4" downpipe dual exhaust
 

I am running a LQ4 with a t6 1.10A/R s475, with a goal of 650 to the wheels, with a 4" downpipe and need/want to split it into duals to be able to run it out the back of my truck. It is going to just be a street truck and don't want the noise and drone under the cab. If I split it into dual 2.5" will that kill the flow? I could possibly step it up to 3" but will have to get creative.

IMO yes it will hurt flow. Where do you plan on splitting it and how long will it be 2.5inch? I think you'd be better off with dual 3"

Do the dual 2.5 with a 4" cutout.

Full tailpipes with enough power will kill some horsepower but a loud valve before the dual exhaust would make quick work of that. I don't really think you'll lose that much power, 650 or even 500whp on the street is going to destroy tires anyway.

Dual 3" or single 4 or 5" all the way to the back. Consider a cutout after the downpipe if you must run full exhaust so you can open it up if need be.

I think you'd also be hard-pressed to find a pre-made single 4" to dual 2.5" merge pipe. You might need to have that custom made, if so. Not sure what someone would charge for that. I think you could find 4" to dual 3" though.

are you talking tubing volume, exhaust velocity, or a balance of the two?
because dual 2.5" = 38% more volume than single 3"

overall though, dual 3" is still about 13% more volume than single 4", so it won't restrict it (particularly if running straight-through mufflers instead of chambered)

Why not find a take off Duramax exhaust or a take off Denali exhaust and run a Borla muffler? I have the left over Duramax exhaust pipe from my dually with a Borla that's not terribly long on my Yukon and it sounds good with O drone. Where my Y pipe merges is where the Duramax was l
flattened so it would seal around the cut. It was done a year ago and even with my Yukon dropped 5" it isn't a problem.