Intake Port Size.
The factors to consider when modifying an intake port is the size of the intake port area that will be required to make power at the projected engine rpm, and that the intake port area is suited to the size of the valve in the cylinder head. It is better to have an intake port that is too small than one that too big, due to the fact that the performance of an intake port will drop off quicker on a intake port that is too big, than one that is too small.
The initial step towards modifying the size of the intake port area is to make an approximation of the intake port's parallel section that is upstream of the expanded intake valve’s bowl area. The size of this section should be about the same area of the valve, and is multiplied by the intake valve's maximum lift flow efficiency. This is dependant on how tight the intake port’s turn is into the intake valve’s bowl and seat area. The parallel part of the intake port does not see a certain intake valve diameter but instead experiences a flow value that is limited by the intake valve.
The lower the flow value of the intake valve and the final section of the intake port, the smaller the parallel section needs to be. By controlling the parallel section size the intake port’s ramming capability is maximized without compromising the overall intake port flow.
The action between the parallel section of the intake port and the intake valve’s throat is very significant. When you expand the intake port to form the intake valve’s throat, two situations are accomplished. First the air is slowed so as to give it more time to make it around the short-side turn of the intake port. Secondly, slowing the air / fuel mixture at the intake valve throat entrance will convert some of the air/ fuel mixture’s kinetic energy into pressure energy. This is very important when the piston reaches bottom dead center (BDC) on its induction stroke. Increasing the pressure next to the intake valve means that this additional pressure will push more of the air / fuel into the cylinder even though the piston is moving up on the start of its compression stroke. The more kinetic energy available to accomplish this, the later the intake valve can be closed. Pressure energy can develop as much as 7 psi above atmospheric pressure just before the intake valve closes.
In conclusion do not remove metal from an intake port unless it produces a flow gain within the intake valve’s lift range that is going be used.