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 a manifold system in which the exhaust gases are used to heat a rich air-fuel mixture supplied to the auxiliary combustion chambers and is also used to heat the lean air-fuel mixture supplied by the primary carburetor barrel to the main combustion chambers, but the lean air-fuel mixture supplied by the secondary carburetor barrel is not heated. The secondary carburetor barrel comes into action only when the engine is operating under heavy load, so that the unheated additional lean mixture improves the volumetric efficiency and thereby improves the power output of the engine. Thin wall liners and heat transfer plates having low thermal capacity are provided for the exhaust passages in the engine, the interior of the exhaust manifold, and the heat exchange surfaces for heating the rich mixture and that portion of the lean mixture provided by the primary carburetor barrel.
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