I know I am going to start another round of discussion with the enclosed subject, so I thought I would capture the facts first.
• I wrote a program that would write 1000 records on the CT 729 II number 1.
• I used 80 character records with a blocking factor of 1,2,4,8,16 and 32.
• I timed how long it took to write a 1000 records at the various record lengths.
• The attached spread sheets shows the results.
• Saving tape by blocking is a small savings compared to the effective bandwidth of the tape drive at larger block sizes.
If you have any questions, let me know.

Transfer Rates
Sheet1

Sheet 2: Sheet1

 Magnetic Tape No. of units that can be connected 6 Units No. of chars/linear inch of tape 556 Char/inch Channels or tracks on the tape 7 Tracks/tape Blank tape separating each record 0.75 Inches Tape speed 75 Inches/sec Transfer rate 15,000 to 62,500 Char/sec Start time 7.3 or 10.8 Millisec Stop time 7.3 or 10.8 Millisec Average time for experienced operator to change reel of tape 60 - 120 Seconds Physical properties of tape Width 0.5 Inches Length of reel 2,400 Feet Composition Mylar Two tape units can be specified: 729 II or 729 IV. Records Record Length Blocking Factor Total Data Record Time Transfer rate 1000 80 1 80,000 12.0 6,667 1000 160 2 160,000 14.5 11,034 1000 320 4 320,000 19.5 16,410 1000 640 8 640,000 26.5 24,151 1000 1280 16 1,280,000 43.0 29,767 1000 2560 32 2,560,000 73.0 35,068 100% transfer rate 75 ips * 556 bpi = 41,700 Records Record Length Blocking Factor Record Length IN Record Time Records Time Per Cent Transfer Capstaan Time 1000 80 1 0.14 0 1.92 15.99% 10.08 ms 1000 160 2 0.29 0 3.84 26.46% 10.66 ms 1000 320 4 0.58 0.01 7.67 39.35% 11.83 ms 1000 640 8 1.15 0.02 15.35 57.92% 11.15 ms 1000 1280 16 2.3 0.03 30.70 71.38% 12.3 ms 1000 2560 32 4.6 0.06 61.39 84.10% 11.61 ms 11272.78 ms