Successful tuning of BMW M42 should start with purchasing of M50B25 / M54B30 . If you still want to improve your M42 motor, then you need to buy Dbilas camshafts with duration 264 or 272 deg. In addition you will also need to buy stronger springs, cold air intake, performance exhaust system and ECU modification to suit all these upgrades.
Such modifications will provide us with extra 30 HP and sag and low rpm. If you need to obtain even more HP, you can buy individual throttle bodies and add productive camshafts and cylinder head porting.
Our M42 will eventually become more powerful but at the same time more expensive for maintenance and rather uncomfortable for city driving.