Originally Posted by gasman
Well, It's a good thing that I didn't read this fifteen years ago.
I installed a 1.5 inch scat to the inlet of my heat muff with no noticeable change to outlet temps or comfort in the cabin. What I do notice is way more service room to the back of the motor. I do live in mild climate.
I sure can't argue with the educated. Please disregard my response to the OP.
Wasn't supposed to come off that way but it's my superpower and I had a few minutes before a meeting to get my geek on.
Think about the iterations/improvements of GA heat muffs over time. Smooth surfaces gave way to corrugated-ish ones to increase area and turbulation (air mixing.) Air is a poor heat conductor, an insulator actually so it's important to have a good bit of turbulence/recirculation/etc. to bring as much of the quickly passing air into contact with the hot surface. Studs were added to increase both of the aforementioned; A and a component of U in the equation.
Putting steel wool into the heat muff could help; the steel that has contact with the hot surface specifically. The heated steel wool would increase the hot surface area plus the extra turbulation would bring more of the air into contact with the hot surface. Too much would probably be detrimental relative to the optimal amount. Bronze would probably be great; at first. It's a better conductor but once the surface started to oxidize, that would quickly diminish.
A decent design shouldn't need it. As Vlad mentioned, another HEx in series will help but it is diminished returns because of the MTD in the second muff.
As mentioned, the overall Heat transfer is more important than the outlet temp alone. The air passes into a cabin that is basically a big heat sink.