Porsche 911 GT3 Owner's Manual page 129

Starting the timing
f Push operating lever forward.
The selection field is switched on.
f Select CHRONO with the operating lever.
128
Instruments, On-Board Computer
f Push operating lever forward. f Push operating lever forward.
The time runs on all stopwatch displays.
The on-board computer display changes to the
"Stop timing/Intermediate time" selection.
PCM information
The performance display in the "Trip/Sport
display" menu must be selected in order to
analyse the data in the PCM.