LS1 x-pipe EXUAST drama's 3" xpipe or 2 1/2" xpipe PLEASE HELP!!!!!!

[matqld]

HeLLo ALL,,,, Got a question, i got ls1 cam otr mafles tune ect, just got x-force headers with 3" dumpers into 3" cats but thing is beforehand all i had was x-force 2 1/2" cat back and stock headers and cats, now its been suggested to keep the smaller cat back rather then step up to 3"catbak as exhaust gasses shrink as they go through the system and cool,
And thats kewl i understand that, and will never force induct the motor so wont ever need 3"catbak,
BUT where the 3" cats join the 2 1/2"catbak its been suggested to just grind down the flange on the smaller side to help with a smoother transition, BUT
Its also been said the only real disadvantage in the x-force desighn is the 'x part' where the 2 pipes are welded together. They have been cut down then welded together, thus reducung the overall flow through that point.
BUT i can buy whats known as a true x-pipe from either sureflow or magnaflow and have it welded in-thus increasing the flow through that point and that would make the catbak system as good a flowing one just like a better quality brand one would. ($123Aus delivered for the x-pipe part to weld in. "special deal though")

BUT the biggest part im stuck on is if i should buy a 3" (or) 2 1/2" magnoflow x pipe, see they can come in different sizes for eg, 2 1/4", 2 1/2", 3" ect.
And this would mean the stepdown from 3" cats to the 2 1/2" catback would either be infront of the x pipe or after the x pipe based opon which size i get, Which size would you suggest i get?
I still need to get this engine's ecu re-tuned from recent rebuild so ill hold out on tune to get the exhaust sorted.

Are you with me on this? cheers anyone that can shed some light on this, many thanks in advance anyone that takes the time to offer advice.

[Not_An_Abba_Fan]

Hi Matt, sorry I didn't get a chance to reply to your PM, but this question is pretty relevant to a lot of people so it's good to be abswered here.

There will be a restriction to flow where the 2 1/2" system meets the 3" cats. Look at the flange on the rear of the cat to see if it is actually 3", as some of them are only 2 1/2". If it is 3", the best theng to do is to step it down to 2 1/2" just before the flange using a tapered flare. This will smooth the transition from 3" to 2 1/2". It will also utilise a selling tool from Difillipo, "pinched" collectors. This is a venturi design that increases exhaust flow where the pipe is too large to begin with.

The next thing is put the X pipe as far back in the system as possible. Fit it as close to the front of the mufflers as possible. The X pipe will also quieten the note as well, so you can even remove the front mufflers and fit it there, but keeping the rear resonator. This will, of course make the note a bit louder, but as I said, an X pipe does quieten the note. Oh, and use a 2 1/2" X pipe.

You will also have to cut out the restrictive merge just behind the flanges on the front of the system, this is the worst part of the X force systems, apart from the rubbish mufflers and resonators that souind like crap.

[matqld]

Thanks not_a_abba_fan for taking the time to reply and offer your opinion.
Absouloutly everything i was looking for, thankyou very much.
Trully appreciate your help on this one.
And hope others can benefit from this also,
The smarter thing for me to do would have been to buy a better system originally.
But lack of knoledge and money at the time prevented this.
My advice to others would be to buy a better system to start with if possible.
Not_a_abba_fan would be great to help you out with that.
Again trully appreciate the advice not_a_abba_fan..