here is something interesting I found about the S55 HPFP control scheme in boost addicts site:
boost addicts/showthread.php?68823-S55-N54-HPFP
what I understood is that it is similar to an N55 engine up to 3000rpm then the second piston pump starts to cover the extra fueling needed for the M3/M4 engine to achieve its power.
also the difference between N55 & S55 HPFP is one is single piston the other is double piston pump.
I hope someone who is more experienced in bmw engines can help us out here on the possibility of installing it in our N55
