The engine of the 956 was a six-cylinder boxer engine with water-cooled cylinder heads, air-cooled cylinders and four-valve technology. It was based on the Indycar engine developed for the Porsche Interscope project in 1979. Designed at the time for methanol, this 2.65-litre engine was derived from the unit in the 935/78, which had a displacement of 3.2 litres and produced more than 700 PS. In order to be used in Group C, the Type 936/81 engine was heavily revised to meet the Group C fuel consumption formula. The maximum engine speed was reduced and the boost pressure and compression was adjusted.
The Type 935/76 engine of the 956 had a 92.3 mm bore and 66 mm stroke, resulting in a displacement of 2649 cc. Boosted by two KKK Type K26 turbochargers with a maximum pressure of 1.2 bar, it delivered its maximum output of 620 PS at 8,200 rpm and the highest torque of 630 Nm was available at 5,400 rpm. In the years that followed the car’s debut, this engine – which in essence was barely changed – was modified in terms of bore and stroke to produce further engine versions with displacements of 2994 cc (Type 935/79, 935/82, 935/83) and 3164 cc (Type 935/79, 935/86).
Racing engines based on the 911’s ‘Mezger’ engine
As was the case for all six-cylinder boxer racing engines built up to that point, the engine designed by Hans Mezger for the first Porsche 911 in 1963 also formed the basis for the Type 935/76 engine. The crank case, which was divided vertically into two parts, was based on the 911 (930) Turbo’s unit, but modified for racing use. For example, the crankcase was optimised in terms of flow towards the cylinder sides and had larger windows between the individual cylinder units to ensure more favourable pressure conditions and minimise undesirable pumping losses. As in the series-production engines of the 911, the cylinder stud bolts were made from Dilavar, a high-alloy steel.
Classic racing engine design was reflected in the six connecting rods, which were made from titanium and polished to a mirror finish. From the mid-1980s, the engine featured shot-peened titanium connecting rods for improved durability. The shot peening process was used simultaneously in series production in the TAG turbo engine in Formula One and shortly afterwards in the 959 super sports car, the engine of which was closely related to those of the 956 and 962.
The cylinder barrel set was a technical highlight. It is worth noting that the pistons had circumferential cooling channels in the area of the ring grooves for improved cooling. The ‘blind cylinders’, in which the cylinder and cylinder head form a single unit, were a particular technical delight.
This unusual technical solution was developed in response to practical problems encountered in competition. This is because cars sometimes came in for pit stops and then burned out the cylinder-head gaskets on the way back onto the track. The reason for these defects was that the cylinder head fittings made of Dilavar cooled down more slowly than the aluminium cylinders and cylinder heads, which resulted in lower preload on the cylinder-head gasket. Porsche countered this by welding the water-cooled cylinder head and the 956’s air-cooled (and later water-cooled) cylinders together using an electron beam process. This design, in turn, placed increased demands on the honing of the Nikasil-coated cylinders, as the honing tool had to manage without a runout towards the combustion chamber. However, this process was mastered without any issues at that time.
Twin overhead, gear-driven camshafts
In the Type 935/76 engine, the camshaft housing served as the overlapping component of the cylinder heads – this also applied to the very first ‘Mezger’ engine in the 911. The same applied to the intermediate shaft for the camshaft drive, with the two gears being placed at the front and rear to account for the cylinder offset of the boxer engine due to its design. However, the racing engine had two overhead camshafts in each case; these were also driven by a very precise gear drive rather than by chains. On the outlet side of the cylinder heads, the large water pipe guides to the cylinder heads directly at the outlet channels, which were subject to extremely high thermal loads, were striking. Worthy of note were the separate and therefore completely self-sufficient cooling circuits for the left and right engine sides, each with its own cooling water pump.
While all air-cooled series-production engines of the 911 have an overhead camshaft for each cylinder bank and associated valve actuation via rocker arms, the Group C racing engines of the 956/962 had direct valve actuation via tappets. The valve clearance itself was adjusted via overhead shims. This type of valve actuation was used for the first time in a 911-based Porsche series-produced vehicle with the 959 super sports car.
