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A 1.8 litre four-cylinder engine with a slice between the head and the block carrying instrumented plugs has been used to study the growth of combustion chamber deposits and some of their effects on engine operation. Different techniques for measuring deposit thickness, knock onset and deposit effects on the thermal characteristics of the cylinder have been developed. Deposit growth as measured by deposit weight on the plugs is reasonably repeatable from run to run and cylinder to cylinder. The presence of deposits already in the cylinder does not affect deposit growth on clean plugs introduced into the combustion chamber. Deposit thickness and morphology vary substantially at different locations, the thickness being greatest at the coolest surfaces. Deposits increase the flame speed and reduce the metal temperatures just below the surface. They also reduce the mean heat flux away from the cylinder.