best mods for a limited budget

Guard Dad

Test pipes can generate a check engine light because the ECU can't locate the cats. Test pipes will flunk the smog inspection, which I think is at 6 years.

High flow cats might be OK as far as the ECU is concerned but may or may not pass the inspection.

It's a violation to replace a functioning cat in California but people do it anyway. Be aware that cat replacement can be difficult. some of the bolts are hard to access and they tend to break off.

Intake mods can cause you to flunk the test but they are easy to reverse so lots of folks do them anyway, I like the Stillen G3.

The NST underdrive pulley is cheap horsepower but none of the pulleys that I've seen so far have harmonic balancers (lots of discussion if it's even needed on the Z) so I haven't pulled the trigger on a pulley, yet.

A used cat-back exhaust is good horsepower but check for sound clips as some owners have had issues with resonance or exhaust rasp with some combinations of cat-backs and test pipes/high flow cats.

If you do all three you will probably need a custom tune which is 400-600 bucks but it's worth another 5-10 horsepower.

I'd suggest a Stillen G3 intake and a cat-back exhaust, minimal headaches and about 30 HP.