Guide: Porsche 924 Turbo (931) — Supercar Nostalgia

Dec 30

Dec 30 Guide: Porsche 924 Turbo (931)

Porsche - Porsche 924 - 1970s Supercars - 1970s German Cars - 1980s Supercars - 1980s German Cars

Background

Although the 924 was well received and sold in a substantial quantity (around 50,000 had rolled out of Audi’s Neckarsulm factory within two-and-a-half years of launch), the low cost Porsche’s most glaring deficiency was a perceived lack of performance.

To rectify this, Porsche’s options were to either fit a bigger engine or add a turbocharger.

Having trialled a normally aspirated V6 in a 924 mule during 1977, for reasons of weight and performance, the decision was made to go the forced induction route instead.

By this time, Porsche’s technical department was well versed in the art of turbo technology. The all-conquering 911-based 935 and 934 models campaigned since 1976 had dominated the GT racing scene. In parallel, the 936 had proved itself the era’s most capable Group 6 Prototype.

Porsche had initially pioneered on-track forced induction with the likes of the Group 7 917/10 and 917/30 from 1972-1973. These Can-Am and Interserie machines were followed the wild Prototype class 911 RSR Turbo of 1974.

The firm’s first turbocharged production car, the 930 (aka the 911 Turbo), had also proven a great success. Initially conceived to homologate the aforementioned 934 and 935 racing cars, the 930 proved such a hit that Porsche quickly decided to make it a permanent fixture in their model line up.

Since going into production in early 1975, the expensive and extremely fast 930 had improved the image of the rest of the 911 range. It was hoped the new 924 Turbo (internally dubbed the 931) would have a similar effect on Porsche’s entry level model.

In addition to a forced induction engine (built at Zuffenhausen rather than Audi’s Neckarsulm plant), the 924 Turbo came with uprated brakes, suspension and steering. It also incorporated a special new gearbox, custom wheels and a higher level of interior equipment.

Sales began in the spring of 1979 after the 924 Turbo had made its debut the Geneva Motor Show in March.

Chassis

As per the standard 924, the 924 Turbo was based around a unitary steel bodyshell with a 2400mm wheelbase. The engine was mounted at a 40° angle to allow the lowest profile hood.

Through careful design and the adoption of a transaxle layout, the normally aspirated 924 had a near perfect 53/47 weight distribution. This was improved to 51/49 for the 924 Turbo.

The fuel tank (located above the rear axle) was kept at 62-litres. For the Turbo, an extra fuel pump was fitted inside the tank and larger fuel lines were installed.

The basic suspension layout was retained: MacPherson struts with lower wishbones, coaxial coil springs and shock absorbers at the front, transverse torsion bars with semi-trailing arms and shock absorbers at the rear.

Special equipment for the 924 Turbo included stiffer front springs, thicker rear torsion bars and firmer shock absorbers all round (Koni front and Bilstein rear). A thicker anti-roll bar was fitted at the front. The rear anti-roll bar was made softer as the new gearbox was lighter than the original.

21mm spacers were fitted to the rear hubs for a wider track.

There were also strengthened wheel bearings and Porsche fitted a higher rate steering ratio to cope with the wider wheels and tyres.

New multi-spoke wheels had five instead of four-stud fixings and measured 6 x 15-inches all round (up from the 6 x 14 alloys that were an option on the regular 924). Pirelli tyres were originally fitted.

Instead of the existing 256.6mm solid disc / 228.6mm drum arrangement, the 924 Turbo came with ventilated discs all round. 282mm front discs were imported from the 911 SC while the 289mm rears came from the 928. The 928 parts bin was also raided for its beefier calipers.

Engine / Gearbox

At the heart of the 924 Turbo was its Porsche-built Type M31/01 engine.

Although the bottom end of the M31/01 unit was left virtually intact, the top end came in for considerable attention.

Porsche devised a brand new aluminium-silicon cylinder head with dished combustion chamber, a new gasket and bigger exhaust valves (up from 33mm to 36mm). The inlet valves were kept at 43mm.

