The 1640 supports faster +R DL speeds (8x) and supports -R DL at 4x with firmware update. It also has increased RW writing speed 6x -RW and 8x +RW. It also has increased CDR writing speeds with speeds increased to 48x CDR writing and 32x Rewriting and 48 reading for CDR.
Specs below the ones with the star beside them are the ones that have changed / Been added since the BenQ 1620.
8X +RW * (4x on BenQ 1620)
6X -RW * (4x on BenQ 1620
8X +R DL * (4x on BenQ 1620)
4X -R DL * (Not available on BenQ 1620)
48X CD-R * (40x on BenQ 1620)
32X CDRW * (24 on BenQ 1620)
48X CDROM * (40x on BenQ 1620)
The BenQ 1640 also has a much better cooling system than the 1620. The 1640 uses a Dual Cooling System (description below)
The BenQ exclusive Dual Cooling System(DCS) features Anti-Dust Cooling System(ADCS) and Air Flow Cooling System(AFCS) to effictively reduce the potential for over-heating during high-speed reading and writing. With an integrated heat chimney design, ADCS redirects the heat flow out of the drive and still provides for protection from dust. AFCS accelerates the circulation insie the drive and speeds up the heat elimination through the drive's steel housing.