Honestly, not much. By a seat of the pants (butt dyno) feel you'll barely notice a difference. I have them and think that whatever I noticed was mostly my imagination. I would skip them and spend the money elsewhere, like maybe an off road midpipe. Even a CAI is money better spent in this situation imo.
i wouldnt do bigger than 75mm throttlebody and plenum since its n/a. you will also need a tune cause it will run lean cause of alot more air going in. also, the better it sucks air in, the better it needs to let it out, put an x-pipe on it. then there are also gears that can be put on it.
I have a BBK 75mm throttle body and C&L plenum I will sell cheap if you interested. The only reason I took them off was for the KB SC. How does $125 sound only has a few thousand miles on it:bigthumbsup
PM me your address and I can find out what shipping would be.
As for swapping out your h pipe, it depends on what your looking for sound wise, hp wise your only looking at a couple horses if that. X pipe is more raspy like Nascar and H pipe sounds more like the classic muscle car...plenty of sound clips on youtube
Anything you do to your car as far as performance goes is going to affect your gas mieage. If your just driving it not so much , but if your into the throttle all the time than ofcourse. One thing it will do is give it a deeper tone and better acceleration
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