Exhaust temperature difference

My 68 4.2l engine has been rebuilt with new rings and valves with good compression on all cylinders. It is tuned so running warm and
About 900 rpm
Good manifold vacuum
Airflow equal on 2 ZS carbs
Correct air/fuel.ratio on both carbs per piston lift teat

All well but surface temperatures significantly different from front to back. For example number 2 header at 180C and number 5 at 130C

So I have two questions:

  • why temperature difference?

-and given that temperatures probably reflect amount of combustion in cylinders, is it better to adjust air flow (with idle speed screws) to obtain similar exhaust temperatures, or do conventional thing and adjust to get same air flow to both carbs?

I simply would not fret about it: if the plugs read well, all cylinders are close in compression, it’s really a nothing-burger.

These engines/induction/exhaust systems are not that precise, especially at idle.

Phillip - just in case you have a faulty spark plug, take the # 2 spark plug and change it with the # 5 spark plug and then see if the temperature range moves with the spark plug - Tex.

That’s exactly right.

The temperature difference is because there is different combustion in the cylinders. Primarily, this will be down to gas flow, i.e. air distribution and exhaust gas emitted being more unequal at lower revs. For example, if the runner lengths are different, then there will be a lower pressure in the longer inlet and that now changes the gas velocity. This means the column of air in that runner is accelerating and stopping differently to the others when the inlet valves open and close, so you have a different compression. Add to that, if you have different inlet runner temperatures, a different amount of fuel is condensing onto the insides of the manifold and also being sucked back into the airflow. All of these effects even out a bit as the revs pick up, so your measuring a discrepancy at idle rpm with no load bears little relation to how the car performs under load and at roadspeed rpm. A variation of 50’c at idle isn’t a great concern and if it can be tuned out to just 25’c, then you have done well.

Here is a fuel injected v12 etype pulling from 0-80+ mph in a few seconds. The EGTs are within about 50’c of each other but they do wander around - some cylinders breathe better or worse than each other at different loads and rpm. If you consider that having multiple carburettors and a distributor which distributes ignition sparks based on springs and weights adds to your variabilty of when combustion actually happens, you probably aren’t doing too badly.

kind regards

The top graph are just general parameters, the middle graph is EGT and the bottom graph is EGT spread.