I think it is better to just source a complete RS4 engine ~5k

What do you call a "good clutch" as example? SRE? Southbend? SPEC?

What about downpipes/Intercoolers?

Two questions.  How deep are your friends pockets?  How skilled is he with repairing cars?

The S4 to RS4 conversion is a huge ordeal.  Everything will break at least once.

I'd recommend against getting just K04's.

Get FT's or 605's or RS6-R's.  They can push the car to the limit and potentially beyond, K04's are a bit under the engine mechanical limit.

That's my advice, unless you come across a full setup that somebody has already, you should just toss in K03's and sell the car - then rebuy a done car.

There is not really a need to change the whole engine.

* OEM K04 turbos, I would personally advise against using china stuff like FT, but that's just me ...
* RS4 intercoolers with meth injection OR wagner IC's.
* RS4 MAF or hitachi MAF in big housing
* 550cc EV14 injectors
* Bosch 044 fuel pump in tank
* 2.5" downpipes into 3" or 3" downpipes into 3.5".
* Clutch...
* Upgrade the brakes.

Honestly, for a few thousand euros, just fit an Aquamist HFS-3 water/methanol injection system, replace the CHRA's on the K03's and have it mapped accordingly.
It will make about 350hp...
Upgrading a S4 to RS4 spec is pointless, because you are going to spend tons of money, and when you are done, you could have just bought an RS4 in the first place, which is going to hold it's value much better.
Not to mention it looks much better than an S4...

I might be a bit biased though, as I own one

Thank you all for your various opinions and advices, this is highly appreciated.

I have toalked to the owner of the car and told him accordingly which options he has and that he will not be very happy spending tons of money on the S4 engine, ending up NOT having a RS4 engine ^^

The cylinder heads from RS4 seem to have bigger ports also, so he will run from one restriction into the next....

My advice was to fit 2 new k03 turbos, use some decent downpipes with removed pre-cat and some exhaust system, maybe intecoolers upgrade and a remap and call it done.

The clutch will start slipping sooner or later (I guess sooner) so he will want an better clutch shortly after.

For an daily driver I guess its not bad to have the "small" k03 turbos, as they make full boost at around 2000rpm, whereas the k04 spool a tad later (*2500rpm)

As I run a turbo business I think I can make him a good offer for brandnew k03 turbos, a repair with CHRAs is not possible anymore because the housings are damaged also.

The k03 cost about 630€ ex. VAT per piece as a brand new part, the k04 about €1.000 per piece....

You're almost 1000 RPM off on this. Full boost in 3rd is closer 3k on K03 and 3.5k on K04.
Of course stock is a different story, but they barely run any pressure stock

Basically, i agree with PRJ:


personally, I have had good luck with knockoff K04s but unless you have a reliable supplier who is willling to work with you, I can understand why avoiding them would be a good idea.

I've also had one car with 044 issues; ended up with different pump (there are a lot of options here, ymmv, don't know what is good and available in the UK).

I've only logged/dynoed one car with wagner ICs so far, but seem to be an excellent, high quality, well performing product.

the only head/motor work i'd suggest is thicker head gaskets to reduce C/R if you aren't running water/meth and/or have to run crappy fuel - 91oct (95 ron) here in the US is the norm.

also suggest replacing throttle body boot with something more solid (or something like APR's bipipes), it almost always fails, along with the f-hose... but now we are talking about stuff easily found discussed on other forums...

the b5s4 is going to get harder and harder to maintain; recommend your friend start educating themselves on problem areas that need to be watched carefully from here on out..