1984 Chevrolet C10 Diesel cold start
6.2 diesel, around 20 degrees Fahrenheit. No block heater, only glow plugs, I think it did pretty good.
For reference: This truck originally had an automatic solenoid for the glowplugs, it would switch on with the ignition when the engine was cold and turn off after around 8 seconds. However, with 80's electronics being what they are, and these engines' reliance on the glowplugs to start, it now has a push button on the dash for the glowplugs, and it uses a ford starter solenoid for power. That way I don't have to worry about the auto system going out (like it tried to do a few times) and potentially leaving me stranded (yes, this is a daily driver). I normally don't hold it for this long but it also doesn't normally get this cold either. (SW Georgia)