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Why do bored engines overheat?

 
  #1  
Old 11-03-2011, 02:46 PM
CougarJohn
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Why do bored engines overheat?

We all know that they do but nobody has ever explained the physics of why they do. Anyone know the answer?
 
  #2  
Old 11-03-2011, 04:43 PM
jim henderson
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Don't know the exact physics, but essentially the metal of the cylinder is thinner and there is slightly more surface area, rember "2 pi R x stroke"? R increases by a tiny bit but enough to make a difference, why else would you bore it eh? except due to wear. Actually the water jacket is usually longer than the stroke.

The thinner metal has less thermal resistance and the extra cylinder surface area has more area to absorb heat. Also, the engine was probably hot tanked which removed any deposits(insulates) in the water passages around the cylinders, so more heat gets into the water.

There is also the possibility that you improved the performance of the engine, this creates additional heat.

So anyway, more heat is absorbed by the bigger cylinder and is transferred to the water more quickly, and the engine may just be making more heat since with most engines HP= more heat.

This is what my boss told me waaaaay back when I hotrodded my first car, 0.060 overbore, plus the works. Had to go to a 5 row radiator from the 3 row stocker.

Jim Henderson
 
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Old 11-03-2011, 05:03 PM
AZ Bill
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Basically, jim henderson covered it, but let me give you an example you can try out at home.

Heat a burner on your stove up until it it red hot. Now place a sheet of aluminum foil on it, and see how hot it gets in one minute.

Now, lets put a heavy cast iron skillet on that same burner for one minute. The cast iron pan is not as hot to pick up.

Same thing with an overbore. You have removed 0.015" from each side of the bore on an 0.030" overbore, thereby decreasing the amount of metal impeding the transfer of heat from the cylinder to the water jacket through the cylinder wall.

At least that is the way I was taught about overbores and heat transfer from an early hot rodder an mechanic.
 
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Old 11-03-2011, 07:04 PM
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Great gys, thank you for our replies were pretty much what I had figured out: There is less metal to conduct heat up to the cylinder heads and upper water jacket to get the heat to the water. Hence, local boiling.
But it is still a head scratcher to this old hot rodder.
Semper Fi
 
  #5  
Old 11-05-2011, 06:45 PM
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Sorry to disagree, but an engine that has been bored (within proper limits,) overheats for the same reasons that any other engine overheats. But with the following caveat: Engines that have been bored often are subject to any number of other modifications. These efforts to get greater performance from and engine, and the way it is then operated, often lead to overheating for any number of reasons. But simply being bored twenty or thirty over is not going to be the cause of overheating. If that were the case, there would be no race cars running around the tracks.
 
 
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