Usually open space makes it worse. When I had the Borlas on my truck (before they wore in), they were very mild. My buddy had them on his Blazer and they were much louder.
Length of pipe can go either way, can make more reverberation or make it quieter, depending on bends/straights/etc. Tailpipes always help, but in both those instances I stated above, the trucks had full tail pipes out the back and swept to the corners.
I had a dual-in/dual-out Borla muffler on Mean Green, the guy routed the exhaust to the outside of the frame, so the muffler was where an auxiliary tank could go, and he ran the pipe out to the corner of the rear bumper. It was loud-ish, but not too bad. When I got Crusty Biscuit, the PO had it as dual exhaust with the mufflers tucked up to where they cleared the trailing arms, and the pipes routed over the axle, but not out the back. That thing droned really bad when driving low speeds. I had a single exhaust system installed, but the guy made a weird y-pipe that wouldn't allow an oil change, and he flattened the pipe and cut the cross member to install the pipe. When I saw all of this I was pissed! The truck did not drone with this setup, though.
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Needing an exhaust system that allowed me to work on things if I needed to without dropping the exhaust, I dragged the old Borla exhaust out to install on Crusty. Well, I didn't take into account that the HO52 axle is narrower than the Dana 60, and the pipes wouldn't go in between the tire and frame. This was exhaust swap was right before going on a trip, so no trip to an exhaust shop to have the tailpipes rerouted out the back. Off to autozone, pepboys, oreillys, to find exhaust parts and pieces, and then cut the pipe just aft of the muffler. Voila!
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While this worked, it was LOUD!, and it droned way worse at low speeds! It was miserable on that trip, and on the way home there was a fire, so we had to go out through Nevada from Lee Vining and back in at Bishop. 13 hour drive that day with that loud exhaust and droning. I was exhausted!
LOL...you can see the new exhaust behind the Borla muffler, I had forgotten that I had to leave that muffle and tailpipe in place for the trip...lol...
When I got back I took it in, and had the exhaust shop do a blended system, utilizing the muffler in the background and removing the Borla. No more drone.