924 Turbo
Porsche executives soon recognized the need for a higher-performance version of the 924 that could bridge the gap between the basic 924 and the 911s. Having already found the benefits of turbochargers on several race cars and the 1975 911 Turbo, Porsche chose to use this technology for the 924, eventually introducing the 924 Turbo as a 1978 model.
Porsche started with the same Audi-sourced 2.0 L I4, designed an all new cylinder head (which was hand assembled at Stuttgart), dropped the compression to 7.5:1 and engineered a KKK K-26 turbocharger for it. With 10 psi (69 kPa) boost, output increased to 170 horsepower (130 kW). The 924 Turbo engine assembly weighed about 65 lb (29 kg) more, so front spring rates and anti-roll bars were revised. Weight distribution was now 49/51 compared to the original 924 figure of 48/52 front to rear.
In order to help make the car more functional, as well as to distinguish it from the naturally aspirated version, Porsche added a NACA duct in the hood and air intakes in the badge panel in the nose, 15-inch spoke-style alloy wheels, four-wheel disc brakes with 5 stud hubs and a five-speed transmission. Forged 16" flat wheels of the style used on the 928 were optional, but fitment specification was that of the 911 which the 924 shared wheel offsets with. Internally, Porsche called it 931 (left hand drive) and 932 (right hand drive), much like the 911 Carrera Turbo, which had been "Type 930". These designations are commonly used by 924 aficionados.
The turbocharged engine allowed the 924's performance to come surprisingly close to that of the 911 SC (180 bhp), thanks in part to a lighter curb weight, but it also brought reliability problems. This was in part due to the fact that the general public did not know how to operate, or care for, what is by today's standards a primitive turbo setup.
A turbocharger cooled only by engine oil led to short component life and turbo-related seal and seat problems. To fix the problems, Porsche released a revised 924 Turbo series 2 (although badging still read 924 Turbo) in 1979. By using a smaller turbocharger running at increased boost, slightly higher compression of 8:1 and an improved fuel injection system with DITC ignition triggered by the flywheel, reliability improved and power rose to 177 horsepower (132 kW; 179 PS).
After a successful sales run of both naturally aspirated and turbo models, in 1980 Porsche surprised everyone and release the 924 Carrera GT, making clear their intention to enter the 924 in competition.
By adding an intercooler, increasing compression to 8.5:1 as well as various other little changes, Porsche was able to develop the 924 Turbo into the race car they had wanted, dubbing it the 924 Carrera GT.
Visually it differed to the 931 in that it had polyurethane plastic front and rear flared guards, a polyurethane plastic front spoiler, a top mounted air scoop for the intercooler, a much larger rubber rear spoiler and a flush mounted front windscreen. It lost the 931's NACA duct in the hood but retained the air intakes in the badge panel. This more aggressive styling was later used for as motivation for the 944.
In order to comply with the homologation regulations, the 924 Carrera GT and later 924 Carrera GTS were offered as road cars as well, producing 210 and 245 hp (157 and 183 kW) respectively. Clubsport versions of the GTS were also available with 280 hp (209 kW; 284 PS), and factory included Matter rollcage and race seats. 924 Carrera GT variations were known by model numbers 937 (left hand drive) and 938 (right hand drive).
The ultimate development of the 924 in its race trim was the 924 Carrera GTR race car, which produced 375 horsepower (280 kW; 380 PS) from a highly modified version of the 2.0 L I4 used in all 924s. In 1980 Porsche entered three 924 GTRs at the 24hrs of Le Mans, which went on to finish 6th, 12th and 13th overall. Also building a 924GTR Rally race car, and 2 other GTR's (Miller & BF Goodrich).
Lastly, in 1981 Porsche entered 1 of 2 specially built 924 Carrera GTP's (Aka: 944GTP Le Mans) which Porsche Motorsports introduced a new prototype highly modified 2.5 liter I4 engine. This engine sported 4 valves per cylinder, dual over head camshafts, twin balance shafts and a single turbocharger K28 to produce 420 hp (313 kW; 426 PS). This last variant managed a 7th place overall finish and spent the least time out of any other car in the pits. This new 2.5 liter configuration engine is the predecessor of the 944 platform(s) and the later quick 944S 16V M44/40 power-plant.
Production of the 924 Turbo ceased in 1982 except for the Italian market which lasted until 1984. This is due to the restrictions on engines larger than 2 liters, putting the forthcoming 2.5 liter 944 into a much higher tax category.
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