The engine block drain plug is below the #4 exhaust. It is way below and just forward of the engine serial number stamp. It just looks like a lonely bolt that holds nothing on. Don't over-tighten it (stripping the block threads) when putting it back; it doesn't require much torque.
See location photo:
https://www.*****************.com/v/vspfiles/assets/images/29422.jpg
When changing coolant one should always drain the block to get as much of the old coolant out, not draining the engine block leaves a fair amount of spent coolant in the system. Good luck.
__________________
1988 Silver 924-S Original owner
Porsche 924S: The 944's cheaper, faster little sister.
