Ok, Robert Gisolf's heat riser is insulated. I didn't thing this was possible with such a tiny tower. But he used something like
superwool, and a
"metal heat riser"
Anyway, you can have many cases, for temp differences. Either you have a perfectly running rocket without mass going to the stack directly, and it will draft a lot, releasing it's
energy faster than another one. Or a crapy rocket implemented th esame, but not burning as hot.
But on a mass system, when the mass gets closer in temp, to the gases inside the flue, heat exchange goes at a slower rate, thus giving higher flue temperature.
While, when it's cold, a lot of heat exchange happens between the two, giving very low flue temps. Sometimes stalling the system. The cooled gases can't go up the chimney.
A difference in top and side gap with the barrel also gives a different heat exchange scheme, up to a point that i would say, making two realy identical rockets is complicated.