Abstract: A coolant temperature control system of an internal combustion engine is composed of a thermostat disposed in a coolant passageway leading to a radiator. The thermostat is operatively connected to a diaphragm actuator so that valve opening temperature of a thermostat valve is controlled in response to engine loads. Additionally, the thermostat valve is arranged to be opened in response to the discharge pressure of a coolant pump, thereby lowering the opening temperature of the thermostat valve at a high speed engine operating range.

Abstract: A reed type valve is operatively secured on the outer surface of the casing of an engine air filter assembly to supply the exhaust system of an engine with secondary air. The reed valve is fluidly connected to an air induction passage formed inside the casing of the air filter assembly. A resonance chamber is also formed inside of the casing and communicates through a communication passage with the air induction passage so as to attenuate a relatively low frequency component of exhaust noise transmitted through the reed valve. The air induction passage, resonance chamber and communication passage are formed as follows: ##EQU1## where, S.sub.o (cm.sup.2)=the cross-sectional area of the air induction passageS(cm.sup.2)=the cross-sectional area of the communication passagel(cm)=the length of the communication passageV(cm.sup.3)=the volume of the resonance chamberM=the number of the engine cylinders of the engine.

Abstract: A reed type valve is operatively secured on the outer surface of the casing of an engine air filter assembly to supply the exhaust system of an engine with secondary air. The reed valve is fluidly connected to an air induction passage formed interior of the casing of the air filter assembly. A resonance chamber is also formed interior of the casing and communicated through a communication passage with the air induction passage so as to attenuate a relatively low frequency component of exhaust noise transmitted through the reed valve. The air induction passage, resonance chamber and communication passage are formed as follows: ##EQU1## where, S.sub.o (cm.sup.2)=the cross-sectional area of the air induction passageS(cm.sup.2)=the cross-sectional area of the communication passagel(cm)=the length of the communication passageV(cm.sup.3)=the volume of the resonance chamberM=the number of the engine cylinders of the engine.

Abstract: An internal combustion engine is disclosed in which each pair of cylinders has a Siamese exhaust port. Each of the exhaust ports is designed so that the ratio of a surface area (F) exposed to the exhaust gas discharged from one of the cylinders to the minimum cross-sectional area (S) through which the exhaust gas flows (F/S) ranges from 3 to 6.