cannot.. it's really useless anyway, since your rocker arm wear will reduce the duration anyway. A lot of factors go into getting that ACTUAL duration number, including if you've ground the valve seats before, etc. etc. The manufacturer measures the duration on a machine, that may or may not replicate the actual engine enviroment...
What you can do is compare it in the sense that you will see that if crower have the same duration number than the japanese, then you can consider it as having a slightly higher duration than that japanese cam. Normally you'd notice by now that the americans seems to have smaller duration numbers than japanese.
Anyway, when talking about duration numbers.. read the paper that comes with the cam.. some of the advertised duration is at 0mm lift, so it not really an accurate way to gauge exactly what of duration the cam has.
Thing is.. no one really know exactly how much duration to get peak power at XXXX rpm, unless by experimentation, due to various factors that affect duration and lift (that rocker arm wear, valvetrain works factor again). The formula helps you make an educated guess, but flow is not really calculate-able.
Higher durations better power at higher end of the RPM range... so if you buy a cam that the manufacturer CLAIMS makes +30hp at 9.5K rpm, and you plonk it in a B20B rev limited to 8K rpm.. you'd probably not be making power efficiently, and power gains will be minimal, if any.
That's why crower does make custom grinds for those people that really have the money to try out a couple of cams and make a closer estimate to what kinds of cam they need.
If you're still wondering what cam to use, email/call crower.. they can help give suggestions too.
Toda and other japanese company also.. but I don't know if they reply to english speaking customers. I didn't get a reply when I sent an email in english.. (O_o)
