my friend had a 2.4 ss. she want to know what kind header down pipe and cat back should to buy for best gain. can any one help her out thanks.
who made and how much HP gains.

 

An aftermarket catback wont do much because the stock piping is 2.5 inch which is what most aftermarket ones are. The only time the pipe goes to 2.25 is the bend over the rear axle. Most header/downpipe combos net around the same whp, you wont get much from them. The best thing to get more HP for the money is to get a really good dyno tune, find a shop that uses HP tuners and has a dyno

 

What transmission does the Cobalt SS 2.4 have?
If it is a manual you can use the LSJ DP/Cat -- it is 2 3/4 OD diameter pipe with a 60% bigger Cat on it.
If it is auto the LSJ DP/Cat doesnt' fit without having a muffler shop cut it apart and welding in pieces of pipe to get it to fit.
The LSJ DP won't cause you to get a CEL/SES light either...aftermarket high-flow or catless DPs will cause a CEL/SES light because the HO2S-2 sensor will read more O2 in the exhaust stream -- unless you can tune with HPTuners...

The Auto LE5 DP is 2" OD pipe. Don't know what the manual tranny DP pipe size is.

She can use the GMPP performance Cat-Back exhaust.

Tell her to buy the K&N Typhoon 2.4L Air intake too!

 

A turbo header.

With a turbocharger .