I know that it's better to keep the stock air box and put in a better filter but I have no money for a full exauhst system. Any suggestion on a cheap modification?

 

You got the wrong car if you want cheap HP. Unless you're going to go forced induction, it'll be pretty hard to get much power out of these engines. Plenum spacer, and a tune are "cheap" mods for HP.

 

test pipes

 

This question is annoying and has been asked so many times I can't count:

https://my350z.com/forum/2003-2009-n...heap-mods.html

https://my350z.com/forum/engine-and-...big-gains.html

https://my350z.com/forum/intake-exha...d-exhaust.html

https://my350z.com/forum/2003-2009-n...or-your-z.html

https://my350z.com/forum/2003-2009-n...-for-350z.html

https://my350z.com/forum/2003-2009-n...heap-mods.html

Please search before creating threads, obviously this platform has been around for a long time and any mod has most likely been already done on the car