A look at thermodynamics and flathead overheating

February 1, 2009 | By Richard Prince


I read in the September issue about problems with flathead engines overheating. One of the readers made reference to the potential problem being associated with the water going “through the water jacket so fast that the water recirculating through the block never had a chance to cool down.” As a licensed engineer and someone that deals with heat transfer on a regular basis, this explanation doesn’t correspond with the laws of thermodynamics. The rate of heat transfer from the block to the water circulating through the block is based on the following equation: Q=mc(TblockTwater), where Q = the rate of heat transfer, m = the circulating water flow rate, c = the specific heat of water, Tblock = temperature of the water jacket walls, and Twater = temperature of the water flowing through the block.

For this purpose it is safe to assume the specific heat of water is constant. Given that, the rate of heat transfer from the block to the circulating water depends on the flow rate and the temperature difference of the block and the circulating water. If the radiator is doing its job (i.e. the temperature difference is constant) and the flow is increased (example, a larger, more efficient pump is installed as you discussed in your response), then more heat is transferred from the block. If the flow is constant then the only way to increase the heat transfer is to increase the cooling capacity of the radiator, either by increasing its size or increasing the air flow across the radiator tubes (such as adding an electric fan—popular with the hot rodding crowd as it helps dissipate the higher heat generated in a higher horsepower engine).

As such, I think your response is technically accurate and corresponds with the established laws of thermodynamics.


It is crystal clear that Auto Restorer has the most intelligent readers in the car magazine world! Thank you for taking the time to share your explanation with us.