I have a 77 300d and an 85 300td. It was 10 below zero this morning. I plugged in the 77 since it had starting problems in cold weather last year before I started using synthetic oil. The 85 was not plugged in.
The 85 started right up as if it was 70 degrees. The 77 did not start even though the temperature gauge showed the block heater was working. The difference was obvious and I knew the 77 would not start the minute my wife turned the key. The starter simply was not spinning the engine fast enough whereas on the 85 the engine was spinning at its usual starting speed.

Why the difference? I have a new battery in the 77 so I don't think the difference can be attributed to old versus new batteries. Could the difference be in how the alternators are charging the batteries or could the difference be in the starters themselves? Even in warm weather, the 77 does not spin as fast when starting as the 85. Were there different kinds of starters installed on these engines?
It may not be worth trying to solve the problem because we only rarely get temperatures this cold in Denver and the 77 has started regularly without being plugged in at temperatures down to about 5 degrees. However I would like to be able to understand the differences in these starting characteristics. (I know each has different style glow plugs but I don't think this is relevant to the speed at which the engine turns)

Anyone have similar experiences or good hypotheses?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13