Headers tube diameter.

Friend of mine is running a 416ci small block with indy 360-2 heads, solid roller etc. Has 500hp at the wheels through a high stall auto (other cars ran on the same dyno at the same show and the dyno is very close to his tuners dyno, other cars were also consistent). He is running 1 3/4" primary tri y headers, not 4 into 1s. People told him he would need bigger primary pipes.