I have Doug's 3 inch exhaust cutouts. So my headers run to a 3 inch extension, then the dumps...or, if the dumps are closed, it goes to a full 2.5 inch custom exhaust with an X pipe and dual 40 Series Flowmasters. Basically, I've either got the resistance of 3 inch (though much shorter and no mufflers) or the best 2.5 inch system you can buy.
My last dyno session, the most the car would make through the exhaust was 379.8RWHP.
With the 3 inch/dump, it made 407.33.
I like having a switch that can make the car go from civilized to outright insane. Passing a Prius with 469CID of open headers causes some pretty awesome facial expressions. I'd probably give up some of those 28hp if the 3 inch system went through mufflers, but not all of it. My experience confirms everything that Engine Masters already proved. Go with the big exhaust. Don't let a lazy guy at the shop talk you out of your top end performance.
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77 Trans Am, 469 w/ported E-Heads via Kauffman, matched HSD intake, Butler Performance forged rotating assembly, Comp custom hyd roller, Q-jet, Art Carr 200 4R, 3.42s, 3 inch exhaust w/Doug's cutouts, D.U.I. Ignition. 7.40 in the 8th, 11.61@116.07 in the quarter...still tuning.
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