You can optimize the short-side turn of a cylinder’s intake port by expanding the sides of the port. This is necessary in order to address both of the aspects in order to make the turn more effectively and to compensate for the valve guide boss and valve stem which uses some of the available cross sectional area. A well streamlined valve guide boss can enhance results especially swirl rather than hinder it. Expanding the cylinder head walls helps to accomplish the filling of a cylinder when the port and valve is feeding a pair of intake valves in a multi-valve head.
When cylinder head modifications are limited to removing metal dealing with the short-side turn means making the most of whatever is already there. Most production heads have a more abrupt turn than is necessary due to the result of machining the valve throat below the seat. Rounding this off is the best possible solution to what can be done to improve the form of the short-side turn, once the smoothing out of the contours in the valve throat have been completed.
The best way to get the air to move to the back of the valve is to slow it down so that it can make that turn, expanding the intake port’s wall area creates a significant change. When the port is progressively widened and the intake port’s roof is raised in the turn area the slowing of the air just before it reaches the valve can create some substantial HP and Torque gains.
The majority of the air wants to flow in the top half of the intake port, so that area should be favored when removing metal. The increase in cross-sectional area in the valve’s throat area will also create an improvement by converting some of the high velocity into pressure energy, thus intensifying the air / fuel mixture charge into the cylinder.