We have been working closely with truck and engine manufacturers to develop mufflers to control diesel engine exhaust noise without decreasing performance or fuel economy. These mufflers have been tested at a technical center and in many cases by the truck manufacturer, The result is a select line of mufflers that cover all of the major truck engines and exhaust systems. These mufflers will enable most trucks to meet the 66 dB. A total vehicle noise level and many will meet the 83 dBA level. All of the noise level and backpressure information is compiled in the application and performance section. This will enable you to choose the best muffler for your attenuation and backpressure requirements.

In the past years noise control laws have been adopted and enforced by State and Local governments. The Federal Noise Control Act of 1972 was the first nation-wide regulation concerning truck noise, this regulation requires total truck noise levels, not to be greater than 86 dBA, measured at 50 feet for speeds less than 35 mph. This regulation went into effect on October 15, 1975. On January 1, 1978 this noise level was lowered to 83 dBA, The mufflers in our line will enable you to meet this lower noise level when properly applied.


1 . Fan Noise - Excessive fan speed is the biggest noise source in this area. This can be controlled by the use of a temperature controlled or thermostatic fan. Other noise concerns are bent fan blades and broken or missing shrouds,

2. Air Intake System - Both the type of system and the location of the inlet affect the truck noise level, Air inlets that open to the side are generally noisier than those that do not. Intake air silencers are available for problem applications,

3. Mechanical Noise - The engine and drive train are the major source of noise. Operating speed, type of engine and drive train all affect the total noise level. Shielding the exposed areas with acoustic barriers will reduce noise levels. Generally, any part of the engine or drive train that you can see while standing away from the truck will generate noise.

4. Tires - Many times this is the major noise source at high speeds. The condition of the tires and the tread pattern affect the noise level. At low speeds this is not a large noise source,

5. Exhaust System - Worn or inadequate exhaust systems are the largest and the most frequent contributor to high noise levels. A visual inspection and repair of leaking connections or holes in the system components will noticeably reduce noise levels, If the noise level is still too high, the addition of resonators, packed stacks or a change in mufflers is recommended. The muffler application and performance section of this catalog will give you the recommended mufflers for your particular engine and system configuration.


1. Worn or leaking flexible tubing is a common noise source. Replace the part if necessary. The service life of flexible tubing can be improved if it is installed in a relaxed position. Sending, stretching and compressing all reduce life because they limit the tubing's ability to absorb the vibration and thermal expansion.

2. The entire exhaust system should be well supported. This will reduce the noise generated by the exhaust pipes and the muffler shell. Care must be taken to isolate engine vibration away from the exhaust system and to provide for expansion when the system is hot.

3. The addition of a universal resonator in the system will generally reduce the exhaust noise level from 3 to 6 dBA. Packed stacks would reduce the noise level 3 to 4 dBA. Both items have a minimal effect on engine backpressure.

4. Change from a single to a dual system. For maximum benefit, different mufflers are usually recommended. Lower system backpressure is generally an additional benefit of dual systems.

5. On horizontal exhaust systems, the tail spouts should be pointed towards the center of the vehicle. This reduces the noise reflected off of the road surface.

6. On vertical systems, straight stacks will cause lower meter readings than curved stacks will. Straight stacks direct the noise upward, where as curved stacks direct the noise towards the meter.

The noise levels listed in the muffler application and performance section of the catalogue reflect the exhaust noise only, of the particular engine at rated RPM and full load.

Since engine exhaust is usually a major contributor to total vehicle noise, it is essential to reduce it significantly below the desired total vehicle noise level. The following guidelines indicate how low the exhaust noise levels need to be to meet total vehicle noise levels of 86 dBA and 83 dBA.


Excellent under 77dBA
Good 77 to 82 dBA
Marginal 82 to 86 dBa

NOTE: To reduce the vehicle noise level to 83 dBA, the categories on the chart must be reduced 4 dBA.