how can i make my 3 series bmw go to 400 hp?

Im assuming youre referring to your 330i. The most cost effective way is to sell it and buy an E39 M5. The difference in price of the M5 and your 330i is much less than it costs to go from 250 to 400 hp and you will have the reliability and smoothness of a stock engine. Or get an E92 335i and put a chip in it. It may not be 400 hp but it will have as much or more torque as the 330i would have at 400 hp. If you still insist on tuning your car I suggest you go to a BMW forum such as www.bimmerforums.com or www.bimmerfest.com or www.e46fanatics.com and ask there or just go to/call a shop specializing in BMWs and see what they say.

I agree with your choice to buy an M5. Going from 250 to 400 HP and the added torque affects the entire drive line. I own a 1993 3-series now with mods and it chirps the tires shifting into 2nd gear at 6500 rpm...this is an automatic lol. My solution is ordering a Mustang 5.0 with 3.37 axle and Brembo package followed by a Magnaflow 3" cat-back exhaust, CAI and custom tuning all for well under $40K and mop up M3s and their huge maintenance/repair costs after the first four warrantied years.