Which of these two Y-pipes is best for an 04 coupe with the rest of the exhaust stock? and What makes one better than the other? Also would like to know, will this take away from the tone we all love of the stock exhaust, or will it still sound pretty much the same, just a lil louder?

JIC Spartan or Stillen?

 

With all cars you have manufactures that claim HP gains with almost everything, intakes and y-pipes. Sorry to lay this on ya but it's not true, buying a aftermarket y-pipe is a waste of your hard earned cash! You'll see no change in sound or seat of the pants HP. The only gains with our NA cars can be found with Motor Dyne spacers and actual cat back exhaust systems, Stillen and HKS are the 2 I've been using....
Gary

 

I know with the Altimas the Y-pipe does increase the HP. Not sure about the RWD platform