Platinum-tipped spark plugs were moved to the induction side of the engine and the fuel-injectors were now screwed into the head. New pistons were also fitted.

A single KKK K26 turbo was fitted downstream of the exhaust manifold and set at 0.7 bar.

The compression ratio was dropped from 9.3:1 to 7.5:1.

Displacement was kept at 1984cc thanks to an unchanged bore and stroke of 84.4mm and 86.5mm respectively. Similarly, the eponymous Bosch K-Jetronic fuel-injection system was also carried over.

In this configuration, peak output was 168bhp at 5500rpm and 181lb-ft at 3500rpm.

For comparison, the normally aspirated motor pumped out 123bhp at 5800rpm and 121lb-ft at 3500rpm.

Instead of the four-speed Audi 088 gearbox, Porsche fitted the 924 Turbo with a specially developed Getrag G31/01 five-speed unit. Whereas the Audi ‘box had a conventional H-pattern layout, the Getrag unit found in the 924 Turbo had a racey dog-leg first configuration.

A larger diameter single dry-plate hydraulic clutch replaced the original cable-operated set-up.

Bodywork

Although the 924 Turbo was not equipped with the kind of wide arch look that came to define the 930, it was nevertheless kitted out with a host of special visual features.

Most obvious of these were the bank of four rectangular intakes on the nose.

There was also a large NACA duct carved out from the hood to feed cool air to the turbo.

Less easy to spot, particularly on darker coloured examples, were the banks of nine vertical brake cooling vanes either side of the front apron.

At the back of the car was a polyurethane rear spoiler that reduced drag from 0.36 to 0.33. This also proved useful for opening and closing the tailgate.

Underneath the spoiler on the right-hand side of the tail was a stylised ‘924 turbo’ decal.

Black anodised side window trim was standard.

Porsche also fitted each 924 Turbo with a special undertray to reduce air pressure below the engine.

Interior

Inside, all the basic fixtures and fittings were imported from the standard 924 to include the dash, door panels and seats.

One new feature was a small-diameter leather-rimmed three-spoke steering wheel from the 930.

Another change concerned the instrument lettering which was switched to green (from the original white).

To reflect the faster top speed, a new 260kph / 170mph speedometer was installed.

Kick plates with ‘turbo’ script were added to the door sill panels.

The hard plastic dash was unchanged.

The main instrument binnacle housed the speedometer in the centre while off to the right was a rev counter and to the left was a combined fuel / water temperature gauge that also incorporated a host of warning lights.

At the top of the centre console (underneath two fresh air vents) was an analogue clock, an oil pressure gauge and an oil temperature gauge. These, along with the ventilation controls and audio system, were set within a plastic satin black fascia.

High-backed seats from the Porsche 911 were used up front. The rear seats were only really suitable for children.

Upholstery was vinyl leatherette with a choice of fabric inlays.

Electric windows were standard.

Options

New options offered to 924 Turbo buyers included a two-tone paint scheme, ‘turbo’ graphics, a digital Blaupunkt stereo and several special interior fabrics to include Herringbone and tartan.

Other upgrades were also available such as a limited-slip differential, electric mirrors, Sports shock absorbers, an alarm, a removable sunroof, leather upholstery, metallic paint, headlight washers, a rear wiper, a four-spoke steering wheel, a choice of audio systems, an electric antenna, tinted glass and air-conditioning.

If air-conditioning was specified, the oil temperature gauge was deleted and the clock moved to its position. A rotary control switch for the a/c system was added where the clock originally resided.

Weight / Performance

At 1180kg, the 924 Turbo weighed 100kg more than the original 924. Around 30% of the weight gain was a result of the turbo installation, 50% was down to its beefier suspension and the remaining 20% was due to the generally improved specification.

Despite the weight gain, performance was substantially improved.

Top speed went from 121mph to 140mph.

The 0-62mph time dropped from 11 seconds to 7 seconds.

