Harry,Turbo6 His reply, As a general rule any increase in duration on the intake will lower the velocity so you must balance the area of the port relative to the demand (engine size/RPM) If one starts out with an engine that has a port designed for limited RPM/HP as in the Chevy 6 cylinder it takes a larger than ideal cam to extend the torque curve (at least with stock type heads) to get to the higher HP numbers.
Most engines of these designs will push the torque up the RPM range about 500 or so as you add 7 to 10 deg. but it also depends on the lobe separation as you start to get to a point of having so much overlap that the type and tuning of the intake and exhaust will narrow the range that runs well in.
Any normally aspirated engines possible power output is limited by the cylinder heads ability to get air into the cylinder and of course the block,crank,rods and pistons capacity to hold together. But even a well designed engine with the wrong tuning cam,carb and or ignition can run like crap.
As to exhaust relative to the intake it comes down to how well the exhaust leaves. Many 4 valve head motors and small engines that have large breathing capacity like motorcycle engines or the air cooled 911 Porsche motors you see cams that have less timing and lift on the exhaust side as they "blow down" very quickly.
On engines like the 6 cyl Chevy a single pattern can sometimes be used to help make a FAT torque curve but at the expense of top end. adding exhaust duration will extend the torque curve but will soften the rise some.
You will have to decide the real RPM range the motor needs to run well in based on weight,gearing,converter/clutch and how you will be using it.
Remember that the cylinder pressure needs to be within an effective range and load/use at the RPM you want it to pull well at. So a camshaft that might work at 5000 RPM with 9 to 1 compression on pump gas will not be the same as one with 15 to 1 compression even at the same RPM.
The standard for race engines is as much compression as the fuel can take and cam to make the highest torque within the range it runs the most. Use the lobe separation for width of the powerband (wider will make less peak torque over a broader range) The intake duration and lobe center to tune the peak torque RPM and exhaust duration for over-rev peak HP
