The 931 - What is it?
The 924 Turbo, aka 931 model. Probably the most economical Porsche with the greatest performance and performance up-gradability of all the cars built in the late 70s to mid 80s. The 924 was the platform which brought Porsche back from the brink of bankruptcy during the oil crunch of the early 70's. The 931 first proposed in 78 was built for the euro market in 79, with a few US spec cars brought to the US in that year. Porsche having learned a lot from its turbo race cars, put the technology to work. At the same time, the factory was planning a series of race cars to promote the 924. The Carerra GT GTS and GTR were built in different stages and configurations from 80 to 81. The GTS was the factory prototype for the soon to be 944 Turbo. The story goes that one of the 924 GT Lemans cars in 80 was equipped with the Prototype 944 engine. After the development of the 928 motor, Porsche took the technology and engineering and built the 944 2.5 liter engine from the ground up, some writers state that Porsche split the 928 motor and came up with the 944 motor, hogwash, I've seen no facts to that.
More than likely due to the dwindling demand for 924 series in the US and the extra effort in making the 924 Turbo; Porsche pressed on with getting the 944 to the European market in 82 and the US market in 83. There are a few 81 944s built by the way. The 944 engine in itself is an interesting design and a separate story.
The 924 continued production until 1988 in Europe. I'm not sure when the crossover to the 2.5 liter engine occurred, maybe our euro counterparts can have an answer o that question. For the US market Porsche offered the 87 and 88 924S with a down-rated 2.5 liter engine. The 924 body style has a better drag coefficient and thus a higher top speed than the 944, that is the reason for the detuned 924S motor (keeping things fair).
Back to the 931. The European 931 kept the same specs throughout production until it ended in 83. The 83 models allegedly went to Italy only. I have yet to see one advertised or an actual photo of one. The US market 80 model has a lower compression and a Porsche designed transmission (aka snail shell G31), this was the same trans used in the GT unit with a few internal parts uprgraded for the extra GT HP. The 80 also had a hybrid half vacuum half digital timing system. In 81 and 82 the 931 went to the same trans found in 944s built up to 85.5 . The engine received a compression bump and full digital timing, a few more options here and there.
The debate over brakes on the 931 is ongoing. The 79 931 were equipped with 924 brakes, disc in the front and drums in the back. The brakes were a little under powered when taking the car into performance driving, and besides there was a push from owners and enthusiasts to have four wheel discs. The 80 cars, at least for most of the US market, I feel were equipped with full disc brakes (as an option). It may be a regional thing but I haven't seen any drum equipped 80 931s. There was an option which delivered the cars with full disc, sport suspension and 16 inch wheels, all the 80's I have seen have had this option. If you find a drum brake 80 931 don't worry, the 944 brakes bolt on with a little work. The 81 and 82 931 came equipped with full disc, the same system used on 944s.
There are so many common parts used from the 76 924 to the end of the early 944, that you can be surprised what can be swapped and upgraded.
The 931 itself can be modified for extra HP. Without going into those mods, I just point out the progression from one model to the other. US 931 154 HP euro 931 177 HP GT 210 HP GTS 245HP GTR 280-345+ God and Ferry Porsche only know.
The blocks stay the same all the way through The crank is the same until the GTR. The pistons change on every model as that is how the compression ratio is set. The rods stay, I believe, the same until the GTR. The head is the same until the GTR. There may be be some valve size differences on the GTS.
The GTR and GTS start getting wild on the intake system. The GT adds an intercooler and the GTS adds a 944 style intercooler in the nose and a 928 fuel distributor. The GTR has a Kugelfisher mechanical fuel injection pump , ram induction intercooled intake and a dry sump oil system.
The turbos on these cars vary and I can't remember the specifics, other than there were like six different numbers for the stock us and euro 931. And it finally got settled down to two numbers, a euro version and a us version, both interchangeable. They spool up at different rpms.
I'm going from memory on this so the facts may be a little different and if anyone knows something for sure, please correct me. Time for me to get back to work.