How much he can I get out of a 3800 serious motor
I mean horsepower
Lots. The only real limit will be your budget. There are a number of companies that specialize in parts for the 3.8. I would start by joining a Pontiac GP forum and check out the performance threads to see what others are doing and who they are using for parts. Stay with companies with proven parts and reputations. There are lots of junk parts, and especially junk electronics out there. As with most things you get what you pay for. Do your homework so you get the most bang for your buck. Good luck with your project.
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