I was wondering, everybody out there that has gone dual exhaust with their v6's... What size piping did you use? 2.5" or 2.25"?? What will be better because I dont want the pipes to be too big that I will lose power? and secondly, did you guys reuse the old cats? or did you buy new ones? If you bought new ones where did you get them from?
Mine reused stock headers and cats. We cut the y-pipe out behind the cats and replaced it with a Pypes 2.5" x-pipe and 2.5" midpipes with Flowmaster American Thunder GT mufflers.
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*1998 Ford Contour SVT* EH 2.5L Duratec v6
Stock as can be, just trying to reverse the damage the previous owner caused.
I used GT takeoff mufflers,cut the pipe just behind the "Y" purchased a left side GT exhaust pipe and hanger from Ford. Had the muffler shop fab up an H pipe and install the left side pipe. Put the GT mufflers on and exited through a GT takeoff bumper. Very clean and simple. Sounds good to,not to loud but still noticeable. Later I put on JBA shorties,a simple bolt on with no additional cutting or bendig,almost like stock, Easy
Xcharger with water injection, JBA headers
JBA duals with an H pipe, Roush rear springs, Che K member brace with torque limiters, Cervini's quarter window deletes, Canton polished expansion tank, Roush scoop, Street Scene Elenor grille, custom GT500 chin spoiler, PJ rear louvers, Black rear spoiler, BSM custom stripes, Hurst shifter, Billet interior trim, custom door welts, Autometer dash pod, AEM wideband, Autometer boost & Fuel pressure gauges
Been putting my order list together for several days now and am just about ready to start making the calls and smoking my checking acct.
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2005 V6Mustang coupe. Automatic. Black with Charcoal Interior
Mods: JLT CAI, Bamachips 91 race tune, single Mustang V8 muffler, Mustang V8 swaybars
Looks: smoke front turn signals, sequential taillights, 14" shorty antenna, MRT rear window louvers and a little chrome for the interior 2000 C5 Corvette convertible, Torch Red with Black Interior. Full of options, Corsa exhaust 1990 Ford Bronco 351ci engine- it runs.
Most kits will be 2.5 inch pipe. My Pypes kit w/ x-pipe had 2.5 inch x-pipe and mid-pipes. Just leave the cats alone. Do everything AFTER the cats. The cats will supply the necessary back-pressure. If you remove the cats or go to higher flow cats then you will probably loose power (unless you are turbo or supercharged of course). Otherwise, there is no reason to remove or replace the cats.
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"Their are two types of pedestrians: The quick and the dead"
-unknown
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05 Satin Silver V6/Automatic/NO Traction Control NO anti-lock brakes (can't stand either one)/TI CAI w/ SCT Xcal2 93 Octane Bamachips Torque Tune/Pypes true dual w/x-pipe and Violator mufflers/Alpine Head Unit w/ iPod jack/XM Delphi SKYFi2/Alpine rear deck speakers/Molded splash guards/Steeda Dead Pedal/GT takeoff front and rear sway bars.
I used 2.25" pipes on my exhaust. Maintains more backpressure. You won't encounter any problems with 2.5" pipes, but I wanted to go with the 2.25" pipes. Stock pipe is 2.5", so to maintain roughly the same back pressure with greater exhaust flow, you would have to go down a size in pipes. I say go with 2.25" pipes. Oh, and do all the work after the cats. No need to modify anything before the cats. It is already running dual before the Y.
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4.5L Stroker Kit, SSM Custom cams, Wiseco forged pistons, ARP 2000 head studs, Port and Polished Intake Manifold, Mac Long Tube Headers, FRPP Oil Cooling System, 4.10 Gears, K&N Performance Filter, StangWires 8mm Plug Wires, Bosch Platinum 4+ Spark Plugs, 39lb Ford Racing Fuel Injectors, ASP Underdrive pulley kit, MAC Performance Pro Chamber, Flowmaster 40 Series Mufflers, 2.25" dual straight pipes w/o cats, 3" chrome tips, Custom SCT XCalibrator 2 Tune w/ Jet Chip Module, 4" Aluminum Driveshaft
Questions about going to dual exhaust 
