The results of a comprehensive test program using a 1969 383-CID V-8 engine at two compression ratios-9.5:1 and 7.6:1-are reported. Compression ratio changes were effected by piston changes only. Except for necessary ignition timing modifications, no other changes were made in the engine. The effects of compression ratio changes on exhaust emissions and fuel consumption were studied in steady-state dynamometer tests and in vehicle tests.At MBT ignition timing or at the same percentage power loss from MBT timing at each compression ratio and with identical carburetion, decreasing the compression ratio from 9.5:1 to 7.6:1 produced the following results: 1. In steady-state dynamometer tests, NO (ppm) and CO (%) emissions were unchanged, HCs (ppm) were decreased, and fuel consumption was increased when equal power was developed at both compression ratios. 2. In vehicle tests using the 7-mode Federal Test Procedure, NO and CO emissions were unchanged and HCs increased somewhat. Additional vehicle tests showed 6% higher road-load fuel consumption and 6% longer full-throttle acceleration times for the lower compression ratio. Thus, tests in this engine show that decreasing compression ratio cannot be justified on the basis of reducing NO emissions.