We get asked constantly, "How do I get my n55 to make 400+ hp?"
Its honestly not too difficult to reach that goal. Most n55's come with enough fueling and turbo range to hit that number so we only recommend just a few modification to hit that juicy 400whp goal. Back when the shop car was less modified, that was our initial goal. It didn't take much to hit that goal.
Mods I had done:
Chargepipe - This is a must on n55 powered vehicles. They all came with plastic ones that can be damaged on stock boost levels. We personally went with Evolution Racewerks but any would work.
Downpipe - To help the turbo breathe better, give the exhaust a better sound and add power, we recommend getting a downpipe upgrade. Whether you go with a catless one or high-flow is up to you regarding the sound you want to hear but either will give you gains. We went with a VRSF catless downpipe on our shop car.
Intercooler - Keep those AIT's down and help build boost. An intercooler upgrade is highly recommended as it helps your n55 keep cool and avoid damaging anything when pushing high boost numbers. We put a VRSF 5" in our shop car and has been working great.
Intake - Help your turbo take in air easier and increase boost. Getting a custom cold air intake can help your turbo breathe better on the intake side just how the downpipe helps it breathe on the exhaust side. Injen was our choice as it shows the best gains and looks the best in our opinion.
Tune - Last but not least, you need to find a good tuner and platform that works best for you so you can take advantage of the parts you installed on your n55. There are many tuners out there right now who are doing great work and on a FBO(Full Bolt On) engine is wont be to hard to get a good tune dialed in.
All of the above were able to yield me 400 whp and 400 ft-tq on my 335i xDrive and helped me win a lot of races. We took her to the dyno and watched our work pay off. The itch to modify started there and got us to where we are now with the shop car.