it is twin 3 inch then single 3.5 over the
irs cradle, back to twin 3.
This far back in the system you dont lose any flow - at least up to 500rwkw
I made 18rwkw extra with the big boy and BPR cold air, compared to a previous full custom system and YT cold air, that was 2.5inch though
The headers are longer tube 1 7/8 primaries with a trick little cinch for the right backflow from the 3 inch collector