What was done:
To get an idea of how much heat the exhaust manifold gives off during routine driving I placed a high temperature thermocouple close to the passenger side exhaust manifold surface.
Initial driving was easy to allow the engine to warm up and then I conducted a few short third gear pulls up to the 5000 rpm range to see what would happen with the near exhaust manifold temperatures under a brief heavy load.
The chart below show the changes to the near exhaust manifold temperatures during this drive.
I was not expecting such cool temperatures near the exhaust manifold, cool being a relative term, but in comparison to what I have logged as exhaust gas temperatures (below) the temperature outside the manifold is not relatively high.
I’ve made a point to call out the temperature as near surface because of the alignment of the temperature probe and the curvature of the exhaust manifold surface it was not possible to have the probe flush against the exhaust manifold exterior. With only a small segment of the probe in contact with the exhaust manifold there was some length of the probe that had an air gap.
It looks like the double walled OEM exhaust manifolds do a good job of keeping interior exhaust gas temperatures from reaching the engine compartment.
Temperature spikes in the exhaust gases during high load conditions do not register at all at the temperature probe. It is possible that during these high load conditions, where the vehicle is accelerating rapidly, that there is additional airflow through the engine compartment which can carry away the additional heat. If that were the case I would expect the exhaust manifold to remain at an elevated temperature for a short time after the high load condition, and with the vehicle decelerating the temperature at the probe to rise. Because that does not appear to happen I believe the exhaust manifold surface temperature does not fluctuate rapidly with the interior temperatures.