In the late 1920s, International Harvester already had the model M engine, which was an enclosed version of a flywheel engine. Their next step was the model LA, which was a totally enclosed engine (except for the valve system) featuring self-lubrication (oil in the crankcase), reliable spark plug ignition, faster-speed operation (up to about 750-800 RPM) and most of all, light in weight compared to earlier generations. While the 1½ HP (1.1 kW) model LA still weighed about 150 pounds (68 kg), it was far lighter than the model M 1½ HP engine, which is in the 300-350 pound (136 – 159 kg) range. Later a slightly improved LA, the LB was produced. The models M, LA and LB are throttle governed. As time passed, more engine manufacturers moved to the enclosed crankcase engine. Companies like Briggs and Stratton were also producing lightweight air-cooled engines in the 1/2 to 2 HP (.37 - 1.5 kW) range and used much lighter-weight materials. These engines also run at much higher speeds (up to approximately 2,000-4,000 RPM) and therefore produce more power for a given size than slow flywheel engines.