An engine's exhaust silencer must suit that particular engine's noise requirements. It must also suit the rate of exhaust flow. Too small a silencer will insufficiently muffle an engine's exhaust pipe. Too large a silencer will magnify the engine's back pressure, hurting engine performance. Determine your silencer's size from the gas's flow area. A number of factors affect the flow area, including the gas's temperature, the back pressure, the gas flow rate and the target silencer pressure drop coefficient.
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Square the engine's gas flow rate, measured in cubic feet per minute. If, for instance, 3,000 cubic feet of gas flow per minute: 3,000 ^ 2 = 9,000,000.
Multiply your answer by the pressure drop coefficient. If you engine has a pressure drop coefficient of 4.2: 90,000 * 4.2 = 37,800,000.
Multiply by 530: 378,000 * 530 = 20,034,000,000.
Add 460 to the exhaust gas's temperature, in degrees Fahrenheit. If the gas has a temperature of 593 degrees C: 1,100 + 460 = 1,560.
Multiply by the engine's back pressure, measured in inches of water. If the back pressure is 20 inches of water: 1,560 * 20 = 31,200.
Multiply your answer by 16,040,025: 31,200 * 16,040,025 = 500,448,780,000.
Divide 20,034,000,000 by 500,448,780,000 = 0.04.
Find the square root of your answer: 0.04 ^ 0.5 = 0.2. This answer is your flow area, measured in square feet.
Divide your answer by pi: 0.2 / pi = 0.064.
Find the square root of your answer: 0.064 ^ 0.5 = 0.253.
Multiply your answer by 2. 0.252 * 2 = 0.506. This answer is your silencer's diameter, measured in inches.
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