Engines with a very small squish region, such as those with a wedge-style cylinder head, are prone to a knocking sound, especially when the coolant is at ambient temperature. The high level of CCD along with the minimal piston crown to cylinder head clearance at TDC in the squish or quench region causes mechanical contact between the two. This results in a knocking noise, which has come to be called carbon knock or carbon rap. It has nothing to do with detonation. The sound usually disappears after the engine has been warmed-up. Excessive carbon build-up causing this problem may often be mistaken for a failed rod bearing since it is rhythmic and may not occur in every bore.
The Cooling System
In an engine the radiator is used to remove heat from the coolant but the liquid has the job of cooling the engine and especially the cylinder head and combustion chamber. This fact is often not recognized since a temperature gauge reads the amount of heat in the liquid and not the surface temperature of the combustion chamber. The metal surface temperature is where detonation begins and needs to be addressed.
There are three reasons for the existence of a cooling system on an engine: to promote a high volumetric efficiency by limiting the amount of heat transfer into the charge air; to ensure normal combustion takes places in lieu of abnormal combustion known as detonation; for mechanical operation and reliability of the components and the complete engine.
High cylinder head metal surface temperatures, be it either local or general, can affect the performance of an engine. Excessive heating can lead to a loss of strength. As an example, aluminum alloys soften at temperatures over approximately 400 degrees F and the piston ring grooves can then deform by a phenomenon known as creep. Furthermore, if detonation occurs and is severe the piston can either melt or erode the top land region. Where the damage occurs on the piston usually is the hottest region and coincides with the area that the end-gas spontaneously ignited. The second concern is the top piston ring groove temperature must be limited to 400 F degrees if the oil is to do its job. Above this temperature the oil can degrade, leading to a loss of lubrication and clogging the rings and groove with decomposed oil. Finally, failure can result from thermal strain. This phenomenon is directly proportional to the temperature experienced over time. Failure is not likely from a single occurrence of overheating, but from repeated exposures. The regions most likely to suffer from thermal fatigue are those within the combustion chamber that have both a high temperature and a high gradient. A good example of this would be the valve bridge region, which is the area between the intake and exhaust valve seat.
Heat is transferred from the cylinder bore and the cylinder head walls to the liquid coolant in a number of convection or semi-convection phases. These phases are dependent on the rate of heat flow through the metal-per-unit area along with the temperature difference between the metal surface and the liquid coolant. As the coolant reaches the hottest part of the cylinder head, which is usually around the combustion chamber and exhaust valve, the coolant will actually start to boil. This phase change is identified as the nucleate boiling point and allows the efficient transfer of heat from the metal surface to the liquid coolant following Boyle's gas law. The coolant media's chemical and thermal reaction is responsible for how efficient this process becomes. When the coolant first comes in contact with the hot metal it will boil, changing phase, and then due to the pressure in the cooling system, the gas bubbles will be pushed from the localized boiling spot and carry with them the heat and recondense into a liquid again.
 The next step was to introduce...  The next step was to introduce the Run-Rite system. The applicator connects to a vacuum hose (the PCV line on a Buick GN works well) and hangs from the hood latch. The tool has a control valve to adjust the flow rate and the chemical is introduced at a slow drip, at a fast idle. |  Anthony checked all of the...  Anthony checked all of the intake manifold bolts to make sure there were no air leaks. All were nice and tight. |  We did not do this but a good...  We did not do this but a good precursor to the Run-Rite system is to pour GM Top Engine Cleaner on the piston top through the spark plug holes and let it sit overnight to loosen up very hard deposits. |