2000/11/01 Caterpillar

Truck Engines:

All

Reference: Truck Application and Installation Guide, LEBT5109-01, June 98, "Exhaust Backpressure Calculations" page 24

Exhaust back pressure can vary and possibly exceed the maximum allowed limit of 9.95 kPa (40 inch of H₂O) under rated conditions depending on the location of the pressure tap. The pressure tap may be located in a straight length of exhaust pipe or in the turbocharger outlet elbow.

The following test procedure is from page 24 of the referenced publication:

"Exhaust backpressure is measured as the engine is operating under rated conditions. Either a water manometer or a gauge measuring inches of water can be used. If not equipped, install a pressure tap on a straight length of exhaust pipe. This tap should be located as close as possible to the turbocharger or exhaust manifold on a naturally aspirated engine, but at least 12 in. downstream of a bend. If an uninterrupted straight length of at least 18 in. is not available (12 in. preceding and 6 in. following the tap), take care to locate the probe as close as possible to the neutral axis of the exhaust gas flow. For example, a measurement taken on the outside of a 90 degree bend at the pipe surface will be higher than a similar measurement taken on the inside of the pipe bend. The pressure tap can be made by using a 1/8 NPT half coupling welded or brazed to the desired location on the exhaust pipe. After the coupling is attached, drill a .12 in. diameter hole through the exhaust pipe wall. If possible, remove burrs on the inside of the pipe so that the gas flow is not disturbed. The gauge or gauge hose can then be attached to the half coupling."

Illustration 1	g00754245

If the pyrometer port on the turbocharger outlet elbow is used to measure exhaust back pressure, this will cause a high back pressure reading. The lowest back pressure reading would be obtained if the pressure tap was located on the inside radius of the elbow. The pressure tap at the neutral axis of the turbocharger outlet elbow, which is located near the center of the bend, provides a reasonably accurate measurement of the exhaust back pressure. Refer to Illustration 1. In this example, the reading taken from the pyrometer port leads the technician to believe that the exhaust back pressure is 8.5 kPa (34 inch of H₂O) when the actual back pressure is only 6.5 kPa (26 inch of H₂O) when the measurement is taken at the neutral axis of the elbow.

If the exhaust back pressure exceeds the maximum allowed limit of 9.95 kPa (40 inch of H₂O) with the pressure tap located either in a straight length of pipe or at the turbocharger elbow neutral axis, perform the following procedure:

1. Inspect the exhaust system for damaged or plugged piping and rusty or broken muffler baffles.

2. Make sure that the configuration of the exhaust system (muffler and piping) is suitable for the engine rating.

3. An engine uprate may require a less restrictive muffler and/or exhaust piping and a recheck of noise emissions (Title 49 CFR, Part 325.7).

If you are unsure about Steps 2 or 3 above, please contact your local OEM dealer.