[dariusjonsson]

I removed the rubber elbow that goes into the airbox and dremeled out about a 4 square inch hole on the bottom piece of plastic between the battery and airbox. Butt dyno says it made a non-negligable difference. As you know, power falls flat on its face after 8k rpm and this seemed to help extend the torque curve a bit. Anyone have other mods to help it breathe better up top?

 

[DannoXYZ]

Note that total flow volume is dictated by Helmholz resonance variables (pulse tuning). Air just doesn't flow straight through engine, but bounces back & forth. It's an entire combined system, so every part of system needs to be tuned to give final effect. If you want better high-RPM flow and peak-power, best mods start from inside engine outwards:

• big-bore pistons, no replacement for displacement
• higher compression 12.5-13.5:1 pistons, depending upon highest octane fuel available
• larger valves, back-cut and waisted stems
• fully radiused valve-seats, flows way, way better than 7 or 9-angle "competition" cuts
• ported and polished intake & exhaust ports
• port matched to intake & exhaust (need min. 1mm step to prevent exhaust pulses coming back)
• variable diameter exhaust that increases in diameter as gases cools and slow down
• high-lift, long duration cam to tie all above together. Cam profile must be customised with all these other components in mind
• airbox & filter? They're same size as on my VFR, bike with 5x power and 5x airflow. So I doubt there's any kind of restriction that can be freed up

This would be my very 1st mod since it makes biggest different. Variable radius seats would be best depending upon section of valve circumference. For example, air flowing towards center takes different shaped path than air running into edge of cylinder wall.

Basically Honda engineers thought of everything. You're not going to outsmart them. It was designed to perfrom exactly this way for cost of production, retail-pricing and target market (end-users). To make significant and noticeable gains, you'll need to go back in time and get on Honda's design-team when they're drawing up initial specs.

Or buy their 250 product designed with high-output in mind. Get one of these engines, they're sold as spares. Just swap engine into CBR (makes double power of CBR engine).

https://www.hondaracingcorporation.com/product/nsf250r/

REFERENCES
Internal Combustion Engine Fundamentals - Heywood 1988 McGraw-Hill
Engineering Fundamentals of Internal Combustion Engine - Pulkrabek 2003 Prentice Hall
Flow and Combustion in Reciprocating Engines - Springer, Arcoumanis, Kaminoto 2009 Verlag Berlin Heideberg

 

[Tamir]

I removed the rubber elbow that goes into the airbox and dremeled out about a 4 square inch hole on the bottom piece of plastic between the battery and airbox. Butt dyno says it made a non-negligable difference. As you know, power falls flat on its face after 8k rpm and this seemed to help extend the torque curve a bit. Anyone have other mods to help it breathe better up top?

It's really fun to play with engine improvements, and as Denno written to you, your improvement to be noticeable has to come with re-tuning the map in the ECM. I have been trying for a while to figure out how to get free maps for our CBR250R, and if there is a cheap (amateur) tool with which I can install the new map to our ECM. If you will know the subject, before me, then you are welcome to sharing us about it.

The guy in the attached video (Yamaha MT-7) "removed the rubber elbow that goes into the airbox":

 

[Tamir]

Besides as Danno wrote to you our motorcycle comes out of the factory almost perfectly. Our motorcycle, which is sometimes defined as a motorcycle for beginners, is also a motorcycle that professionals know how to derive great pleasure from her. On the track with the right tires, your personal riding abilities at laps time may be no less noticeable than an extra few percent in power.
In any case the effort for improvement is certainly worthy and justified.
But it is usually better to switch to a motorcycle with a larger engine, instead of spending time and money on a motorcycle with a smaller engine.

The basic and cheap actions have already been done?:
1. New and improved plug.
2. New and improved air filter.
3. Change engine oil, to quality oil (fully synthetic).
4. Adjusting valves.
5. and cleaning of the fuel system, combustion chamber, and valve mounts with a substance such as BG-44K at a concentration close to 100%.

 

[Tamir]

Anyone have other mods to help it breathe better up top?

If you have accumulated many Km like me, over 66,000 km, probably a noticeable improvement will come with addressing the issue of the catalytic converter, which is probably at this point already quite blocked.
The options:

• Cutting the converter from the original muffler (frame work).
• Replacing the muffler.

 

[DannoXYZ]

Weigh-savings is major improvement for these bikes:

• lightweight exhaust -5-10kg
• CF bodywork -5kg
• rear-seat delete -2kg
• CF wheels -3-5kg
• 415 chain & sprockets -2kg
• aluminium or CF petrol tank -3kg
• race tyres -3kg
• alloy swingarm -6kg
• titanium wheel-axle and swingarm pivot bolts -4kg
• titanium con-rod & bolts -0.5kg improves engine revving too
• clip-on handlebars -1kg
• tubular chromoly or alumnium sub-frame -3-5kg
• race-harness with smaller-gauge wires and tiny connectors -2-3kg

 

[SCALECRAFT]

I removed the rubber elbow that goes into the airbox and dremeled out about a 4 square inch hole on the bottom piece of plastic between the battery and airbox. Butt dyno says it made a non-negligable difference. As you know, power falls flat on its face after 8k rpm and this seemed to help extend the torque curve a bit. Anyone have other mods to help it breathe better up top?

Larger rear sprocket would do that.

 

[B.C-Rob]

At about 8:25 they cut holes in the airbox and the bike ran significantly worse so they had to cover them up again. Im not sure about the airflow engineering but its not going to be simple

 

[Tamir]

they cut holes in the airbox and the bike ran significantly worse

In our engine, the computer control unit can balance supercharged air intake (with certain limitations) by reducing the fuel injection times in the injector (mainly related to closing the control loop with the oxygen sensor). The old engines, without "smart" controls, even a change in altitude of over 1000 meters required a mechanical change in the carburetor nozzle. To balance the supercharged air intake they had to reduce the nozzle hole. Tuning engines to improve performance requires additional specialization over the knowledge of "normal" motorcycle mechanics.

 

[DannoXYZ]

To get any significant amount of extra power to be worth effort, you'll want forced-induction: