A five-cylinder variable displacement research engine has been designed, built, and tested for over one year. Displacement is varied by changing the piston stroke using a four-bar linkage. By proper choice of the linkage dimensions, compression ratio can be held constant as the stroke changes, or it can be tailored to optimize economy and performance. Before the engine was built, its dynamics were thoroughly analyzed, and a five-cylinder configuration was selected to minimize vibration. Test data show that BSFC is, as expected, less dependent on load than in conventional engines, and averages less than 0.5 lb/bhp-hr over the load and speed range of ordinary automobile operation. Several methods for estimating urban fuel economy from dynamometer data predict a significant improvement compared with conventional engines. Emission controls have not yet been applied to this engine.