raw vs block speed difference?

["Aaron J. Grier" <agrier%poofygoof.com@localhost>]

I've got 5.1_RC3 on a dell poweredge 2650 I've been eyeing for a backup
server:

ahc1 at pci3 dev 6 function 0: Adaptec aic7899 Ultra160 SCSI adapter
ahc1: interrupting at ioapic1 pin 14
ahc1: aic7899: Ultra160 Wide Channel A, SCSI Id=7, 32/253 SCBs
scsibus0 at ahc1: 16 targets, 8 luns per target
scsibus0: waiting 2 seconds for devices to settle...
sd0 at scsibus0 target 0 lun 0: <SEAGATE, ST373453LC, 0006> disk fixed
sd0: 70007 MB, 31310 cyl, 8 head, 572 sec, 512 bytes/sect x 143374744 sectors
sd0: sync (12.50ns offset 63), 16-bit (160.000MB/s) transfers, tagged queueing


rsd0a: 1342177280 bytes transferred in 17.588 secs (76312103 bytes/sec)
sd0a : 1342177280 bytes transferred in 40.806 secs (32891664 bytes/sec)

why is the raw device > twice the speed of the block device?

-- 
  Aaron J. Grier | "Not your ordinary poofy goof." | 
agrier%poofygoof.com@localhost