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How to Calculate an Intake Runner

An engines induction system can be designed to improve the location of peak torque within its RPM range by altering the intake manifolds plenum volume, runner length and runner diameter. Long and narrow intake runners will result in lower RPM ratings, while short and wide runners will result in higher RPM ratings. Determining the RPM rating at which you want peak torque to occur is up to you, but once you do decide, calculating the intake runner size is straightforward.

Instructions

Calculating Intake Runner Diameter

1

Decide the RPM rating at which you want peak torque to occur. For example, youd like your cars engine to reach 5,000 RPM at peak torque.

2

Multiply the RPM rating from Step 1 by the engines displacement volume and volumetric efficiency. For example, the cars engine has a displacement of 1.5 liters and a volumetric efficiency of 95 percent. Multiplying 5,000 by 1.5 by 0.95 equals 7,125.

3

Divide the result from Step 2 by 3,330. In the example, 7,125 divided by 3,330 equals 2.140.

4

Calculate the square root of the result from Step 3 to determine the intake runners diameter. In the example, the square root of 2.140 is 1.463. The intake runners diameter should be 1.463 inches.

Calculating Intake Runner Length

5

Subtract the result from Step 1, Section 1 from 10,000. In the example, subtracting 5,000 from 10,000 equals 5,000.

6

Divide the result from Step 1 by 1,000. In the example, dividing 5,000 by 1,000 equals 5.

7

Multiply the result from Step 2 by 4.3. In the example, multiplying 5 by 4.3 equals 21.5.

8

Add 17.8 to the result from Step 3 to calculate the intake runner length in centimeters. In the example, adding 17.8 to 21.5 equals 39.3 cm.

9

Multiply the result from Step 4 by 0.3937 to convert the intake runner length to inches. In the example, multiplying 39.3 by 0.3937 equals 15.472. The intake runner should be 15.472 inches long.