N38143 Chapter 2 Exhaust systems The exhaust system should conduct exhaust gases from the engine to the atmosphere with acceptable back pressure at the same time reducing exhaust noise to the minimum, avoiding gas leaks and excessive surface temperatures while accommodating engine movement on flexible mounts. There are two types of systems used, wet and dry. Caution:
In all types of exhaust system, when measured within 305mm (12 inches) of the exhaust outlet from the engine, the exhaust back pressure must not exceed the amount stated in the relevant data section at the back of this manual Wet Systems Wet exhaust systems, where the raw water used to circulate through the heat exchangers on the engine is finally injected into the exhaust pipe to cool the exhaust gases, are the most common choice for small craft. Their principal advantage is that a rubber exhaust hose may be used, with a fairly low surface temperature, which presents no risk of fire. A general arrangement for such a system is shown on (A).
In many cases the exhaust outlet passes through the transom, just above the waterline (A1). It will be seen that a minimum fall of 10 degrees is required (A2), and that the point of water injection must be at least 200mm (8 inches) above the waterline (A3), although the actual height necessary for a particular boat can only be decided in the light of the exhaust system design, and the pitch and roll which may be encountered in service. Caution: It is essential that the exhaust system is designed so that water from the exhaust does not enter the engine under any conceivable operational condition.
Page Exhaust pipes Engine model Exhaust bore Engine mode Exhaust bore 415GM 50mm (2.0ins) 4.4GM 76mm (3.0 ins) 422GM 4.4TGM 422TGM 4.4TWGM 100mm (4.0ins) 700GM 63.5mm (2.5ins) 4.4TW2GM 4GM 76mm (3ins) 6TG2AM 4TGM 6TWGM 125mm (5ins)