It's called a reducer. An alternative would be to run a single pipe from the muffler to the tip and just using the tip as the decorative shell sheet metal. TIG welds thin metal like this very easily without damaging it.
OP: Most, if not all, of the aftermarket offerings leave the muffler at one size (usually 3") and taper up to a larger tip (usually 4"). This is completely useless power wise and only function is to produce a larger sound (which I do not want). Furthermore, a catback is only going to flow its weakest link (smallest choke point). If it's a true 3" catback, that point is either at the tightest bend radius/greatest angle or inside the muffler (not to mention if you are still running the factory downpipe and/or cat. Just look at that picture of the Borla exhaust and try and tell me that smaller tips would be a choke point.
I want the stock tip back to be more stealth to both cops and other enthusiasts - I also think the dual round tip just looks dumb with the factory 15 rear bumper I have...honestly I wouldn't mind finding a focus titanium guy to swap bumpers with and really flying under the radar. These are just images I pulled of the internet to edit quickly as to what I plan to do. I currently have a Flowmaster (aka DroneMaster) on the car that I can't wait to swap for something more practical like the Borla, but I also gave the option for someone with a Milltek-type rear muffler that is set up like the stock one (single in dual center side out versus most aftermarket offerings single in single out and then bends).
I do not claim to own any of these images. I ripped them off the internet and quickly MS Paint edited them for demonstration purposes. Stock. Once cut square to the back face of the tip (the stock muffler-tip pipes are on a slight angle up/down and side/side) decide if you are going to use reducers to mate to the factory dual 2" or you are going to just gut the tip and use it as a decorative shell.
Borla or any other type that has a single pipe out the muffler and Y's at the tip
Milltek or any similar type that has dual exits out the side of muffler...carry both to the stock tip shell or Y them together and carry that single pipe to it. The Milltek is a 3" single inlet catback. 3.00" ID tube estimated cfm is 747. Tapering down to the dual 2" tip (if you chose to do that and not either Y their exits into a 3" to the tip or carry their tips to the stock gutted tip shell), would cost you 111 theoretical maximum cfm (2x318). If you think in the real world you are flowing 747 at the entrance to the catback, though, well, lol. If you care that much about horsepower, run a straight 3" off the turbo with the largest diameter radius single bend you can and just enough pipe to get accurate wideband readings then open to air.
If anyone would like to donate (read: sell me) their stock tips where I marked the cut line, PM me and I'll make a how to. Hell, I'll even do it on this Flowmaster if I can't find a Borla quickly - cutting off these ricer trumpet tips should shut this stupid thing up some. I hope the pics give other people with access to their stock CB and want to do this the motivation/ideas to do it. I won't lie though, that JBL is a nice piece, V bands and all.
