Your Onan generator service tech friend is probably right. "Anything" one does to make any changes to anything will likely void the warranty. It seems to be the way all companies are headed these days, and one of the two reasons we bought a gently used, 5 (now 6) year old unit. I have nothing to lose by making modifications and lots to gain.
That said, I'm willing to gamble because mine is a 2012 unit. The generator is already past the 3 years they warranty, and they won't sell an extended warranty at this point because the original period has lapsed.
One of the things I noticed while testing the unit after wrapping the pipe was that it was considerably more "steady", even with the AC on as a load. The engine firing rhythm was very regular, something that was not always true before wrapping. It may have nothing to do with wrapping, since I have recently dumped a few cans of Sea Foam into the gas tank.
I have also wondered if I could put a small, downward facing exhaust extension on the end of the generator exhaust pipe. As it is now, the exhaust blows straight out and is noisy, but below the 60 db federal park limit at 50 ft. In fact, ours is now 55 db at 10 ft., but it seems to me that a slightly curved, small extension which pushed the sound into the ground would make it even quieter at 10 ft., probably something like 50 db, which is pretty near normal conversation level.
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