Jim, have a read of the America/Speedmaster section
HERE. The dyno's are all from EFI bikes so the figures from a carb bike will be a little less, but the percentage changes in torque/power from mod to mod will be about the same.
We usually recommend that America/Speedmaster owners keep the airbox in place unless you're carrying out internal engine mods, otherwise you can lose too much mid range torque which takes the fun out of riding a cruiser.