Careful assembly was vital with this type of engine. This applied in particular to the tooth flank clearance within the gear cascade for the camshaft drive. Too little clearance here would lead to bearing and tooth flank damage, while too much clearance would result in increased running noise and imprecise timing – and therefore in negative effects on performance and durability.
As was customary in racing engines, the combustion chambers were gauged. This gauging was used to control the combustion chamber volumes and therefore the compression ratios. For optimum performance and the best possible running behaviour, the values had to be identical on all cylinders. This measurement was also used to rule out compression ratios that may be too high and have a fatal effect on a turbo engine.
Numerous components, such as the wheel housings of the camshaft drive, the two magnesium oil scavenge pumps for the twin turbochargers and the cylinder head suction system, each driven by the exhaust camshafts, or the nine-blade fan impeller made of carbon fibre laminate, also bore witness to the consistent lightweight construction of the engine.
From mechanical to electronic fuel injection
The Type 935/76 engine featured mechanical fuel injection from Kugelfischer. “However, we used the Bosch Motronic MS2 for the first time as early as at the end of 1982 during test drives at Paul Ricard,” recalls Hans Eckert, who worked in the Porsche motorsport department from 1976, and was responsible for the body control system of the Group C cars. He was also chief mechanic for Stefan Bellof in 1983/84 and for Hans-Joachim Stuck in 1986/87. The Bosch Motronic MS2 was used for the first time in the engine now known as the Type 935/79 in September 1982 during free practice for the 1,000-km race at Spa-Francorchamps. The next use of the new fuel injection system dates from the 1,000-km race at Monza on 10 April 1983.
Porsche began working with the Bosch Motronic engine management system as early as 1979 as part of the Indycar project, and used a similar system that was almost ready for series production in the 924 GTP Le Mans in 1981. When Porsche applied the Motronic to the 956 with strong support from Bosch, Bosch developed a datalogging system with the aim of perfect engine tuning. This system could record values such as engine speed, boost pressure, throttle valve position and accelerator pedal position as a basis for programming the ignition and fuel injection maps.
Initially, the creation and modification of the maps was very time-consuming. It was based on a hexadecimal code that had to be created on a computer and then stored on a chip. This chip was then inserted into the Motronic control unit.
All the effort was worth it though. The Motronic not only improved fuel efficiency, which was particularly important in Group C, but also provided more power. In the years that followed, the Motronic 1.7 took a major leap forward with the development of knock control and the option of two different maps that could be activated by the driver.
Racing gearbox modelled on the 911 (930) Turbo
The gearbox with five forward gears and one reverse gear was based in part on the gearbox of the 911 (930) Turbo and was designed for a maximum torque of more than 800 Nm. Like the standard gearbox, it had helical gear pairs. Porsche went for lightweight construction in the form of a magnesium housing and rear axle drive shaft flanges machined from solid titanium.
The power transmission, which was equipped with a separate gearbox oil cooling system, was divided into three housing sections: the clutch housing, the ‘ox horn’, was a large magnesium component on the vehicle that also served as a carrier for the rear suspension. The rear axle drive with limited-slip differential was located in the middle section. It had a 100 per cent locking effect with rigid through-drive. On the 956, the differential housing was made of lightweight magnesium, while on the 962 C it was cast from aluminium for stability. Finally, the rear section housed the gearbox, with an input shaft at the top and the output shaft at the bottom.
As was the case with the engine, it was vital that the mechanics took the greatest possible care during gearbox assembly. After tightening the nuts of the input and output shafts, the shift forks in particular had to be adjusted very sensitively and precisely with the aid of an adjustment housing in order to ensure optimum shiftability during driving.
