I have a straight pipe and a presilencer, but from there it goes to the stock Y-pipe and stock mufflers. How much power am I losing with these stock parts? is it worth upgrading?

 

Stock Y-Pipe isn't THAT bad... but combined with the Stock mufflers, they are probably costing you 5-10 HP at High RPM over a catback system.

Remember, you unload the exhaust too much on an s4 and you'll have to find alternate ways of operating the six ports.

BUT.... its worth it for the sound of an RX with a pair of raqcing beat muflers on the ***!!

 

I used to have that setup, but with different mufflers, N1's, dont get N1's they are so loud.

 

How much power would you be losing with stock cats and header and aftermarket catback system?

 

on a s4 N/A btw...

 

*BIUMP*

How much power would you be losing with stock cats and header and aftermarket catback system?

 

Neither are the mufflers! This must be one of the urban myths always stated but never proven!

This is not a flame... IMHO the reason a lot of people do the mufflers is #1 the wow factor and #2 the ease of changing the last link in the system. Adding a cat-back system without taking care of the exhaust manifold and the cats is going at the problem from the wrong direction.

The biggest restriction in the stock exhaust system, possibly greater then 80% is the exhaust manifold and cats. Once these are gone the gains from just replacing the mufflers is minimal at best.

Having said all of this, the biggest improvement to power (wow) is the stainless steel 4" tips!, without a doubt the biggest performance gain for the price!

I know this for sure, because I just welded some on this weekend, the car goes faster just sitting there!

 

I have trouble reading your post 'cause you avatar is distracting me

 

 

You are losing at least 14hp per inch smaller than 4" exhaust tip.