Two turbocompound diesel engines were assembled and dynamometer tested in preparation for vehicle tests. Both engines met the 1980 California gaseous emission requirement and achieved a minimum BSFC of .313 lb/bhp-hr and a BSFC at rated conditions of .323 lb/bhp-hr. These engines were then installed in Class VIII heavy-duty vehicles to determine the fuel consumption and performance characteristics. Fuel consumption testing showed a 14.8% improvement for the turbocompound engine in comparison to a production NTC-400 used as a baseline. The turbocompound engine also achieved lower noise levels, improved drive-ability, improved gradeability, and moderately increased engine retardation. The second turbocompound engine was placed in commercial service and accumulated 50,000 miles on a cross-country route without malfunction. Tank mileage revealed a 15.92% improvement over a production NTCC-400 which was operating on the same route.A number of component modifications were incorporated into the turbocompound engine which resulted in fuel consumption reductions which exceeded the expected benefit from turbo-compounding alone. Previous laboratory testing showed a 6% reduction in fuel consumption for a turbocompound engine over an equivalent NH engine operating along the torque curve. Using these test results, the incremental fuel consumption improvement strictly due to the turbocompounding alone was estimated at 4.2% to 5.3% depending upon the terrain or mission load factor.