Outstanding musings on exhaust configuration for PPonk 470-50 from Paul at Acorn:
"John,
The engines are tested at 2 stages, 1) With 6" straight exhaust straight down from the cylinder on a test stand 2) with the attached exhaust as defined on the Type design. I cannot speak as to how Steve tested his engines but I know he was thorough and I cant tell anywhere that it needs a different exhaust. Fact of the matter is that when Cessna Introduced the O-470-R engine they went to a 1.75" OD exhaust(at the cylinder) while the O-470-A, O-470-J, O-470-K, and O-470-L powered aircraft used 1.5" flanged exhaust.
Our STC specifically SA97-33/SA00780NY started off as a safer muffler in responses to several poorly designed onfigurations on the market at the time. The tailpipe to Can joint is completely outside of the heat exchanger area (It wont break anyways) and was suggested by the engineers to make a fail safe solution to the tailpipe cracking or falling off. As you know everyone wanted a longer tailpipe and various extensions exist but... are they actually legal? Did anyone actually test this?
Anyways... over the years it became apparent that there was a issue with a useless strap cracking on the muffler and a update to the STC was required. Since the original STC also had model errors on it (Canadian showed different models that US did) this was also corrected. In the spirit of updating the aircraft to the latest configuration and making them safer and by customer request, the 182/185 Late model exhaust was certified all the way down to where the cowling can take the single exhaust. Since testing was required again anyways to prove that the strap that cracked was useless (It cracked on cold starts without preheat from tailpipe hitting cowling FYI), I decided to remove the flame cones from the muffler and adjust the design to meet the FAR's. We also force guys to comply with a unknown Cessna SL that removes carb heat from the muffler and puts it on the collector. The old school thought was to melt ice that had formed and the new school is to prevent it from forming in the first place.
Testing is a bit "Difficult" as the original Cessna Exhaust that the STC and PMA collectors were based on really isn't that "quiet" and was exempt from noise testing at the time. A new STC would require the FAR36 noise test. We went to Cessna and bought a Factory set of exhaust and returned the aircraft to"Factory configuration" for initial tests and then tested the modified exhaust. We found that ours was quieter than Cessna but louder than the version we have of the same exhaust with cones. Performance wise, the aircraft didn't care.
Do we claim a increase inperformance? Well no legally you can't claim that without a STC. But 3 inputs with a area of 1.61 Sq" each on the old exhaust (4.83"SQ), vs 3 at 2.15 Sq" Each(6.45Sq" total). Do some math and it is like adding a completely additional two headers to the engine (6.45/4=1.6125). On the muffler end, the inlet went from 2.087"ID(3.42Sq") to 2.63"ID(5.43Sq"). This is where any kind of choke point is but a interesting thing is that the design of the formed stampings on the newer collector actually has some reason to the madness and accelerates the exhaust into the muffler. So is there a performance increase when removing the cones? Likely... Computer modeling for a tuned exhaust showed the exhaust was accelerated and then crashed into the cones. Now think about the PMA designed ones that do not use the formed shells that Cessna designed... they claim performance but do they really perform better? Cessna didn't do a bad job designing the parts. The had the opportunity to redesign it and they did,
Now lets talk header type(IE LEES). Everyone talks performance... but what does that mean? Why do the LEES headers work, does it have anything to do with the actual exhaust at all? not really. Dane was a brilliant hot rod guy that used venturi and balanced flow to extract heat out of the cylinders. As you mention your CHT temps...that is why the aircraft performs better hands down. I am not sure that the added weight benefits but I do agree with you that the LEES systems work very well. We actually tried to develop our own and then said why bother spending millions to certify something that exists and few people would buy. The same cooling effect happened on our exhaust. Back to the Performance thing and cooling. This really only matters at higher density altitude on really hot days. The low sea hugging guys wont notice as much as the guys in Arizona or Colorado. When we did some initial testing on our exhaust, you could take off with cowl flaps closed on hot day and have a negligible impact with a tuned header.
To sum it up, the exhaust port size did change on the engines. Unfortunately TCM link is down right now so I cant compare actual cylinder configuration between the O-470-A andO-470-J, and the O-470-K and O-470-L engines. The O-470-50 engine is achieved by several different ways. If you have a O-520 Native block then you really are choking the exhaust (no TSIO-520 engine ever used small exhaust). The STC does not require it and it probably works fine but we sell the STC kit as areplacement because it is cheaper than buying the 0750130-XX exhaust components. We have them and sell them piece here and there but it makes no economical sense to keep them when you do a complete exhaust. Since the cooling really only matters on climb (no one runs engine full power on cruise), investing in better baffling is the more economically practical way to get good cooling."
