So my future in-law has a '97 Dyna 80ci, high flow heads, cams, ignition, carb, some minor changes. Another guy we ride with has the EXACT same work done at the same time, the tow are identical except for air cleaners. The in-law was always faster, then bought a 2 into 1 pipe, thinking it would increase horsepower, and he actually LOST power and now is slower than the other guy? Any suggestions as to why?

Did he rejet and compensate for the 2-1? If he did and still lost power, then I would take a look at the cam specs, they may not work well with a 2-1, for example if he had it custom made...The cam should match the pipe closely, and the carb jetting too....
When I was a youngster I gave myself a headache, going through three different cams, to get a decent amount of power our of my 80"...All this because I wanted to run straight/drag pipes....Not to mention the tuning was a well remembered lesson...

My guess is the back pressure of the 2-1 has to be compensated for...his bike is carbeurated, right? Maybe adjust the AFR?

Yeah I think the carb needs to be adjusted too. He had some really short Vance 7 Hines, really loud straight duals, then went to a 2 to 1, so I'm sure something with backpressure/air/fuel changed? Do you think it would make a big difference though, I mean like from smoking another bike to losing pretty bad to the same other bike?

If you ask in the tech forum, under engine/exhaust, you are gonna find a lot more help with this question, guaranteed.

Cool thanks!