Performance Automotive Engine Math by John Baechtel

Author:John Baechtel [Baechtel, John]
Language: eng
Format: epub
ISBN: 978-1-61325-021-1
Publisher: CarTech Inc.
Published: 2011-01-15T00:00:00+00:00

This cutaway view shows the valve curtain area so you can visualize the flow window of the open valve.

Calculating Optimum Port Area from Valve Size

The primary objective of all performance cylinder heads is to produce the maximum possible volumetric efficiency across the broadest possible range of engine speed. That’s why port and valve sizes are so critical and so easy to mess up without careful deliberation. The intake port cross-sectional area (c/s) describes the smallest area of a port in a perpendicular plane to the flow upstream of the valve.

There are two schools of thought on this. Depending on the cylinder head, the smallest cross section may actually be the venturi diameter or throat area directly above the valve seat. This is particularly true if you also consider the additional obstruction of the valve guide and valve stem. Others define cross-sectional area as the choke point farther upstream near the bump in the port wall adjacent to the pushrod. To determine this you measure the vertical and horizontal dimensions at that point and multiply to find the area.

Port c/s Area = height × width

To find the area of the valve throat (venturi) simply measure the diameter of the throat opening above the valve seat and calculate the area as follows:

Throat Area = Pi × radius2

Throat Area = diameter2 × 0.7854

Head porters contend that the upstream cross-sectional area (in the port itself) should be 90 percent of the flow diameter of the intake valve for a race engine and 0.85 percent for a street engine. Some feel that 90 percent is good across the board. This is based on the valve’s flow diameter at the inner edge of the valve seat. It’s a reasonable assumption although the throat diameter directly above the valve seat may be even smaller and that is what the air actually sees. And it doesn’t account for the partial blockage caused by the valve guide and stem. For now however, we are simply relating port cross-sectional area in the port itself to the flow diameter at the valve seat.

For example, a 2.02-inch intake valve has a flow diameter of 1.717 inches if we’re going by the 85-percent rule. To calculate the equivalent port cross-sectional area, use the following formula.

Flow Diameter for Street Engine = valve diameter × 0.85

2.05 × 0.85 = 1.7425 inches

Port Area = (flow diameter2 ÷ 4) × 3.1417

(1.74252 ÷ 4) × 3.1417 = 2.38 square inches

At 85-percent flow diameter ratio, the flow diameter is calling for a minimum cross-sectional area of 2.38 square inches in the port. To keep that in perspective, note that a 195-cc Air Flow Research cylinder head for a small-block Chevy has a 2.02-inch intake valve and a 2.21-square-inch cross-sectional area in the port. This is smaller than our calculation, but close. If we were to use the 90-percent rule it calls for a port c/s of 2.59 square inches, which is even larger.

It can be surmised that they’re keeping it tight to preserve port velocity and they may also be considering the restriction of the valve stem.

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