Compressors require a substantial torque to get things rotating, and some smaller engines can get bogged down trying to start up a compressor even with all drains open. The 4 cfm Kidde needs about a 6 hp engine to make for a reliable start "in gear". However, with the clutch, the Kidde portable can use the low profile, light weight Briggs 5 hp. Those engines are not EPA friendly and no longer made. Lately, guys have been using the 5.5 Honda for most of the small machines and this is understandable. Right now, I'm working on a direct drive application which will use the 6 hp Kohler. With a six quart tank, this engine should run all day without topping up. Technical issues are several but one interesting problem is reversing the rotation of the Kidde block and this requires modifications to the oil pump, piping and fitting a CCW rotation fan.