Only if very significantly LOP.
During climb, the CHTs are ~1350F so ok. The problem occurs when you inadvertently fly too slowly but with cruise settings: peak EGT or LOP. And it probably needs very little time (seconds) for damage to happen.
LOP produces less heat and pressure, right?
I’m still thinking the situation is as I described above in post #11. Same gas temp before and after the collector but higher velocity downstream, therefore higher heat transfer to the pipe wall and higher wall temperature. I can’t prove it 
In the photo of the naturally aspirated BMW engine in post #05 the wall temperature is hotter before the junction too, but the marked change is at a weld in every case. I’m guessing the pipe wall thickness or material changes at the weld.
Perhaps 2x the gas pulse rate, against a fixed flow rate of cooling air?
Why would the exhaust gas increase in temperature when the volumetric air flow per unit area doubles? There is no substantial change in pressure.
When two or more exhaust pipes merge, the temperature of the gas does not in general change as a result.
That is what should happen but I think it happens only if the post-join pipe is bigger, which it obviously should be, and then the gas will cool naturally by expansion.
These GA exhaust systems were designed by idiots.
When two or more exhaust pipes merge, the temperature of the gas does not in general change as a result. What changes is the speed of the gases through the downstream pipe and sometimes the character of the flow (for example meaning increased turbulence). This then changes the ability of the gas to conduct heat into the pipe wall.
Re exhaust tuning, gas is not in general pushed back and forth in the pipe. Instead pressure waves move through the gas, up and down the pipe. At a resonant engine RPM a negative pressure wave arrives periodically at the exhaust valve (or exhaust port if its a two stroke) at the right moment, helping to evacuate the gas out of the cylinder. Given overlap between the exhaust valve opening and intake valve opening, this increased evacuation then helps suck a greater intake charge into the cylinder.
Is this not just as simple as the energy flowing through the exhaust gasses. For a header pipe serving 1 cylider it will see 1 hot gas pulse every 2 rotations and cool by conduction (to the cylinder head), convection to the cowling air and radiation. When 2 pipes join further downstream, the gas has lost some energy into the header pipes but the joined pipe now has twice(ish) the heat energy, less capability to conduct, the same airflow so gets hotter.
On the F1 engine you can see that after the 4 pipes join the pipe gets cooler towards the exhause. At the 4 way join its clearly the hottes BUT this is likely due to turbulence and other factors. On a high performance engine the header pipes are tuned so that at the design point (RPM & throttle) the exxhause pressure pulse passes down the header, and the “reflects” at the junction and pushes some fuel-air mix (which escapes towarsd the end of the exhause stroke) back into the cylinder. this is/was a big issue with 2 stroke motorcycle engines.
Exhaust design was/is an art and it does impact on an engines performance.
I could be wrong on this so if I’ve missed something, or are barking up the wrong tree, please let me know.
Yes exactly.
The actual tube temperature depends on how much air is passing through the engine cylinder fins and is available to cool the exhaust.
On the TB series, there is an oil cooler on the RHS
which is stealing a lot of air, so not only do cylinders 1,3,5 have higher CHTs than 2,4,6 but also there is less air available to cool the exhaust.
The problem with GA is that this stuff is designed mostly by less than bright people. Look at Quatrelle’s exhaust pipe above. And then they make it out of an unsuitable (cheap) material: stainless steel.
During climb, the CHTs are ~1350F so ok. The problem occurs when you inadvertently fly too slowly but with cruise settings: peak EGT or LOP. And it probably needs very little time (seconds) for damage to happen. The pipework gets eroded and gets thinner and thinner and before many years the pipe is paper thin.
Re higher temp where exhaust pipes are joined, my guess would be that when exhaust pipes combine the doubled or tripled internal velocity at that point increases the internal heat transfer coefficient, with no effect on the external heat transfer coefficient, thereby raising the wall temperature to a value closer to the internal gas temperature.
Aircraft engines and in particular those driving fixed pitch propellers need to have a flat torque curve to climb well at relatively low airspeed, with prop loaded up at high blade AoA. So exhaust tuning is not greatly appropriate unless it’s done so as to be effective over a wide enough speed range. There are companies selling tuned aircraft exhausts regardless.
