I want to replace the stock 2.25" intermediate pipe (from cat to muffler) with a 3" pipe but my exhaust guy can't bend a 3" pipe so I'll have to go with a 2.5" one. How much of a power gain should I expect with either the 2.5" or 3" pipe? And will it sound different from stock? My mods are my sig.

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'88 IROC 305 TPI
Crappy 700R4 slushbox
Gutted airboxes
180 degree T-stat
Advanced base TPS voltage
Relocated IAT sensor
Momo steering wheel (gotta luv it)
Ram-air setup coming soon
Flowmaster muffler (puke)
Taylor SpiroPro wires
Accel cap and rotor
Ported plenum
Kills: '94 Z28, Olds Aurora V8, bunch of Mustangs, T-birds, ricers, and others who assumed a 12 year-old car would be too slow.

 

If you really want 3" exhaust,, go for a dynomax cat back from JCWhitney for $134, it's mandrel bent (no pinch or wrinkle bends) and then replace the muffler with whatever you want(if you must have loud),,, it will run you under $200 (w/new muffler) and it all clamps together so you can do it yourself.
Almost anything does better than stock and it will sound better too.

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My Firebird

Part Diagrams

 

My exhaust guy wanted $80 for a mandrel bent aluminized 2.5" pipe. He can't bend a 3" pipe because it won't fit in his machine.

How much power does a 3" pipe typically free up?

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'88 IROC 305 TPI
Crappy 700R4 slushbox
Gutted airboxes
180 degree T-stat
Advanced base TPS voltage
Relocated IAT sensor
Momo steering wheel (gotta luv it)
Ram-air setup coming soon
Flowmaster muffler (puke)
Taylor SpiroPro wires
Accel cap and rotor
Ported plenum
Kills: '94 Z28, Olds Aurora V8, bunch of Mustangs, T-birds, ricers, and others who assumed a 12 year-old car would be too slow.