"The 4-inch Peacemakers system has no more overall change than any other slip-on mufflers. The air/fuel ratio difference between open and closed is less than the change you would see by going from a #180 main jet to a #185 main jet. The bigger change in A/F ratio comes from using a high-flow air filter. You need to tune to the air intake first, because the pipes cause little change (I remember learning this back in the '70's).
"Bottom line: you don't need to tune for a difference between open and closed diverter valve positions on the Peacemakers! Just tune it to the 12.8-13.0 to 1 air/fuel ratio at wide open throttle (best power) and you are good to go.
"Congratulations to National Cycle on getting the Peacemakers to have such a small A/F ratio difference between the opened and closed positions. They worked hard to get the change minimized and it shows in the facts."
~ Rick Botting
Total Fuel Systems