Adding HFCs changed the Stillen catback from smooth but somewhat quiet (still a vast improvement over stock) to perfect loudness. However, there is rasp that really only noticeable under WOT. I think it's a fair tradeoff for the improvement in sound at idle and low revs as well as butt dyno gains.
I recall a track day last year, before I learned about some of this stuff, where a car kept making the WORST mechanical wail as it went past. Turns out it was probably test pipes or something similar in his exhaust setup.
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2013 Cadillac V-Wagon, RIP Z
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