Upgrading to a K04 turbo, aim of 300bhp!

Maverick touched a lot of the points but some clarification:

The stock turbo will do about 200hp, and a chip will see you most of the way to that. Probably 190ish. 210 simply isnt possible without a very aggressive tune and methanol injection.

You need to understand the differences between these engines and the A3/Golf units when planning mods. A lot of subtle differences add up to make them quite different.

The S3 injectors you have wont fit. You need EV1 style injectors. C20LET yellows are a common option, but probably arent big enough for 300hp. I'm currently using injectors from a Vauxhall Astra VXR. There is no bolt-on k04 turbo that will do 300hp on these engines either. The K04 from the S3/TT BAM engine doesnt fit, as the exhuast manifold positioning is totally different on the A4 engine. The name "K04" is a family name that describes a model of turbo, and within that model are hundreds of variants and power levels. There does exist a "K04" that bolts on, but they're pretty weak things and will top out around 230hp.

Unfortunately, there is no easy bolt on options for these engines due to the fitment differences.

Many people look to hybrids, where they take the internals from another model and fit them into the standard turbo housings, and these do well on golfs and A3's, but again with the A4 things are different. The turbine housing is more restrictive, and as such a hybrid that will make a comfortable 300-330hp on an A3 (like mine!) will be choking on itself at 280hp.

Your best bet if you really do want 300hp, is to get a K04 from the TFSI 8P S3, (K04-064). This comes as a complete unit with a built in exhaust manifold. However this doesnt fit the 1.8T cylinder head, and thus requires an adaptor plate. It will also then require a custom exhaust and custom intake and boost pipe work. Its a lot of work. But done right with the appropriate parts, you'll see well over 300hp. 350ish is likely.

The engine internals are ok, but the rods are a weak point. Various people have various ideas as to how far you can push things. Ofcourse these are old engines now in varying levels of health, so its not really possible to say xxx torque is safe. I'm running 280hp/290lbft on a stock engine. Many would suggest thats living on borrowed time. Clearly if you go down the K04-064 route, your probably pushing things quite far with a stock engine. One option is to have it tuned conservatively without a large torque peak etc and it might well survive.

You will also find getting these engines tuned is difficult. Again, unlike the A3/Golf, where tuning is as simple as reflashing the ECU, these cars require a physical chip soldering in. This is fine for a simple stage 1 type map, but not feasible when you've got lots of custom parts and are needing a custom tune for 300hp. Luckily, certain A3 ECU's are a straight swap, and can be flashed just fine. You'll need to find a custom tuner though, your local speed shop that does flash maps wont be any use.