A fixed-geometry configuration of Pinnacle Engines' 4-stroke spark-ignited 250cc single-cylinder opposed-piston reciprocating sleeve-valve engine is presented. A test cell has been developed with a custom crank-angle based data acquisition system to allow friction testing and indicated cycle development. Incipient knock criteria are defined and test results are presented to compare the opposed-piston's indicated cycle performance to conventional single-cylinder engines. At high loads, the Pinnacle opposed-piston engine utilizes the Cleeves cycle, an operating mode in which spark timing is highly delayed from best power spark advance, to enable knock-free operation at an elevated 15:1 compression ratio using 87 octane fuel. Data is presented for sleeve-valve operating temperatures, peak cylinder pressure effects, and exhaust gas temperatures. LogP-logV plots at key drive-cycle operating conditions highlight indicated cycle advantages of the opposed-piston sleeve-valve engine over two configurations of a comparison poppet-valve engine. Data is presented to show that extended lean operating limits enable reduced drive-cycle emissions in the target vehicle. Indicated cycle performance using gaseous CNG/methane is also presented. The engine has been developed such that, with the most basic configuration, light-load indicated efficiency improvements of 15-30% can be realized over conventional poppet-valve technology.