Intake Valve Closing
A good flowing intake port can take advantage of later intake valve closing. At higher rpm, where a good flowing port is still unrestricted, the later intake valve closing is allowing the charge air to fill the cylinder and build cylinder pressure. A poor flowing port that is choked at a lower rpm can benefit from earlier intake valve closing. It's important to build as much cylinder pressure as possible when closing the intake valve. If the intake port is choked and can't keep filling the cylinder, it's advantageous to close the intake valve earlier so the cylinder pressure is maintained. As you can imagine throwing a long duration camshaft that closes the intake valve late on a poor flowing engine combination will most likely hurt low to midrange power without making any peak power gains. Intake valve closing is very important and can have a dramatic impact on power production and the power curve.
Exhaust Valve Opening
A restrictive exhaust port can benefit from earlier exhaust valve opening. Opening the exhaust valve early bleeds off the pressure from the power stroke that is pushing down on the piston and producing torque, but at the same time it gives the exhaust more time to evacuate the cylinder through the exhaust port. It takes power to push the exhaust gases out the exhaust port, this is called pumping loss. The earlier exhaust valve opening on a poor flowing exhaust port can reduce this pumping loss and increase power. The trick is for the reduction in pumping loss to outweigh the power lost in the power stroke. With a good flowing exhaust port and little backpressure we can open the exhaust valve later because it's easier for the exhaust gas to get out of the cylinder (lower pumping loss). The later exhaust valve opening also keeps more cylinder pressure in the power stroke. My initial thoughts told me that since the turbocharger is a restriction on the exhaust side, a turbo car, especially with a stock turbo, can really benefit from opening the exhaust early; conducting a test on a stock turbo car changed my thinking. I installed an aggressive exhaust camshaft on a stock turbo car and it lost a lot of low-end torque, and didn't make much more peak power. I remembered that a turbocharged engine makes power a little differently than a naturally aspirated engine. The cylinder pressure that pushes down on the piston from the combustion process last longer, meaning it pushes down longer over the rotation of the crank. Imagine giving a naturally aspirated combustion stroke a quick and violent shove and the turbocharged a steady long push. This steady long push can be cut short when you start to open the exhaust valve too early. Remember that the exhaust valve starts to open well before the piston reaches the bottom of the power stroke to give the exhaust gas time to get out. Well, now we have a dilemma. If we open the exhaust valve too early we lose low rpm power but if we open it too late we lose top-end power due to pumping loss. This led me to another thought. The restriction (turbine) is after the exhaust port/valve and at lower mass flow rates (rpm), the exhaust port to turbine flow ratio. Remember that a turbocharger acts like a restriction after the exhaust port so the exhaust valve opening event requirement will be different than that of a naturally aspirated engine.