For Long-EZs we have used the Brock prefab, plans-type stainless exhaust headers with "ball swivel" joints. On the prototype, as well as Dick Rutan's and Mike and Sallys Long-EZs (well over 3500 hours total time), we have never had a single problem with this setup. For VariEzes, we have always recommended this same system for Lycoming 0-235 powered EZs; for Continental 0-200, we recommend the exhaust system originally designed, fabricated, and sold by VariEze builder, Herb Sanders, through his company, Sport Flight. Generally, we have had very little trouble with either of these systems.
In the interest of cooling the engine, we have always recommended a rather small clearance hole around each exhaust pipe where it goes through the cowling. Recently, we were testing another system consisting of four separate exhaust headers that exit the cowling two on each side, one on top of the other. We wanted to see if there was any power advantage using four headers instead of a two-into-one system. In fact, there is a small power gain, between 50 and 75 RPM at full throttle at 8000 feet in level flight, but that was not the interesting discovery. During the testing, we simply cut enough cowling away that the exhaust did not touch the cowl. We inadvertently cut more cowling away then we had intended, and during the flight, we noticed a considerable drop in cylinder head temperature!
Yes, lower CHT'S, in spite of a relatively large leak in the pressure cowling. We sealed up the leak using engine baffle neoprene/asbestos glued to the cowling and fitting almost perfectly to the exhaust pipes. The CHT's went up higher than ever! We cut away the neoprene a little at a time. The more we cut away, the larger the high pressure air leak, the lower the CHT's became! We ended up with a full 112 1. of clearance between the cowling and the four exhaust pipes. Amazingly, this amounts to about 7 square inches of "leak" area on each side of the cowling! In spite of this leakage, we have excellent cooling.
How could this be? Perhaps the leak so far aft, even aft of the engine, gets the cooling air mass moving aft at a fairly high velocity. Then, of course, all of the incoming cooling air cannot escape out through the "leak" area, so it has to go through the cylinder cooling fins just as the engine baffling intended it to. For whatever reason, this does work, and not on just one airplane. We know of at least 3 EZ owners who have tried opening up the clearance holes around the exhaust pipes and they saw the same results - cooler cylinder heads!