58 in the outer wall 48 of the respective cylinder heads the insert is then retained in place by a spark plug 59 received in the threaded opening 58 spark plug 59 includes electrodes 60 extending within the ignition chamber 54 the cup insert 55 has a restricted orifice 62 connecting the ignition chamber with its respective main combustion chamber 35 an opening 63 is also provided in the wall of the insert member to permit the induction of a separately supplied air fuel mixture into the ignition chamber supply of ignitable air fuel mixtures to the auxiliary ignition chambers is provided for by means of auxiliary intake passages which comprise a laterally extending passage 64 in the intake manifold 31 which connects with the small throat 40 of the carburetor transversely extending passages 66 in the cylinder heads 26 27 generally longitudinally extending passages 67 in the exhaust manifolds and auxiliary intake port means 68 formed preferably by drilled passages in the cylinder head and connecting with the opening 63 in the ignition chamber wall passage 64 in the intake manifold extends in heat exchange relation with the crossover passage 52 in order to provide for heating of the intake mixture additional heating is provided for by the location of the passages 66 in the cylinder heads in heat exchange relation with the inwardly extending exhaust heat passages 51 as well as with the central exhaust ports 47 additionally the passages 67 in the exhaust manifolds extend at their inner ends in heat exchange relation with the exhaust passages 50 of the exhaust manifolds accordingly exhaust heat is applied to the air fuel mixture supplied to the auxiliary chambers under all engine operating conditions the amount of heat being.