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).
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