An internal-combustion spark-ignition V-8 piston engine has a main exhaust gas reaction chamber positioned between the two banks of cylinders. Each cylinder has a main combustion chamber and an auxiliary combustion chamber connected by a torch nozzle restriction. Valved intake passages supply lean mixture to the main combustion chambers and rich mixture to the auxiliary combustion chambers. A spark plug is associated with each auxiliary combustion chamber. Valved exhaust passages supply hot exhaust gases to auxiliary exhaust gas reaction chambers, each of which projects through an insulated wall of the main exhaust gas reaction chamber, and discharges into it. Each of the auxiliary reaction chambers is substantially smaller in volume than the main reaction chamber and helps to minimize unwanted pollutants such as unburned hydrocarbons and carbon monoxide in the exhaust gases by maintaining the exhaust gases at relatively high temperature and for relatively long time periods before discharge into the atmosphere, and over a wide range of engine speeds and loads. A separate air pump is not required.