If you put on a bunch of stickers, weld on the biggest spoiler you can find, and get 4 12 inch Rockford Fosgate woofers with a good amp: it will increase horsepower.

Trust me.

That works on every Honda Civic I see around here.

 

By warrantied I meant needed, sorry. If you do some searching you will find a lot of tech stuff that pipes 3" or bigger can actually have a negative effect on a stock motor. Causing loss of torque and HP...

 

I added a 3.5" stainless tip to my "pencil pipe".

 

Flowmaster SI/DO 50 series with 2.5inch pipes. If I lost any power I couldn't tell it. The darn truck is slow to begin with. Love the sound though.