The best HP for the money would be found by adding a cold air intake and a free flowing exhaust. After that, you could look at getting a programmer, headers, supercharger, nitrous, etc.... It all depends on how much you want to spend and what you are trying to achieve.
Visit ZZ Performance. You'll likely find some goodies you like there.
M90 or gen 5 m90 is what I hear everyone does. I want one with an m112 but can't seem to find where I could purchase the plate for it to fit an l67.
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