In my book, chicks in 'stangs are always better than d*cks in 'stangs
Right now I concentrating on the looks of my car, so I don't know an awful lot about cheap power upgrades. It's kinda hard finding parts for the V6, especially the 99-00. Seems like everything is still being designed for them. I'm waiting until spring to start on that (winter sucks).
One of the cheapest upgrades is to get a K&N air filter, about $40.
You can also remove the air intake silencer, although, I took mine off and it felt as though I lost something, so I put it back on.
Go to mustangworld.com for details.
Cold air induction is also pretty cheap. You can go to the url in the previous post to find what's involved with that.
Those and the duals are the only power cheapies that I know of at the moment, sorry I can't give you more.
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Black V6
K&N
Cobra R's
BFG T/A 275/40,245/45
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near future:
GT duals