On the other hand, at high lift, we have large flow separation from the valve and valve seat walls leading to a reduced flow area along with dead recirculation zones (aka, the combustion chamber) that do not contribute to fluid flow. As the restriction to fluid flow is now minimized, the air/fuel mixture now has a high velocity in the port, leading to higher friction losses against the wall and the potential for choked flow. Compared to low lifts, the mixture velocity past the valve at high lifts is relatively smaller, leading to non-ideal mixing and a reduction in torque.
So then why would anyone want to run higher valve lifts? Let's throw rpm into the previous discussion. At low rpms, increased valve lift improves performance (torque) since the intake system and port are not limiting air flow. Although the discharge coefficient might drop from a lower lift specification, the increase in theoretical mass flow more than compensates for this.
At high rpm, larger lifts can be a detriment to performance. The intake system and port now become the bottleneck due to high gas velocities increasing friction and leading to possibilities of recirculation and choked flow around the valve. This is where valve sizing and valve seat profile come into play and exactly why Cosworth's CNC ported cylinder heads use larger intake valves and more valve angles and radii for smoother flow transitions.
Valve Event Duration
Valve event duration for peak power is more straightforward than valve lift. Longer valve event duration is usually sought after as it increases the time for gas exchange and consequently increases the time to open and close the valve. Mechanically, acceleration ramps can be less aggressive, leading to lower forces throughout the valve train. Lower forces are beneficial by increasing durability and reducing mechanical friction, allowing more power to end up at the crankshaft.
Keeping things constant, a longer valve event duration at low rpm can lead to decreased performance as the fresh air/fuel mixture can"short circuit the cylinder and go straight out of the exhaust. A longer valve event duration at high rpm is advantageous as it exploits the inertia of the air/fuel mixture through the ram-air effect. This forces more of the fresh air/fuel mixture into the cylinder, increasing power output. Traditionally, this trade off of lumpy long duration cams, reduces bottom end torque in exchange for top end power.
Valve Event Phasing
Intake Valve Opening (IVO) usually occurs 10 to 25 before top dead center (TDC). It should open before TDC so that cylinder pressure does not dip early in the intake stroke. This timing is usually set to optimize full load output. Intake Valve Closing (IVC) is one of the principle factors that determine high rpm breathing. IVC usually occurs 40 to 60 after bottom dead center (BDC) in order to take advantage of ram-air effects. However, at low rpm where the charge has less inertia, IVC can affect torque output by allowing the piston to push air and fuel back into the inlet port.