Originally Posted by Rshane
The best way is to supercharge. The other way is to make small gains by buying many little bolt ons that will net you about a 5th of the horsepower at about half the price of supercharging.
I dont know if "best" is the correct adjective "most" power?..sure..but then you are going FI..completely different animal..
OP is looking for cheapest ways and they have been suggested..removing intake/exhaust restriction is the cheapest..lots of parts are available for sale second hand to minimize cost.