goes, your early work was great. You got a bit repetitive after that.

Nah, it's great when folks do stuff and then post 'How To' vids - I love 'em.

Looking forward to hearing about the results.
 
#39 ·
So just got done riding and the first thing I notice is how quiet it is. Still deep but quieter, at 50mph could barely hear it with the wind. Feels smooth and strong. How much stronger hard to say, it has been about a month since I rode this bike and have been riding my other bikes. With higher compression it should not only have more hp and tq but better fuel economy too for the same displacement and air/fuel ratio.

https://youtu.be/wWz-WKencgs

https://youtu.be/-uJLWPKxp_c
 
#40 ·
Wait did you say you were using the cbr250 cylinder? If so did you just put your 300 head on top of it? This whole thing is really interesting, if it's not toooooo much to ask can you make a part list? I kinda skimmed through partzilla and figure the whole project would be less than $300 in brand new oem parts. This is a really cool mod. :laugh: you're a pioneer
 
#42 ·
I looked at your latest video and Holy Smokes that Scorpion exhaust looks very close to the stock Honda one...sounds similar too. Thank you for keeping us updated on your progress.
 
#43 · (Edited)
yeah I was really surprise at the sound too

The reason I didn't do a video of taking things apart and build is that it is more involved and folks that don't have enough experience can screw up. Folks that do have experience won't need the video.

Parts needed for this mod:

Bare minimum (BM) is
crf/cbr 250 piston 13101-KYJ-900
pair of crf/cbr 250 piston circlip 13112-KYJ-900
250/300 (same part number) head gasket 12251-KYJ-901
250/300 cylinder gasket 12191-KYJ-900 (only if damage)*

Highly Recommended plus BM is
250/300 ring set 13010-KYJ-305
Pick one; cylinder 12100-KZZ-900 (CRF250L,grey) / 12100-KYJ-900 (CBR250,black) / 12100-K33-950 (original 300,grey)
250/300 cylinder gasket 12191-KYJ-900 (if stuck to original cylinder)*

*If the cylinder gasket is stuck to the case undamaged then you don't need a new one at all.


Currently I get my OEM parts from WesternHonda out of AZ. They have the lowest cost for parts and lowest cost shipping. Place that I would never buy from again is Partszilla, got burnt by them, one of those places that don't want to man up.
 
#44 · (Edited)
... Currently I get my OEM parts from WesternHonda out of AZ. They have the lowest cost for parts and lowest cost shipping. Place that I would never buy from again is Partszilla, got burnt by them, one of those places that don't want to man up.
I think that to be fair, if you're going to post about an unsatisfactory experience with a dealer/supplier, you should at least include a brief explanation of the actual issue or problem... to simply say that you "got burnt by them, one of those places that don't want to man up" isn't really saying much.

Unlike walking into a "brick and mortar" dealership and ordering parts over the counter, where if the parts guy screws up and orders the wrong parts for your bike (assuming you gave them the correct year & model info) the mistake is all on the dealership, whereas ordering OEM parts online is a completely different animal... you not only need to know exactly what the specific part is that you want, but also that you are 100% sure that you're looking at the correct parts diagram/listing for your specific year & model of bike. If you get it wrong, it's not the online dealers fault. That saying "garbage in, garbage out" definitely applies to placing parts orders online.

While I have no personal or financial interest in Partzilla or any other online parts dealer/seller, I have ordered a fair amount of Honda parts from them and haven't had any issues. Another online Honda dealer I've ordered from without issues or problems is Honda East Toledo.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top