Honda CBR 250 Forum banner

Drilling airbox experiment

1428 Views 9 Replies 5 Participants Last post by  DannoXYZ
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?
  • Like
Reactions: 1
1 - 3 of 10 Posts
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
See less See more
  • Like
Reactions: 3
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
See less See more
To get any significant amount of extra power to be worth effort, you'll want forced-induction:

1 - 3 of 10 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.