A Cummins B55 in3 350 bhp heavy-duty, turbocharged diesel engine was tested in fully cooled and uncooled modes over the EPA transient emission test cycles for comparison of gaseous and particulate emissions. The results are presented at the same fuel injection timing and at similar NOx emission levels. Also, steady state emission measurements and analysis of real-time transient emission data of selected runs are discussed. The uncooled engine does not represent an adiabatic (insulated) engine in its emission characateristics, but may indicate some trends. It may be useful in identifying design and/or operating parameters that need optimization.