This paper describes the development of noise control treatments for two heavy-duty diesel trucks. Noise levels are reduced to 72 dBA when measured according to the SAE J366b Recommended Practice. The treatments, comprising primary dual exhaust systems and partial engine enclosures, reduce exhaust back pressures by 0.3 to 0.4 in. Hg and add about 2 1/2 percent to the tractor weights. Cooling tests show that engine oil, engine coolant, and air-to-boil temperatures meet manufacturers' specifications. Maintenance studies indicate that about 5 min is required to remove and replace three key access panels.