Thread: port verlosity
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Rex Walter Rex Walter is offline
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Join Date: May 2005
Location: Bothell, WA
Posts: 239

Technically speaking, it is called Bernoulliís Equation, which says that an area (A) multiplied by the velocity (V) in one size pipe is equal to the A x V in another size pipe, if a constant pressure is applied to both. For a given engine displacement (3.5 L in your engine) each cylinder will intake a given amount of air (constant pressure). Looking at the intake ports, you are increasing the area from 32 mm diameter to 38 mm diameter. Letís say that with the original 32 mm intake the air flows at a rate of 30 meters per second at 2000 RPM. So you can multiply the area (32/2 squared times PI = 0.08 square meters) times the velocity which we assumed to be 30 m/sec at 2000 RPM, we get 2.4 cubic meters per second of airflow. Now if you increase the size of the intake port to 38 mm, Bernoulli says that A32xV32 = A38xV38. In our example A38xV38 must equal the 2.4 cubic meters per second of airflow. The area of the 38mm port is 0.1134 sq m, so the airflow velocity in the bigger port must be 21 meters per second. So, at 2000 RPM, your intake airflow speed has dropped from 30 m/sec in the original 32mm port, to 21 m/sec in the 38 mm port, or your bigger ports flow speed is only 71 % of the smaller ports.

Not so technically speaking, if you turned on the garden hose, without a spray nozzle, the water would just flow out the end of the hose. Now if you put your thumb over the end, the water would spray out at a much greater speed, because the same amount of water is trying to get out of a much smaller opening, so it must go through the smaller opening faster.

This is probably way too complicated for a Sunday morning, but I hope it helps.

1975 911s
1997 Saab 900s

Last edited by Rex Walter; 10-30-2005 at 07:30 AM..
Old 10-30-2005, 06:50 AM
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