K066838 | Cat® BA-921 Air Compressor For CAT Engines

Peterbilt models equipped with compatible Caterpillar C15 or C18 engines
Freightliner trucks using the same Bendix air compressor
Autocar trucks with compatible Caterpillar engines
Other heavy-duty trucks using a Bendix BA-921/BA-923

1. Increase engine power

A turbo compresses the incoming air, allowing the engine to burn more fuel efficiently. More air + more fuel = more power.

For example:

• A naturally aspirated engine might produce 300 horsepower.
• The same engine with a turbocharger could produce 400–500+ horsepower, depending on its design.

2. Increase torque

Turbochargers significantly improve low- and mid-range torque, which is especially important for heavy-duty trucks.

More torque helps vehicles:

• Pull heavy trailers.
• Climb steep grades.
• Accelerate with heavy loads.
• Maintain highway speeds more easily.

3. Improve fuel efficiency

Because a turbo captures energy from the exhaust that would otherwise be lost, it helps the engine produce more power without requiring a larger displacement. This can improve fuel economy when the engine and vehicle are properly designed and operated.

4. Reduce emissions

By supplying more oxygen for combustion, a turbo can help fuel burn more completely. Combined with modern engine controls and emissions systems, this contributes to lower emissions.

5. Maintain performance at high altitude

At higher elevations, the air is thinner. A turbocharger compresses the intake air, helping the engine retain much more of its performance than a naturally aspirated engine.

6. Allow smaller engines to do more work

Many manufacturers use turbochargers to get the performance of a larger engine from a smaller one, reducing weight and improving efficiency while maintaining power.

1. Loss of power

This is the most noticeable symptom.

You may experience:

• Slow acceleration
• Difficulty climbing hills
• Reduced towing performance
• Lower than normal boost pressure

Possible causes

• Worn turbo bearings
• Damaged compressor or turbine wheel
• Boost leaks
• Sticking wastegate or variable geometry mechanism

2. Excessive exhaust smoke

The smoke color can provide clues:

• Blue smoke: Engine oil is leaking through the turbo seals and burning.
• Black smoke: The engine is receiving too much fuel or not enough air, often due to low boost.
• White smoke: Less common, but can occur with severe turbo or engine problems.

3. Whining, siren, or grinding noises

Healthy turbos produce a faint whistle under load.

Warning noises include:

• Loud whining
• Siren-like sound
• Metallic scraping
• Grinding
• Rattling

These may indicate:

• Worn bearings
• Compressor wheel contacting the housing
• Damaged turbine blades
• Foreign object damage

4. Oil leaks

A bad turbo may leak oil into:

• The intake system
• The intercooler
• The exhaust system

You may notice:

• Oil around turbo hoses
• Oil inside the charge-air pipes
• Increased oil consumption

A light oil film is often normal, but pooled oil is not.

5. Check Engine Light

Modern engines monitor turbo performance.

Common issues include:

• Low boost
• Overboost
• Turbo actuator faults
• Boost pressure sensor faults

Typical diagnostic trouble codes may include:

• P0299 (Underboost)
• P0234 (Overboost)

6. Poor fuel economy

If the turbo isn't producing proper boost:

• The engine works harder.
• More fuel is consumed for the same performance.
• Fuel economy decreases.

7. Increased oil consumption

A failing turbo can allow oil to enter the intake or exhaust, causing:

• Frequent need to add oil
• Blue smoke
• Oil residue in the intercooler piping

8. Boost pressure problems

Using a scan tool or boost gauge, you may observe:

• Low boost under acceleration
• Unstable boost
• Delayed boost response

9. Turbocharger not spooling

The turbo may fail to build boost because of:

• Broken shaft
• Seized bearings
• Stuck wastegate
• Failed electronic actuator (on electronically controlled turbos)
• Exhaust leaks before the turbo

10. Catastrophic turbo failure

If the turbo fails completely, you might experience:

• Sudden loss of power
• Loud bang or metallic noise
• Heavy smoke
• Significant oil loss

In severe cases, oil from the turbo can enter the engine intake. On diesel engines, this can lead to diesel engine runaway, where the engine burns its own oil as fuel and may continue accelerating uncontrollably until the air supply is cut off or the engine fails.