Engine and cooling
The 924 Carrera GT’s engine is designated M31.50 with engine numbers beginning 3150. It generates 210 bhp at 6,000 rpm, or 105.8 bhp per litre, at a low 8.0:1 compression and was apparently the most fuel efficient engine in the 924 range. The M31.50 uses the standard 1984cc cast-iron Porsche 924 block and uprated parts as used on the M31.30 Series II 931, such as the aluminium head, camshaft, and cooling system. There are few parts unique to the 924 Carrera GT, namely the lighter forged pistons made by Kolbenschmidt, a slightly larger version of the 931 KKK turbocharger running at 0.75 bar, and a unique – although still similar to the European Series II 924 Turbo unit – Digital Ignition Timing Computer (DITC).
The intercooler on the GT was situated on top of the engine, where as the intercooler on the GTS was placed at the front of the engine bay.
Apart from lighter pistons, most of the engine internals are common to the Porsche 924 Turbo, which itself shares the majority of its parts with the 924.
The GT shares the bottom end with the NA. The head and turbo ancillaries are squeezed into the bay tightly though meaning some parts, like the starter and alternator are in different places.
The head is from the 931 Turbo model. Valve stem seals can fail, over time, indicated by a smoky engine. Its design means the engine becomes an interference one though so timing belt failure is a big problem on this car. Check when it was last done. It is not an overly tricky job but gets neglected.
High quality mineral oil is a must for the Turbo. In period the oils available were not up to the stresses of the heat generated and quickly broke down, leading to premature failure. Regular oil and filter changes will look after the turbo much better. If buying a Turbo, see how the owner switches off the engine. The turbo needs time to run at idle speed to cool and allow oil flow. A red hot turbo not allowed to cool down will prematurely fail.
Look for smoke on start up and through the rev range on a turbo. Ideally, on the test drive, get someone to follow you to see what comes out of the exhaust. A turbine seals failing will use a lot of oil as will valve stem seals.
Due to the heat generated by the engine, check all electrical connections and wires in the engine bay carefully- they lead a hard life and shorts here can cause intermittent running faults.
If you treat the GT for what it is, an exotic engined sports car, and maintain it well there is no reason it shouldn’t be reliable. Be aware though that unique parts will be pricey and you may have to wait to source one.
