In article ,
(John Galban) wrote:

[snip]
Even assuming that the 170 owner had an STC for the 180
hp Lycoming, wouldn't it be illegal to mount that exhaust on the 170
airframe? I can't find any sign that the Powerflow STC covers a 170.

I've heard of people sneaking in a "not-quite-fully-approved" STC
here and there, but this one sticks out like a sore thumb
(literally!).
[snip]
So, whaddya think? Can this be legal?

one guess: field approval rather than STC.

--
Bob Noel