There should be about 13F outside tomorrow morning, when I will need to start my car ('84 300SD, OM617)...

So far, low 20s F were the lowest temps I tried starting in, and the engine has been catching pretty fast, with proper pre-glow - no hiccups or any need to throttle with the pedal.

My question is, can I get away with a couple 30-sec pre-glow cycles at 13F before trying to start, or should I plug the electrical cord on the engine into the wall? If the advice is the latter, can someone give me an idea as to how long I need to have the thing plugged in before attempting to start?

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84 300SD - bought in April '06 with 237,000 mi