From Larry Vetterman
I asked Larry to respond to this thread as he has far more direct experience with testing heating systems than I do. Clint
This is a response to the post on cabin heat on VAF. First of all there are some 40 different models of exhaust systems for all of the engine-airframe combinations so advising on one system may not be appropriate for others. On the RV-4, the one I have the most time in, I spent a lot of time on heat muffs and getting it into the cabin. The best muff I found is the Robbins Wings muff that goes around 2 pipes, number 1-3. The temperature of the incoming air depends on a number of things like how fast the air is going thru the muff. Too fast and it doesn't have much time to transfer heat and warm it up. Then what are the egt's. The hotter the egt is, the warmer the air will be. Then the OAT is a big one. I flew many times at 0 and was comfortable with a jacket. Lastly, how many air leaks are there? That little draft in the summer time becomes an unbearable arctic blast in the winter. The -4 is a hard one to seal up especially around the canopy. The old subject of tail pumping was really going on in my -4, so I installed a reverse NACA vent in the belly, and it made all the difference. I remember after I had it installed, my wife went on a flight and she asked me what I had changed as her hair was not blowing forward back there. So it worked.
Getting heat to the back is a problem, so after many attempts I settled on a section of scat tubing mounted temporarily. She could move it around at will and it worked.
I also tested the welded on studs on 2 separate systems and did not get any temperature change with or without them, so I never endorsed them. Your experience with them may be different than mine.
On most crossover systems on the 0-320 and 0-360 systems one cabin heat muff usually is enough but the system was designed to install one on each side. Then when we designed the muffler system, heat was never a problem as that setup with the muffler can and shroud puts out way more heat than needed.
By the way, Robbins Wings is still doing muffs, he's just out of town right now. Hope some of the above info helps. Larry Vetterman