Here is a great site / Forum discussion around home made Flow Benches, how to's, and resources for parts and components.
If you are truly serious about high performance, and want to step past the "bolt-on" phase of modding, this is where it all begins.
Flowbench & Dyno Discussion
Always remember, "velocity" Vs. mere "volume". Both have their individual and collective merits, but for a street ride, Velocity trumps seeking tons of "volume" nearly every time.
Maximum port Velocity generates better A/F mixing, better turbulance, better cylinder filling, more complete, and quicker total charge burn. This equates immediately to better Volumetric Efficiency, better overall torque, a smoother, more broadly usable powerband, and in the end, an enjoyable bike to ride.
"More Air" is good as well, but not all by itself, and for different reasons, as in High RPM breathing, and maximum, near redline HP readings. There are reletively easy ways and means to get the maximum amount of both seperately, but to achieve and maintain a selective balance of both takes some technical expertise, and knowledge. "Big Air" is fantastic for Salts Flats record breaking, not so good for relaxed tooling around town, or accelerating from a standing start. Remember our old pal Burt Munro aka "World's Fastest Indian"? Thats Salt Flats racing for sure. A good, well executed balance between the two is riding performance perfection, and what Hot Rodders have been trying to do since the advent of the IC engine.
That said, knowing the why's and where-for's of cylinder head flowing is productive towards an improved bike.
If you are needing to add more initial spark timing (ignition advance), or need to add significantly to Main Jet size(s), this is a clear indication of having achieved "More Air" , but not having achieved improved Volumetric Efficiency, or Cylinder Filling. If done correctly, you should need to adjust for LESS timing, as the better mixed A/F charge will burn quicker, and require LESS fuel metering to result in the same power, not more fuel metering, as you are mixing the original Pilot / Main Jet charge better with increase velocity and turbulance.
This phenomina can also be seen in a chart as LESS (or very little change in) low end torque, and an increase in calculated HP way up high in the RPM chart. This translates clearly as "Very high RPM needed to pump and burn the A/F charge, as the velocity has been reduced through oversized carbs, ports, valves, cam overlap, duration, etc.". Good for "Dyno racing", not so good for the everyday street ride.
Get involved, and "learn all about it!" It's fun, and worth the effort of comprehending clearly. When talking to a Cylinder Head Shop tuner, try to notice if he says "More Air" as opposed to "Higher Velocity charge", the latter is a mark of an experienced, well trained head porter. Don't forget, anyone can actually purchase a "Flowbench" for roughly $5000, and "yer in bizniz".
Not on my bikes yer not!
Some great links for further research and information as well.
Ride Safe!
Bob
Flowbench Technology 
