SYNOPSISIn this paper, feasibility of reducing knock and thereby achieving higher compression ratio in a spark ignition engine has been studied by controlling cylinder head and cylinder block coolant temperatures independently with the aim of reducing the temperature of unburnt portion of mixture during flame propagation, while preventing adverse effects on the engine performance. The study was performed using a 1.3 litre OHV water cooled engine equipped with a dual circuit cooling system.In a fundamental study at the base compression ratio of 9:1, it was found that when the cylinder head coolant temperature was lowered while maintaining the cylinder block coolant temperature at 80°C engine output at WOT was increased significantly, because of considerable knock reduction and increased charging efficiency without increase in friction torque. These results indicated a great potentiality of the dual circuit cooling system for raising the compression ratio.Based on the results of the fundamental study, engine output at WOT and fuel economy at partial load were therefore evaluated at various compression ratios from 9:1 to 15:1 for various cylinder head coolant temperatures. It was found that the decreased engine output caused by the increased knock intensity, with raised compression ratio at the coolant temperature for both circuits of 80°C, was restored by lowering the cylinder head coolant temperature without deteriorating improved fuel economy at partial load.Given controlling temperatures of the cylinder head and cylinder block coolants at 50°C, probably the minimum for practical use, and 80°C respectively, and allowing that WOT engine output must be at least equal to that at the base compression ratio, it was concluded from the above investigations that maximum compression ratio attainable by the dual circuit cooling system was 12:1, when using regular gasoline of 91 RON. With these specifications, WOT engine output increased by about 10% at high engine speeds without any sacrifice of power at low engine speeds and fuel economy at partial load improved by about 5%.