My car is a 01 SS M6 , i am having a forged 347 installed over the winter ,
with 9.25 compression , and i will be running 15 PSI of boost. I am at 10PSI now on stock motor making 540 to the wheels. (methanol injected)

OK my current exhaust system consist of 1 3/4" Long tube FLP headers with 2.5" Y pipe (no cats) and my stock SLP dual dual Exhaust ( 3" main over the axle pipe and slp dual dual muffler.

I am thinking on having a 3.5" over the axle main pipe made up (I think 4" is over kill )and taper it down to fit my SLP dual dual stock muffler , I dont like the way SLP cushed the 3" pipe where it goes over the axle.

has any one done this? any sound clips , performance difference? etc.

I have been hearing my exhaust would limit my HP and power goals.

I might also have a custom 3" Y made up , the 2.5 FLP is ok but where it meets up a the main 3" over the axle pipe , there is a bottle neck there, i hear one guy that cut this off and replaced a 3" spool piece picked up 14 HP. Thats a lot of HP for removing just the bottle neck.
thank you for your help

 

Mine's the same setup as you're looking into. I have a 3" y-pipe into a 3.5" collector catback system with a flowmaster single in/out with a dump on driver side (kinda looks like a V6 on steroids).

I made my whole exhaust setup from scratch and it sounds pretty nice, although the fact that I have twin cats and a muffler kinda robs a lot of power. May sell the setup or hollow out the cats and use a cutout.

Here's how it affected power ... granted my setup isn't optimal right now 'cos of a mismatch of parts ...

TH350 w/3500 stall, true duals with dumps ... 401RWHP
T-56, above exhaust (to make it more family friendly .. ) ... 403RWHP

 

I did almost the identical thing. Custom Y with a proper merge exiting to a 3.5" intermediate pipe. I also put a 3.5" cutout in just before it went over the axle and narrowed down. Without a 3.5" cut-out you aren't gaining much; you will still have a 3" system when you really need more flow.

 

any pictures of the exhaust Ragtop 99 ??

 

Y-Pipe




Underneath car with 3.5' QTP electric cut-out. You can see I have it neck back down to 3" to go over the axle. (some stuff looks messed up because it was post accident.)

 

so a 3.5 wont go over the rear axle, is there not enough room,

looks good otherwise

 

There is enough room for a 3.5" pipe. It knocks around alittle but that's mainly due to my stock MM.

 

You can go 3.5" over the axle, but a D/D is 3" so you still have to narrow down to 3".

Mufflex (sp?) makes a 3.5" catback and maybe a 4" system. I liked my system. Pretty quiet unless I wanted to let everyone know I was coming.

 

I personally don't see why you would want to lower the pipe diameter, unless you're hooked on using the D/D.

My setup is a custom Mufflex. They do sell 4" systems aswell like you stated, that also fit over the axle.

 

Yes i like the stock look of the SLP D/D , and sound , its staying on no matter what i do .
thank you for your help

 

Same here. I wish my SS had come with the D/D. I have a magnaflow now which looks good, but the deep sound can be annoying at times with the top down.