the rotor housing 14 in its tdc portion near bores 30 and 94 however it is this region through which extend the relatively tortuous passages 102 103 104 and 106 while passages 101 105 and 107 pass nearly straight through rotor housing 14 in areas of relatively lower heat flux if not otherwise compensated for this situation could result in lower coolant flow rates just in the areas where lower flow rates are not desirable accordingly the present cooling system includes restrictions in passages 101 105 and 107 this feature of the present cooling system can best be understood with reference to fig7 10 turning to fig7 10 the de housing 16 has a side 17 for sealingly engaging the de side 15 of rotor housing 14 taking into account fig5 and 7 10 it can be seen that passages 101 105 and 107 register with passages 801 and 905 and 1007 respectively in de housing 16 the cross sectional area of passages 801 905 and 1007 is reduced by lips 803 907 and 1009 which project inwardly with respect to passage wall portions 805 909 and 1011 respectively this in effect creates restrictions at the downstream ends of passages 101 105 and 107 in fig7 the lips.