Abstract: An internal combustion engine has a cylinder with a porting ring adjacent the cylinder head to reduce the diameter of the cylinder. The piston in the cylinder has a reduced diameter crown portion complementary to the porting ring and an upper face on the crown provided with a small clearance from the cylinder head to provide a large squish area. A primary combustion chamber of "dogbone" shape is provided in the crown, while a secondary combustion chamber is provided under the crown. On the power stroke, the sparking plug ignites the air/fuel mixture in the primary combustion chamber and the pressure pulse generated pushes the piston down the cylinder. When the piston crown clears the porting ring, the air/fuel mixture in the secondary combustion chamber is ignited and a second pressure pulse is applied to the piston. The slower, staged, burning of the air/fuel mixture in the cylinder applies a more powerful driving force to the piston, generating increased torque.