A multi-cylinder internal combustion engine of the type having an auxiliary combustion chamber associated with each main combustion chamber, and connected thereto by a torch opening, is provided with an improved manifold system and exhaust reaction chamber for maintaining the exhaust gases at a relatively high temperature in order to reduce unwanted emissions discharged into the atmosphere supplied to the auxiliary combustion chambers and also for heating the lean air-fuel mixture supplied by the primary carburetor barrel to the main combustion chambers. Thin wall liners having low thermal capacity are provided for adjacent pairs of exhaust ports in the engine, each liner having a single exit opening. Exhaust collection pipes receive exhaust gases from said liners and convey them to an inner exhaust reaction chamber and then in a one-way flow through an outer reaction chamber which surrounds and encloses the inner chamber. An improved heat exchange device employs the heat of gases in the outer chamber to heat a rich air-fuel mixture supplied to the auxiliary combustion chambers and also to heat a lean air-fuel mixture supplied to the main combustion chambers.