I just received my SLP Dual Dual from another member here and I am going to either my my y-pipe repaired(it broke right where the two pipes are welded to one another) and then have the flowmaster Merge installed into it OR I may jjust buy a Y-pipe from Mufflex since it already has the Merge incorporated into it. My question is this: I noticed that most people who use the Merge have the 4 inch version , does Flowmaster make a 3 inch version. If Flowmaster doesn't make it in 3 inch and I have to use a 4 inch, would it benefit me to have a 4 inch I-pipe then reduce down to the 3 inch over axle pipe or not? I was thinking about utilizing a 4 inch I-pipe and a 4 inch overaxle pipe then reduce done to the 3 inch going into the Dual Duals muffler.
I have been thinking about this setup. What I would do is use the two 3in into a single 4in merge with 4 inch I pipe with a 4 inch cutout and then reduce back into 3 inches over the axle.
it wouldnt pay to do the 3" in 3.5" out. the 3.5" out is simply not enough to handle the 3" in. if you measure for the surface area, a dual 2.5" input is optimized with a single 3.5" exit. a dual 3" inlet is optimized with a 4" or 4.5" single outlet.
just do the math its a simple equation
Area of a circle = πr²
using this method is how i realized when i got my mac mids that the y-pipe on it is simply a poor design. it was two 2½" pipes going into a single 2.75" pipe. The surface area of two 2½" pipes is 9.82. The surface area of the single 2.75" outlet was 5.98 which is way less. What is really needed is a 3.5" outlet which gives a surface area of 9.62 which is close enough to the 9.82. Since I already had an slp loudmouth exhaust which is a 3" system I havent yet wanted to spend the money to replace it so for now I went with a flowmaster merge which converts the dual 2½" pipes into a single 3" pipe. This gives me a surface area of 7.07 which is way better than the 5.98 from the mac y-merge but I am still giving up some surface area which means that until i go with a 3.5" catback, the gain from the mac mids will not be taken advantage of to the fullest. Hope this helped.
4" or go home. I have a couple of posts here with pics of mine if you search. It rocks!
__________________ 02 WS-6 Patriot Stg 2 l 224/228 Comp Cam l FAST 92mm l 96mm TB l Circle-D 3700 l MWC 9" W/3.70's l 3800 lb raceweight 11.46@118 MPH 1.55 60' 400 RWHP
Good thing I read this, I was planning 3" in , with a 3.5" out and into a 3.5" I pipe with cutout.
Guess I'll aim for the 4" I pipe and a 4" cutout. Thanks for educating us.
Will I be okay then stepping down after the cutout to a 3" pipe to go into my GMMG??
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Black 2001 WS6 A4
Lid, TB, CAI, GMMG, Koni, poly bushings & endlinks, relo brackets, HPS, WW GFX & Wing, HID, 10" sub and some small custom touches
Plans include:
LT, E-cutout, pulleys, sways & watts link, and tempted by a stall, 3.73 or a little juice....
Good thing I read this, I was planning 3" in , with a 3.5" out and into a 3.5" I pipe with cutout.
Guess I'll aim for the 4" I pipe and a 4" cutout. Thanks for educating us.
Will I be okay then stepping down after the cutout to a 3" pipe to go into my GMMG??
as long as that 4" cutout is open at the drag strip it doesnt matter what the rest of your exhaust is. just know that when the cutout is closed u will be down about 10-15hp most likely.
On the dyno I saw a HP difference open but at the track it didn't show any improvement on the slip
__________________ 02 WS-6 Patriot Stg 2 l 224/228 Comp Cam l FAST 92mm l 96mm TB l Circle-D 3700 l MWC 9" W/3.70's l 3800 lb raceweight 11.46@118 MPH 1.55 60' 400 RWHP
when the cutout is closed u will be down about 10-15hp
No shit, I thought the GMMG was 1 of the most free flowing catbacks out there. And it's still restricting that much??!
Only consilation I guess is at least the price of the cutout will be worth paying......
Sorry to the OP for
__________________ _____________________________________________
Black 2001 WS6 A4
Lid, TB, CAI, GMMG, Koni, poly bushings & endlinks, relo brackets, HPS, WW GFX & Wing, HID, 10" sub and some small custom touches
Plans include:
LT, E-cutout, pulleys, sways & watts link, and tempted by a stall, 3.73 or a little juice....
I have been thinking about this setup. What I would do is use the two 3in into a single 4in merge with 4 inch I pipe with a 4 inch cutout and then reduce back into 3 inches over the axle.
No shit, I thought the GMMG was 1 of the most free flowing catbacks out there. And it's still restricting that much??!
Only consilation I guess is at least the price of the cutout will be worth paying......
Flowmaster merge?? 
98 Formula 6sp 3.42 gears.Long Tubes Flowmaster Merge. FTRA. MTI Transparent lid. MSD 8.5 wires. Luks pro gold clutch and light weight flywheel. 
I thought the GMMG was 1 of the most free flowing catbacks out there. And it's still restricting that much??! 
