Most of the obvious signs of authenticity are not mechanical. Nevertheless, it is worth checking the options code on the bonnet sticker, and in the car's log book for evidence of factory modifications. Given the age of the cars, many have been modified by aftermarket specialists.
Common mechanical features include:
A Limited Slip Differential (LSD) remained an option on the earlier cars, though most left the factory specified with one. The option code will be in the owners log book.
Porsche offered engine upgrades from first generation slantnose models through to last of the line cars. Most common was a Turbo S kit that increased power to 330BHP and comprised fitment of SC cams, a K27 turbo, a 1.0 bar boost spring and a 4 outlet free-flow exhaust system. The original ignition timing sticker on the fan shroud is a good indicator of factory provenance. If it is specified as 29 degrees BTDC, it is likely to be standard 930, if 25 BTDC it is likely to have an upgrade kit. Cars fitted with the 330bhp kit usually have a larger intercooler and the engine will have a '930/66 S' stamp on the block to confirm that it is specified with this kit. Some cars have modifications that take power to 385 BHP and 400 BHP. However, most with outputs higher than 330 BHP have benefited from aftermarket modifications.
The Euro / RoW slantnose models had a front-mounted oil cooler, with pipes that run through the luggage compartment under the fibreglass covers. The M505 (US spec) cars had an oil cooler fitted behind the rear right air duct due to restrictions in some US states where safety of the front cooler setup was questioned. As a consequence, the M505 models left the factory with the standard 930 front valance and fog/driving lamps, compared to the Euro models that had the deeper front air dam.
More details can be found on:
If you have found what seems to be a genuine car, next you also need to consider a buyer's checklist, find out more