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. There is a good correlation between this mean heat flux and knock-limited spark advance. Deposits in the cylinder head also led to significant reductions in fuel consumption at the operating condition used for deposit build-up. In contrast piston-top deposits have little effect on fuel consumption and on octane requirement increase.DEPOSITS, derived primarily from the fuel but with some contribution from the oil are formed inside the combustion chamber of a spark ignition engine with use. The literature on the deposits that form on all the internal surfaces of a spark ignition engine, including those in the combustion chamber, was reviewed in (1); an earlier source of useful information is (2). Combustion chamber deposits (CCD) have been reported to cause an increase in octane requirement and, to some extent, an increase in emissions, a reduction in volumetric efficiency and power and an improvement in fuel economy - a beneficial effect (1-11). More recently, new field problems like carbon rap (e.g.3,12) attributed to CCD have been recognised. Moreover, fuel and additives technology to ensure satisfactory control of deposits in other parts of the engine viz carburetters, port fuel injectors and inlet valves has matured and is being increasingly mandated. There has been growing concern that some of this additives technology might cause an increase in CCD. The spurt in interest in CCD is reflected in industry initiatives like the 1993 CRC Workshop on Combustion Chamber Deposits (3).This paper describes an experimental study of combustion chamber deposits and some of their effects in a four cylinder engine modified and instrumented for research. Deposit growth was monitored by measuring the weight and local deposit thicknesses on removable plugs in a slice carried between the head and the block. Deposit growth was reasonably repeatable from cylinder to cylinder and test to test. Knock rating was done on individual cylinders by determining the Knock Limited Spark Advance (KLSA) at the trace knock level. Techniques to measure deposit thickness and to detect knock were developed after assessing many options. Flame progress was monitored by ion-gaps in the head. The effect of deposits on heat transfer from the combustion chamber was monitored by using a plug with thermocouples embedded at different distances from the surface particularly during engine warm-up. Specific fuel consumption was also monitored and, for the operating condition used, decreased as deposits built up. We also assessed the effect of deposits from different parts of the combustion chamber by removing them selectively.