Honestly, make sure you're maintence is up to date. Meaning air filter, oil change, fuel filter, and spark plugs. Then when all things are taken care of you have exhaust and intake. Really when looking at those two things, BMW's aren't all that restrictive in those area's.........although when doing those mods they sound nice . If you don't mind bumping up to premium fuel, try to get a simple shark chip........it's not all that pricey ($375 i think)......get the conforti one.
You won't get a lot of power out of that engine without a lot of mods. For a lot less money you can do awesome things with the suspension and tires and be faster (except in a straight line) than you would be with more hp.
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