The reason for an intake opening point after top dead center and some time after the exhaust valve closes is because of exhaust backpressure.
With an appropriately sized turbine wheel, a street car will have a considerable amount of exhaust backpressure, sometimes in excess of twice the intake pressure. When this is the case, it is undesirable to open the intake valve when the exhaust valve is still open, since exhaust pressure will push back into the intake manifold (reversion). I see this all the time on high backpressure combinations; the intake manifold will be sooty black, sometimes all the way up to the throttle blade! Opening the intake valve right after the exhaust has closed is still counterproductive, because exhaust gases are trapped in the combusion chamber, and the combustion chamber pressure is still greater than intake pressure. So the opening point of the intake valve is delayed until sometime after top dead center; as the piston starts down the hole and the intake valve is closed, pressure in the cylinder diminishes. When the pressure in the intake and the pressure in the combustion chamber equalize, the intake valve can be opened with little or no negative impact on the rpm range of the engine, and a much more powerful engine all the way beyond peak power.
TQ
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