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| THE READ FOR SPEED by Ned Erkman |
| CHAPTER 3 |
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Port Shape versus Port Volume
The shape of the intake and exhaust ports is more important than size. It is the shape of the intake and exhaust ports and the shape of the combustion chamber that helps to flow air, fuel and exhaust around the valves.
It is often believed that just making intake and exhaust port passages larger is the method to use for increasing airflow. This is simplistic thinking, as bigger is not always better. In this case, the correct shape is more important than size. Making an intake or exhaust port bigger, just to increase the size of the flow path does not always contribute to moving more air around the valves and into, or out of, the cylinder. Increasing port volume by removing material can actually decrease airflow and efficiency when material is removed from the wrong area, thereby creating an inefficient flow path. A properly sized port, with the right shape to promote efficient airflow around the valve, is best.
The relationship of port shape and increasing airflow around the valve, dictates that the more valve area used to flow air, the better the results of cylinder filling. This means that a port shape must be configured to flow more air around the entire diameter of the valve. This is why shaping a port to flow in a specific direction is more important than simply removing material to increase port size. A larger port configuration does not necessarily help to direct airflow around the valve, but a port designed to move air around the entire radius of the valve can flow more air and thereby increase power and performance.
Air speed is another item that requires consideration. The bigger the port, or more specifically, the wider the port, the slower the air moves. To illustrate this principle, consider what happens when you open a window or door. When either is opened by one or two inches wide, a noisy draft usually occurs. However, when they are fully opened, the noise generally disappears. The noise of the draft, generated by the narrower opening, is the sound of air rushing through, or more appropriately, the airspeed. The louder the noise, the faster the air is moving, and conversely, the quieter the sound, the slower the air speed. A narrower open area creates a faster movement of air, and thereby increases air speed.
Cylinder heads with proportionally larger intake and exhaust port sizes can reduce air speed, resulting in a lower volume of air entering the cylinder before the valve closes. It is the opening and closing process of the valves that requires faster, high velocity airspeed to complete the cylinder filling process before the valves close.
The key to obtaining superior performance is to have the best port volume with the best airspeed, and the best camshaft valve timing events for a specific engine combination.
Ned Erkman
Author of The Read for Speed
Copyright © 2005 The Read For Speed. All Rights Reserved.
Reproduction in whole or in part without expressed written permission from the author
is strictly prohibited.
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