A Summary of tuning the BMW N54

We last talked about what tuning is, and how it's potential depends on the strength of the engine.
Today we'll talk about one strong engine, the BMW N54. Some explanations may be over simplified, but is written to appeal to the novice.

The N54 was used in many BMW's from 2006-2009, for example the 3 series.

A 335i is a rear wheel drive sedan, or 'saloon' as they say in Europe. A 335ci is a rear wheel drive coupe. A 335iT is a rear wheel drive Wagon or 'Touring'. They all have the same engine. The N54.

This engine makes around 250-290 horsepower at the wheels in stock form. -Source

2 of the most common tuners for this engine are the JB4 system or a COBB accessport. 

They are similar, just offer some different features and work a little differently. 

A few comparisons: 

http://www.e90post.com/forums/showthread.php?t=688565

http://www.e90post.com/forums/showthread.php?t=659748

http://www.e90post.com/forums/showthread.php?t=513705

Tuning can bring you up to near 400hp with no internal modifications. That's remarkable considering that's around 50% more power than it originally had. 

(With other engines, if you start to make more than around 25% more power than it originally had, the engine would be very stressed and you would have to strengthen internal components. It's very expensive to take an engine apart and put it back together).

All it really needs are bolt on mods. Bolt on mods are things external to the engine. Things like the air intake and filter, exhaust, intercoolers. They are supporting mods. They keep the engine cooler and make up for the heat and stress of running the turbos harder than normal.

A bigger intercooler will help alot, as cooler air is more dense. An exhaust system and an intake system will allow the engine to breath better and will spool the turbo up quicker. Imagine that the stock air filter is like you breathing through 1 straw. It's kinda hard. An aftermarket air filter system would be like you breathing through 5 straws at once- much easier. The better the engine can breathe, the more power it can make.

This guy, talking about the different mods you can do to the N54, sums it up pretty well:

"A Tune- Holy CRAP, best $500 I ever spent. 

Intake System- Good, relatively cheap supporting mod.
Downpipes- AR (3")are about $800, Raceland (2.5") are $220). ARs sound better and feel A little more powerful (butt dyno probably) but not worth the $500 difference IMO. Build quality on the racelands has been proven to last but occasionally someone has a fitment issue with them and the company replaces them. 
Charge pipe- Meh. Holds boost A little better I guess? I got mine so that I'll have bungs ready for when I install meth but if you are planning to stay meth-free then I wouldn't bother.
Intercooler- Makes a huge difference and once you have a tune you'll pretty much need one anyway. 
Sport tires instead of the stock run-flats- Makes a big difference. Saves weight, money (run-flats are super expensive), and handles way better."
-Source (quote was slightly modified for language and clarity)

Tuning 101

This post may be oversimplified for car enthusiasts, but it's meant as a brief description of what 'tuning' is for those who have never heard of it.

Before we begin:
Remember that more air in the engine equals more power.
Atmospheric pressure is roughly 14.7 PSI.
A normal engine can not take in any moire air than atmospheric pressure will allow.
These are called naturally aspirated engines, or N/A.

Now, there are two devices that can force more air into the engine, turbochargers and superchargers.
If your engine is equipped with either of these, it has quite a bit more potential. The extra air pressure created is called 'boost'.
Superchargers are more mechanical, run by a belt.
Here we will focus on the turbocharger.

The engine's brain:
The engine is controlled by a little computer called an ECM, or engine control module.
This runs software written by the manufacturer that controls how much boost your car makes.

What tuning is:
It's possible to rewrite this software, and make a turbo produce more pressure.
It's a delicate process, as every one change requires another to maintain the proper fuel/air ratios for safe engine operation.
The potential can vary from a small increase in power to incredible increases, depending on how strong the internal parts of the engine are and how much it can handle before breaking something.
It's important to note that many factory turboed engines are quite limited by their manufactures, for many reasons (in order to not compete with their more expensive models, or in order to keep fuel economy high, among other reasons). Unlocking this potential, in a nutshell, is what engine tuning is all about.