Ken, not sure if I should but I have a wordy reply to this. Thanks for posting that information. That was a good explanation of stuff I kind of knew but hadn't considered in the big picture. Helps to understand some of the effects from my own muffler experimentation. You see I'm currently running some modified TORs on my 09 America with EFI.
While it was still basically stock I wanted a little more umph and a little more growl. (loved and missed the pep of my carbed Bonneville that I sold) Had short TORs and a UNI brand air filter at that time. I then purchased a set of 16 inch “universal slip on” tapered mufflers from one of the internet stores. Nice sound but had a bit of a hole in umph from 4,000 to 5,000 RPM then that missing power seemed to come on all of a sudden. I then started playing with TuneECU. I loaded the only custom tune available from that site, removed the O2 sensors and pulled the snorkel. Better but not satisfied. I then got a second set of short TORs to mess with. Well then, that led to the 12,000 mile service and what the heck. Since I didn't have carbs to tweak or replace, engine mods might get me some more umph. 813 cams, 904 big bore and some tuning was next. I refused the PowerCommander option pushed by the local shops. Triumph dealer claimed no knowledge of TuneECU but let me bring my laptop in for some dyno runs. He made suggestions and I loaded revised fuel maps and he would do another dyno run. After engine mods and a “close enough for now” tune things were prime for muffler experimentation. (I also changed the front sprocket, went from the stock 18T to a 17T) I completely gutted the short TORs. Great sound but noticeable loss of power in mid and high RPM range. Unmolested TORs were OK but just, 16” set was better but still just with a gap in that 4-5K RPM area. After a few false starts I cut the gutted TORs off at the back of the hanger bracket, ~ 23 inches long. Fitted a solid 20” core/baffle pipe (very close to same OD as the original core pipe, just not perforated with holes) and steel wool for filler packing/insulation. Man they sounded great at idle and kinda mean under acceleration. Thought I was done. Rode several hours on the interstate to a respected tuner for a good custom tune using TuneECU. After a couple hours of interstate travel the drone of the pipes said I was not done! Got the custom dyno-tune done and was VERY HAPPY with the results. No more peak HP or TQ from the “close enough” tune but it is now spread out over a large and VERY USABLE range. Am now very satisfied with the power and ride ability of the America. After several revisions I am now using the 23” TORs with a 16” core pipe that has many slits (instead of holes) in it wrapped with several tight layers of insulation cloth. Great sound and the power delivery is very smooth and strong. I somehow stumbled across a usable compromise in core pipe size, length, back pressure or what ever. I have no test result or professional performance verification of this, just seat of the pants experience. Exhaust is far more complicated than one might think!
