Lanz has very good point.
All depends what you want, this is the most important !
If you want to be unique S38, is the answer. If you want to be cheap throw S62-B50. Delete the Vanos system and put cheap Haltec model stand alone and with nice exhaust you will make near 400RWHP (at the wheels) If you want Vanos put another 5000$ extra for Pectel stand alone with BMW double vanos control as the factory set up.
To reach 400hp out of 3.6 at the engine is impossible unless you do heavy work. I have spoken with
the best engine S38 builders who spent a LOTS of money. They are members hear,
Janne (MM-5) and Jari (Camber)
You need for S38.
- 3.8 parts in order to increase the displacement. 3.535 is not enough.
- Professional Head work 5 angle job and port the intake channel
- 50mm TBS 3.8, 3.8 Intake, or ITG air filter Dyno will show..
- Min 12:1 compression ratio
- Shricks intake minimum 296.. Exhaust...
- Min 7500rpms
- 39mm Intake valves.
- Custom Headers or M88/3 but questions.. You need to see the headers on M49 race CSL engine produce 470hp at the engine. Each pipe is huge
- Haltec stand alone or some other your choice.
- Custom 3 inch Exhaust.
All said above is starting from $7000+ and like a snow bowl you stop were, their no more down hill...
S62-B50 are going dirt cheap. You can get almost 400RWHP with custom long tube headers, exhaust and professional tuning.
My experience to hop up 3.6 for cheap is to use 3.8 cam gears, Mark's EAT chip and dual 2.5 inch exhaust starting from the headers. Magnaflow dual 2.5 inlet and 2.5 outlet in order not to lose the top end power and you will be happy for the money you spent. 280 Shriks will help too. Its all what you looking for man.
Be careful with E39 M5 it can cause you white hair
Just my 02 cents