Can't get near specified burning speed.

My NEC 3500A DVD burner (specified at X16 DVD burn and X48 CD burn speed)
won't burn CDs (52X media from several manufacturers) at larger than X16
speed. Why?
The PC is NOT overloaded. It seems that K3b "uses" DVD setting also for CDs.

OS: Mandriva 2006/ KDE 3.4.2
Burning software: K3b
Hardware: Athlon 3200/64b, 1GB RAM
Max load (according to top) during burn: CPU: 17%, Swap usage (20 MB out of
100 MB available).

Michael Badt