Don't go any bigger than 3" behind a small block gasser. But 2.5" duals would be plenty, and a 3" single would be just fine too. With big pipe like 4" you lose a lot of the scavenging that you get with the smaller pipe. But too small of pipe and you create back pressure, which you want to eliminate as much as possible in a gasser.
If you don't get smog tested, you could just cut the cat off and run a flowmaster or similar muffler. I'm kinda partial to Flowmaster mufflers, but whatever your preference is.
-1989 F250 (RCLB) 4x4, 4speed NP435 tranny, SAS D44HD, 351w, 3.55 (L/S front and rear), F250/350 Hybrid Suspension, Pacesetter Shorty Headers, into 3" Summit Racing Street and Strip Mufflers, 235/85R16 Uniroyal Liberators.
-1994 Thunderbird LX, Moonlight Blue, 4.6L, Automatic, Bone Stock
-2012 Focus SE
just did a 2.5 " Magnaflow on mine also with a new cat from Magnaflow(A.K.A Carsound) .Sounds nice and quiet at idle but when you get on it, gets nice and throaty!
runs alot better too.
Single exhaust out the back
The exhaust in the truck im getting rusted off at the back of the cat, Im keeping it short sweet and simple! walker dual into one universal cat into a cherry bomb extreme muffler with a 12inch turn down out of that, pops out right before the rear axle. everything is ganna be 2.5 inch tubing
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