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Assuming you've got a properly tuned and running engine, and you measure exhaust manifold temperatures ( for example with a non-contact infrared thermometer ) as a proxy for cylinder temperatures, what would be the range of normal variation in temperatures you would see between cylinders?

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    Unless you do this as an experiment, or someone else has, then there are so many variables that a meaningful answer is in doubt. Such points as mass of the metal being the same for each port, the measuring distance, any drafts over, under or around the manifold etc... – Solar Mike Mar 6 '17 at 5:59
  • @SolarMike Then what would be the point of using exhaust manifold temps for tuning, as suggested in the answer to this post? mechanics.stackexchange.com/questions/24272/… – Robert S. Barnes Mar 6 '17 at 8:55
  • as per that post, how are you going to compensate for the angle and reflectivity of the metal affecting the recorded temperatures let alone the inaccuracies that I suggested. If you wish to use EGT to control tuning or engine performance (and it is done, even on tractor engines to avoid excessive combustion temperatures), then I would suggest drilling, tapping and then fitting temperature sensors to each port - obviously at the same distance and depth to improve accuracy and reliability of the readings. And, before you ask I did fit sensors to do similar on research engines. – Solar Mike Mar 6 '17 at 9:31
  • Given the variation you quote in that post of 70 to 100 degrees F, which does not agree with the figures you supply where the max difference is 108, then with such a variation how can you expect to control sensibly the combustion temperature of each cylinder ? – Solar Mike Mar 6 '17 at 9:39
  • @SolarMike I have a bad case of aluminum oxide contamination in the cooling system, and after exhausting all other possibilities ( other than an electrical problem with the ECU ) I am suspecting that my rough idle may be due to clogged coolant passages causing large enough differences in temperature in the cylinders leading to uneven combustion. My exhaust manifold is matte iron, not reflective, and I was pretty careful to try and measure at the same spot on each exhaust port. – Robert S. Barnes Mar 6 '17 at 9:59
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After finishing carb tune up job on Honda CB900C motorcycle I was measuring exhaust manifold temperature at each of 4 cylinders, and during warm up and during idling for 10 minutes the difference in temperature was not more than 2 Celsius.

  • Great bike. In the middle of a rebuild on an '81. – DucatiKiller Mar 6 '17 at 21:32

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