After lurcking and posting here for a while, I've come to the conclusion that 2nd cat by-pass would be the way to go. I want a little more sound but not intrusively so. I still want the quiet refined lack of sound for relaxed daily driving.

How much improvement in mpg might there be?

I changed from Chevron to Shell nitrogen because it gave me a 1-2 mpg increase. Besides, I'm leery of techron additive effects on my oil/engine.

I'm assuming there would a reduction in weight.

 

I don't know about your quality of gas, Chevron gas isn't available here in Alberta, but I run it every chance I get when I go out of province. I don't use Shell gas, never had good luck with it personally. I use Husky/Mohawk as the octane rating is higher on all grades, and I get fairly good returns in mileage.

I can't say about the secondary cat deletes, mine were changed just prior to me buying my CTT, but the previous owner said there has been a positive improvement (mainly power and sound). Extremely quiet inside, never intrusive noise in the cabin.

As far as weight savings, I don't think it's enough to really get carried away over (probably 15-20lbs removed), but everything adds up and it all affects mileage somewhat.

Can't speculate on mpg increase, I would venture to say it would be a marginal gain (less backpressure, easier spooling, etc) I'm always stepping on it a little too often to hear the sound it makes...

 

With my driving habits and my fuel bills, I'm figuring the mod will pay for itself--eventually. Besides, it'll be more fun. I plan upon making this a half a million mile SUV. Passed 86,000 recently and darned beast saved me twice in one week towards the end of October. Can't put a price on safety.

I just ordered and got a bunch of spare parts. I figured the prices aren't going down anytime soon--especially for platinum plugs.

 

I just did this mod over the weekend with a 1" pipe due to the ease of installation - one weekend afternoon, cost- which was $10 (already had everything else) and the fact that I didn't have to drill into my muffler tips.

I wish I did a before and after, but I can tell you that inside the car I was initially disappointed and didn't hear any discernible sound difference (with secondary cat bypass and a tune).

Then I had my wife take it around the block and romp on it - and it is loud!

My expression from my driveway was

The sound was loud in a good way to me..similar to a mustang with Flowmasters on it upon WOT. When you are off of it, it is as quite as factory

I definitely will not be shopping around for a new "sport sound" muffler for close to $1K.

Can't wait to see the surprised faces around my area.

Thanks FastHackem for posting this cheap mod.

 

Looks a little like the PSE hack for the 996 and 997. I'll bet it would sound even better with a larger diameter bypass pipe...

 

I did secondary cat bypass AND straight pipes in place of the rear muffler. I like it LOUD. I should have it back later this week and will post a vid.

 

pics

 

Sick!

Any CEL issues? Any problems passing state emissions test?

 

I've got one of each set left:

The 2011 TT:
http://www.sharkwerks.com/products.php?pid=301

And the 2004-2008 TT:
http://www.sharkwerks.com/products.php?pid=310

 

CEL issues wont be an issue since the o2 sensors are located way way way up stream from this mod.

Ergo, passing emissions wont be an issue since a CEL is not thrown.

 

Absolutely right...O2 sensor connected to primary cat

Njoy!

 

http://s1097.photobucket.com/albums/g359/sinister04ctt/

 

So this may be a dumb question - but does everyone do these cat bypasses and homemade weld jobs just for the sound factor, and there happens to be a performance boost? Or is it the other way around?

 

performance and sound. Removing the secondary cats will make it noticably louder but you will get a nice performance gain as well due to removing the restriction in the exhaust. Turbo cars like as little back flow as possible that is why I removed both the secondary cats and the rear muffler and just did all straight piping. The turbos actually noticibly spool up quicker and the torque curve feels a lot flatter all the way through the rpm band now as well as a noticible gain in hp at higher rpm's

 

i've my cat delete pipes on the way and im stoked.