2003 s40 HP boost
 

I live in California which means my car gets smog checked every other year and as of 2019 I have to keep my exhaust noise below 95 decibels. What are some mods I can do to increase my hp while complying with the law. I have a 2003 s40 1.9t. I would like to start off with something that won't be too expensive, my budget is around $1,500 the lower the better.

Trade it for a 2005? ;-)

Given your car is a turbo, there's three things you can do: 1) install a bigger turbo. The 2.0s in the US are a low pressure turbo design using a 12T turbo - you can fit a 14T turbo (found on select S60s) to make it the same as the Euro T4s. Its not a direct fit - there's some realignments required but can be done 2) install a bigger downpipe. Generally speaking, performance parts for the 2.0s are hard to come by so you may need to find a European supplier with a US distributor. 3) install an ECU tune. This should not hurt your emissions as mostly this will set a high peak boost and enrich the mix under certain conditions like during hard acceleration. Most emissions are steady state. Again there's not too many vendors for 2.0 ECU tuning, which is also hampered by the ECUs design (no softloading). Check out Rica and BSR. Note you may need to install a celboss for the rear 02 sensor to avoid a check engine light... As Habby noted, the 2005+ models are way easier for finding aftermarket parts.