Posted by Owen Aaland on August 14, 2010 at 19:54:34 from (65.161.86.94):
In Reply to: smta carb frosting posted by don from Indiana on August 14, 2010 at 12:49:11:
It is all about physics and the properties of gasses.
When a gas is compressed it gives off heat. That is why the air exiting an air compressor or turbocharger gets hot. The reverse is also true. When the pressure of a gas is decreased it will take on heat from the surrounding area.
In the case of your SMTA the change in pressure of the air/gasoline mixture takes place across the throttle plate. Air entering the carburetor is at atmospheric pressure, about 29 inches of mercury. The engine is an air pump and the throttle plate is restricting the air flow into the engine. Under low load conditions the vacuum created above the throttle plate is in the neighborhood of 14 - 17 inches of mercury, about one half of the atmospheric pressure below the throttle plate. This change of pressure causes the air to try to take on heat which it gets from the metal of the carburetor and manifold. Given enough time it will cause the temperature of the carburetor and lower intake manifold to become lower than 32 degrees which causes the condensation that has collected there to freeze to become frost.
As the load on the engine increases the vacuum will drop off so that the demand for heat is less, resulting in possible condensation but no frost.
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