Abstract: An exhaust manifold assembly for an internal combustion piston engine employs one exhaust tube for each cylinder and a collection chamber for each pair or larger group of exhaust tubes. Each collection chamber is connected to a plurality of exhaust tubes so that the exhaust timing spacing is the same in each collection chamber. A single exhaust reaction chamber is fed from pipes carrying the discharge from each collection chamber. The effect is to reduce backpressure and to improve scavenging efficiency while maintaining relatively high exhaust gas temperature for purification efficiency. The upstream end of each exhaust tube is supported and sealed within an engine exhaust port and the discharge end of each exhaust tube is slidably received within its collection chamber.