If that were the case then everyone would be running the largest header they could fit on their car. Back when I had the 396 in the Chevelle and it was making about 350hp I switched from 1 3/4 primary tube Hedman headers with 2.5 inch exhaust to 2 1/8 primary tube Hooker headers and 3 inch and I lost power. I went from turning 104mph to 102mph. Back then the car was very consistant. It's my opinion (not a fact just an opinion) that because of the larger size I lost some of the scavenging effect and thus power.
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