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Why does retarded timing = hot engine?

Zira

Moderator
Staff member
Last Subscription Date
03/21/2020
On a throttle governed engine, we all know that running the spark retarded causes the engine run hotter. Can anyone explain the reasons behind this? It seems like the charge would have less time to burn and to transfer heat to the cylinder walls & head.

Just curious.
 

Andrew Mackey

Moderator
Last Subscription Date
05/14/2017
When the engine fires normally, the fire burns completely (or nearly so) within the combustion chamber. The heat is kept within the combustion chamber, and as the pressure drops during the power stroke, the superheated gasses cool. As the power stroke occurs, the cylinder absorbs the heat, and when the exhaust valve opens, the burned cool mixture is blown out of the cylinder and combustion chamber.

When timing is retarded, peak pressure and heat is delayed, and is not confined to the combustion chamber. Delayed ignition causes the peak heat and pressure to occur in the cylinder body itself. The flame actually burns the lubricating oil off the cylinder walls, causing more friction. As the combusting mixture is not under high compression, it is less dense, again causing further delays in the burn. When the exhaust valve opens, extremely hot, still burning gasses are blown out the exhaust port. past the valve. This flame heats the valve red hot, and superheats the head and manifold. Running an engine retarded for any length of time, under load, will cause valve burn out, and excess wear to the rings, piston, and cylinder.

Running a 2 stroke engine retarded can have severe consequences as well. Again, it allows the main flame and pressure to build within the cylinder instead of the combustion chamber. The excess heat in the cylinder burns off the lubricating oil, overheats and distorts the cylinder body, and when the exhaust port opens, allows direct flame impingement on the face of the port and piston surface to occur. A severely retarded 2 stroke, under load will melt the piston and damage the exhaust port in short order!:eek: I have seen engines with the pistons actually blow molten aluminum out the exhaust port, to the point that the rings also get burned off by the flow of burning exhaust out the ports.

An engine, with no load, running retarded may not give symptoms of retarded overheating until it is too late, and damage has been done. This may show up as burned head gaskets, warped or burned valves, poor ring seal, and severe power loss, especially when loaded for a length of time. With the piston removed, a sure sign of retarded timing is a brownish or blued cylinder wall, and a severely varnished, or galled piston skirt. This is directly the result of an over heated piston, and the fact that oil has been burned off the cylinder.

Ignition Timing is not the only reason for retarded timing! Setting the mixture too lean, and or an intake leak (worn throttle shaft, leaking gaskets, broken hoses) will also cause heating due to the lower density of fuel to air within the cylinder. The lower fuel/air density causes the mixture to burn longer, with a similar result in damage.

Andrew:eek:

Andrew
 

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