I bought a 2009 Cobalt SS/TC about 3 weeks ago. The previous owner do a Trifecta tune for it and that is the only non stock for the car. I want to install a downpipe (w/o cat) to start with the mods, but I want to know if I need a re-tune or I can simply install it and keep the current tune?

 

You should retune for it to gain any real noticeable power. Also, you should have the rear O2 sensor disabled to eliminate a CEL.

 

This man speaks the truth

 

if u leave the rear o2 unplugged it won't throw a code, I have had my catless dp on for a little over a month now and have never pluged the rear o2 back in and no code,

 

I have no disable on the rear sensor on my CIA downpipe and no codes!! :P

 

And how about whp gains of the downpipe with my current Trifecta tune? How much can a downpipe can add?

 

I found after i put my downpipe in (after tune) it felt noticeably quicker

On trifecta tune, downpipe and intake i put down 332 whp,393wtq

 

That sound great! I think I will start with a downpipe, then a HP Tuners.