I do not think there is a tach pin on the OBD port. The tach information is encoded in a data stream on OBD. To get a tach signal you should go to either the ECU, the existing tach, or tap onto a fuel injector or coil-on-plug signal.
Unfortunately I have never had the opportunity to work on a Porsche Turbo although I did have the pleasure of driving one once! Every car is different so all I can offer is general advice. You can try posting in the Support Forum and maybe one of our Porsche customers can answer your question.
Thanks again. I did finally figure out how to do it with the help of Suncoast Motorsports. You can use wire 41 from connector C of the brain, then, after some fiddling, I set the no of cylinders to one, which finally gave the correct RPM on the DU. This works because the wire refered to is from the FI and I guess it all makes sense.
Pin 9 goes to the engine speed signal which is the blackk/violet on the 14 pin connector Pin 10 - not used Pin 11 connects to the knock control signal which is the yellow wire (terminal 50 on the DME) Pin 12 - not used Pin 13 - not used Pin 14 - not used Pin 15 connects to the L diagnostic wire which is orange/black and originates from terminal 13 on the DME Pin 16 - constant hot from teh DME relay.