No "algo" in '87. Only program trading, where the program was manually executed. Automation was only operating at the order transfer and communication level.
Most of the '87 crash selling took place 4 days before the crash starting Tuesday. By the next Monday, crash day, in the morning the DOW was down 150 where it bottomed. Then, in the afternoon, the specialists walked off the floor. No one there to make the market so all stocks were artificially marked down. That's when the DOW dropped 350 to down 508, 60% of the 5 days of downside. It literally gapped down intraday.