I have been building racing and high performance road car engines for over 40 years.
I am concerned about the trend in high performance valve springs on the new S55 and S63 engine.
Companies are selling springs with very high spring rates that will rob power and dramatically increase valve train wear.
The goal of a valve spring pressure is to make it as low as possible. This is because compressing a spring takes energy and robs potential power. In addition, more valve spring pressure increases valve train wear. We work very hard at running just enough valve spring pressure to keep the follower in contact with the camshaft but no more. If you increase the RPM you increase the valve spring just enough to manage the inertia from the increased RPM. The valve spring also needs enough seat pressure and the correct frequency not to bounce when the valve is closed. This is to avoid leaking trapped mass (the fuel air charge).
With a boosted engine you will need a slight increase in pressure to push back against the boost pressure in the intake trying to hold the valve open. This amount of pressure added is much less than the pressure to control the inertia of the valve train.
The S55 valve spring pressure is lower than the S65 engine and very close to the S62 engine. BMW worked hard at reducing the mass of the valve train by going to a finger follower instead of a bucket or hydraulic lifter over the valve. This was done to reduce the valve train mass so they could provide extra pressure for the turbocharged engine without increasing the valve spring pressure. This improves long term durability of the valve train.
If you double the boost pressure you do not need to double the spring pressure since only about 20% of the spring pressure is to control the boost pressure and 80% is to control the valve train mass. An additional 20% of valve spring pressure is all you need to run.
Evidence of this is an S63 engine we built that produced 1300 hp and 1300 lb/ft torque at the rear wheels with stock valve springs!
We include high performance valve springs with our engine, however you can purchase the springs separately. Our springs increase valve spring pressure by 20%. We also keep the steel retainers for long term durability.