Weird, have you measured to double check?

After the Y, the Magnaflow dual split is bigger than stock

 

Yep and just googled 2006 monte exhausts size autowharehouse. Com says stock size is 2.25 inches

 

The rear y pipe came with the cat backs I believe the inlet side by the resonator is 2.25 the outlets are 2.5

 

Good to know on the Doug Thorleys. I wonder why they'd do that since they don't provide a complete downpipe solution anyhow, unless that's changed as well?

On the 7th Gen Montes, the 3500 uses a 2.25", the 3900 uses 2.5" and the LS4 uses 2.5".

Here's Walker's website with 3900 exhaust replacement components:


Walker Exhaust Master Catalog

 

Thorley told me they had every thing I needed do a complete header install. Judging by the picture I posted it looks like the down pipe is there
Front header, rear header, crossover and a y pipe going to the cat
Will find out if this is true when we go to install

As for exhaust size Am confused now
But am happy with the magna flows and headers all 2.5 so guess it is what it is

Auto parts Wharehouse show replacment mufflers for the LS4 Monte SS as having a 2.25 in let so i dono conflecting stats

2006 Chevrolet Monte Carlo SS - Auto Parts Warehouse)

 

It doesn't include the full downpipe to the cat-back flange but replaces it up until the cat converter where I assume they expect you to cut and weld.

Looking at your pic, if the crossover pipe is 2.5", the Y collector exit looks like it could be 3", looks bigger. The Thorley headers I helped install a couple years ago were definitely 3" at the downpipe exit because we got it macgyvered up enough for him to drive to Solo and have a full 3" exhaust done.

 

I'll keep you all in the loop when the install happens

What they did and any problems that came up

This is all new territory for me so will learn as it goes

Plan is to have them in place by end of March or beginning of April

 

Guys, go to auto anything.com and search for the catback magnaflòw system and download The PDF installation instructions. Not sure if that will help in figuring out the size

 

Confirmed exit on y pipe is three inches

Iam not really wanting to replace my whole exhaust

Hoping for a work around

 

Beautiful headers... It will be well worth any modifications that are necessary...