Porsche’s dual-clutch gearbox PDK
As early as the late 1960s, Porsche was working on the development of a dual-clutch gearbox with the aim of being able to perform gearshifts with virtually no interruption to the engine’s power delivery. Since the control electronics that were fundamentally necessary for such a gearbox to function perfectly were not available at the time, this system still worked purely mechanically – as did the first PDK that Porsche developed for the 956. “That, however, proved to be a barely viable option. The system sometimes worked very erratically and gave drivers some unpleasant surprises,” recalls Singer. The decision was therefore quickly made to switch to an electronic-hydraulic control system.
The PDK worked with two clutches that alternately established the power connection with the engine via two separate drive shafts. However, during comparative test drives with the 956 at Paul Ricard in March 1984, Jochen Mass was still 2.3 seconds per lap slower with the PDK than with the manual transmission. Two years later, Hans-Joachim Stuck still lost 1.4 seconds with the PDK. As was evident in 1986 during the pre-tests for Le Mans, the PDK also lost out on top speed. Performance measurements on the test stand ultimately revealed that the PDK ate up around 20 PS in high rev regions. The PDK was also very labour-intensive. One contemporary witness remembers it well: “We had to fit and remove the gearbox every day.”
The moment finally arrived a year later, in 1987. In the meantime, the PDK had been reworked in the area of hydraulic control as well as electronics, and the power loss was now only 2.6 PS. Now Hans-Joachim Stuck was able to drive 0.7 seconds faster during tests at Paul Ricard than with the manual gearbox, and the 962 C equipped with PDK was now also ahead of its counterpart equipped with a conventional gearbox when it came to its top speed. The gearbox housing was now made of lightweight magnesium instead of aluminium. With Hans-Joachim Stuck winning the Supercup in 1986 and 1987, the PDK demonstrated its fighting power when it comes to racing.
Engines for the IMSA GTP
According to the regulations, only engines based on those of series production cars were able to be used in the American IMSA GTP series. For this reason, the 962 could not use water-cooled cylinders and cylinder heads for IMSA competition, as Porsche did not yet have a series production car with a corresponding engine in its range. Based on the engine of the 911 (930) Turbo, an air-cooled engine with a single turbocharger was therefore created, a unit that was closely related to the engine of the Porsche 934. This Type 962/70 engine had a displacement of 2,869 cc and was the power source of the Porsche 962 for IMSA GTP races in 1984. Boosted by a KKK Type K36 turbocharger, the engine produced 680 PS at 8,200 rpm, with a maximum torque of 660 Nm. Fuel injection was handled by the Bosch Motronic MS2.
While this engine proved efficient in racing applications, it was not sufficiently powerful for the future. However, since IMSA GTP regulations did not specify a fuel consumption limit as in Group C – and also permitted a fuel tank with a capacity of 120 instead of 100 litres – Porsche increased the engine’s displacement to 3,164 cc. With 720 PS at 7,300 rpm and 830 Nm of torque, the Type 962/71 engine used between 1985 and 1987 proved to be significantly more powerful.
When IMSA limited the displacement for turbo engines in the GTP class to three litres in 1987 and also stipulated a restrictor, Porsche responded with the Type 962/72 engine, which had a displacement of 2,994 cc. Equipped with a KKK Type K32 turbocharger, the engine developed a maximum output of 695 PS at 8,200 rpm and a maximum torque of 710 Nm. As well as increased basic compression, the smaller turbocharger provided improved driveability and a more spontaneous response.
At the end of the 1980s, the 962 C and the 962 for IMSA GTP faced massive competition in the form of new cars from other manufacturers. However, since the 962 was then also allowed a water-cooled engine in IMSA GTP, the final development stage of the Group C engine emerged in the form of the Type 935/86 engine, which was used from 1989 to 1994. Equipped with the Bosch Motronic 1.7, the engine was fully water-cooled, had two overhead camshafts for each cylinder bank and a displacement of 3164 cc. Fed by two KKK Type K26 turbochargers, it delivered 740 PS at 8,200 rpm and 715 Nm of torque.
With thanks to the teams at Porsche Motorsport, Porsche Heritage and the Porsche Museum