How to Calculate Back Pressure in Exhaust Hoses

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How to Calculate Back Pressure in Exhaust Hoses
The size of back pressure is directly proportional to the length of the hose and inversely proportional to the internal diameter of the hose. (Auto Exhaust image by Digital Photique from

Reducing exhaust back pressure in an automobile can increase the engine's power and fuel economy. Turbocharged diesel engines, for example, can lose about half a percentage point in horsepower and fuel economy against a back pressure increase of one inch of mercury. The value of back pressure varies from one vehicle to another, and depends on several factors. You can use an equation to calculate back pressure (P) if you know the total length of the hose (L), gas flow rate (Q), internal diameter of hose (D), and exhaust temperature (T).

Skill level:

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  1. 1

    Define the parameters you will need. The formula you will use has four parameters: "L" in feet, "Q" in cubic feet per minute, "D" in inches and "T" in Fahrenheit.

  2. 2

    Simplify the calculation. Calculate 0.22 times "L" times "Q" times "Q" with the aid of a calculator to obtain a single value, which will be the numerator. Calculate "D" times "D" times "D" times the sum of 460 plus "T" with the aid of a calculator to obtain a single value, which will be the denominator.

  3. 3

    Perform the calculation. Calculate the numerator divided by the denominator with the aid of a calculator to obtain the back pressure value in inches of water.

Tips and warnings

  • 1.0 KPa = 4.0 inches of water = 0.294 inches of mercury = 0.144 pound per square inch.
  • For idle engines and on a few engines, your calculation can results in as high as 76.4 inches of water, but for most engines, 41.7 inches of water or less at idle is normal.
  • Your result should be about 83.3 inches of water for a normal operating engine. If you have a little higher than 83.3 inches of water, the performance is still good, but the reading should not be significantly higher.

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