I am undecided: logic tells me that the 2.7T should handle better (less weight on the front, more balanced weight distribution). The 4.2 V8 is certainly intriguing.

Either way, the project will include upgrading the 2.7T with larger turbos, FIMC, injectors, etc. to approach RS4 levels.

Can the 4.2 also be upgraded? Adding turbos (like in the RS6), or is the block entirely different from the RS6?

Ok, so here we go(But you should really SEARCH)

Handling:
The only reason the B6/B7 is heavier is because they are physically bigger cars than the B5.
The B6/B7 S4 handles better because the 4.2 has an aluminum block which weighs less than the 2.7t which has an iron block, two turbos, and much more intake piping under the hood. The B6 has much less weight hanging over the front wheels than the B5, which makes the car much nicer in the curves. Dont get me wrong though a B5 can be made to handle quite well with some decent tires, coil overs, and sway bars. But a B6/B7 is a better starting point as far as handling.

Power:
The B5 simpaly wins. The power potential of the 2.7t is much better than the 4.2. With just a chip a B5 will be faster than a B6. Even with a supercharger on a 4.2 they make lower power numbers than a stage 3 2.7t.

Headache:
Buying a B5= Buying a project car. Every day is an adventure
Buying a B6= Buying a nice fast dd that you dont really need to worry about leaving you stranded.

I have had my B5 S4 for 6 months and have spent about 3k in maintenance(thats just parts i do all my own work) and am about to pull the engine for the second time, but it is to go stage 3.

Thanks, skerbman, these are helpful comments.
Just this minute returned from a test drive of a 2001.5 Audi S4 Avant, and drove there with our 1998 A4 1.8T (K04 chipped, 18.5 psi).

Handling: the old A4 B5 drives like a go cart. The 2.7T felt heavy (heavy steering) and much less lively. It had 2002 GIAC chip in (no other mods). Power starts coming on at 2500 RPM, then soon starts to fly.

Very solid feel and handling, but clearly feels its 'weight' (not sure if there is actually a substantial weight difference, in total mass as well as weight distribution).

To get into the 400HP range leaves the option:
- B5 - really high output 1.8T (handling!), but 600 lbs curb weight advantage!
- B5 - 2.7T with K04s and upgrades
- B6 - 4.2 (someone told me the 4.2 block is the same as the twin turbo 4.2; so upgrading later to twin turbos is possible?)

Here are some numbers from Yahoo auto and C-D test or Audiworld:

Curb weights and distribution:
B5 1.8T, 170 HP, 0-60: 1.4L 16,4 8.0 (2001) Curb Weight MT 2998 lbs - 61:39
B5 1.8T, 150 HP, 0-60: 8.1, 1/4: 16.2 (2000) Curb Weight MT 2998 lbs - 61:39
B5 S4 2.7T, 250 HP, 0-60: 5.5, 1/4: 14.2 (2001) Curb Weight MT 3593 lbs - 59:41
B6 S4 4.2, 340 HP, 0-60: 5.1, 1/4: 13.7 (2005) Curb Weight MT 3825 lbs - 53:47

Keep in mind a stage 3 S4 build will run you around 10000. I had a 2006 gto befor my S so the S feels nimble
If you get some coil overs it makes the car feel much much better. Being that far in the air these cars really tend to wallow with all the weight.

I hope to skimp a bit and not do a COMPLETE/FACTORY RS4 built without giving TOO much away (but larger turbo for example).

From a cost standpoint (your $10,000):
- deduct cost for turbos
- not sure if factory S4 fuel pump will do. Have Walbro 255.
- deduct labor
- intercooler solution for much less cost than included in the 10,000 price tag.
- straight pipes/exhaust can be done for much less costs.

ECU/chip upgrade probably needed (not DIY)
MAF, Injectors needed
Intercooler/turbo piping needed
Intake ??? Might be able to live with stock without too much sacrifice?

dream come true would be a6 with stge 3 2.7 and another a6 with 4.2 and PES supercharger. Talk about $$$ though.

1. Giving a 1.8T 400hp is much more expensive than an S4 given that you have the car already. You would need a built motor and big turbo setup

2. This is your best option for 400hp, as a tweaked stage 2+ (stock turbos) could do it.

3. The information you got about the blocks is completely false. You can supercharge the 4.2 V8, and that will in fact give you 400hp. We currently have the fastest supercharged B6/B7 4.2L record on one of our customer B6 S4s.


- Why are you deducting cost for the turbos?
- The stock fuel pump should be alrite
- Your intercooler solution at the cheapest will still be $250+.
- You do need software to make the power
- You do need the K04 inlet pipes
- RS4 airbox is fine, or you can run an open element on an RS4 or custom MAF housing