I agree with Andy. I put the 1-7/8” headers with 3” collector dia into 2-1/2” exhaust with an h pipe bcause i drive this car semi- daily and wanted it a little quieter. I ran it at the track with full exhaust, and then dropped the exhaust and bolted on an 18” collector 3” dia. It was .15 sec et difference and 1 mph difference. This is 609 hp and 630 lb/ft tq engine.
I thought the exhaust was gonna rob more power , but it didn’t.

11.70 @ 114.4. Full exhaust
11.55 @ 115.4. Open header