This meant the 924 Turbo was about on a par with the contemporary 911 SC.

USA Version

From June 1979, Porsche began to build a USA specification 924 Turbo which was also sold in Canada and Japan.

These cars came with a catalysed Type M31/02 engine that featured an array of emissions equipment. They also used slightly less turbo boost (0.45 bar) to improve mid-range torque and extend the life of the catalytic converter.

Peak output was 150bhp at 5500rpm and 147lb-ft at 3000rpm.

The gearboxes used in these cars was Type G31/02.

In standard trim they came with a slimmer front anti-roll bar and no rear anti-roll bar.

They also used the inferior normally aspirated 924 brake configuration with 257mm solid discs up front and 230mm drums at the rear.

The vented disc set up could be re-instated as part of the Sport option. Similarly, a rear anti-roll bar could be fitted upon request.

Instead of fibreglass bumpers, these cars came with aluminium bumpers mounted on telescopic energy absorbing dampers. Big rubber inserts were added at either end along with large circular side marker reflectors on each fender.

Inside, several extra warning lights were added to the dash.

Another difference to Rest of World 924 Turbos was sealed beam instead of halogen headlights on USA variants.

924 Turbo USA Edition

To mark the launch of the 924 Turbo in North America, Porsche offered the USA special edition.

600 of these cars (option code 420) were built between June and July of 1979.

They came with special Dolomite Grey Metallic paint and Black leatherette upholstery with black and white tartan check fabric inlays.

1979 Model Year

During the 1979 model year (the first for the 931), 924 Turbo production reached 2932 units.

Of these, 1982 were to Rest of World specification and 950 were to USA specification.

1980 Model Year

For the 1980 model year, Porsche fitted an enlarged 66-litre fuel tank.

A 6 x 16-inch wheel option was also offered. These larger wheels could be specified with Pirelli’s latest P7 low profile tyres.

Total production for the 1980 model year was 5243 units.

Of these, 1803 were to Rest of World specification and 3440 were to USA specification.

1981 Model Year: 924 Turbo Series 2

For the 1981 model year, Porsche brought in a revised 924 Turbo, often referred to as the Series 2 derivative.

Despite a relatively modest boost to power and torque, the new Type M31/03 engines fitted to these cars came with an array of significant updates.

Perhaps the most important development was the addition of Siemens-Hartig digital ignition (the first time this feature had been used on a production offering from Porsche). The Digital Motor Electronics engine management system (DME) improved power, flexibility and fuel consumption.

Another update concerned the KKK K26 turbo which now had backward-facing compressor blades for greater efficiency and added responsiveness. Boost pressure was dropped from 0.7 bar to 0.64 bar.

For greater reliability, crankcase breathing was enhanced.

New pistons were also fitted.

The compression ratio was upped from 7.5:1 to 8.5:1.

As a consequence of these changes, the power output went from 168bhp to 175bhp at an unchanged 5500rpm.

The torque rating was improved from 181lb-ft to 184lb-ft at an unchanged 3500rpm.

The original five-speed G31/01 gearbox was retained, but the clutch was now rubber torsion dampened.

Aside from the engine and gearbox, other updates ushered in for the 1981 model year / Series 2 comprised a galvanised bodyshell now completely hot-dipped in zinc, indicator side repeaters behind each front wheel, white instead of green instrumentation, a new four-spoke steering wheel, a larger 84-litre fuel tank with new sender unit and a ‘turbo’ badge similar to the 930 (instead of a ‘924 turbo’ decal).

Total 924 Turbo production for the 1981 model year was 3312 units of which 1783 were these Rest of World iterations.

USA Version Series 2

Unfortunately for those customers in the USA, Canada and Japan, the new 175bhp Type M31/03 engine did not comply with local emissions legislation. As a consequence, Porsche created the M31/04 unit for these markets.

They came with a catalytic converter, some other bits of emissions equipment, 0.44 bar turbo pressure and a lower 8.0:1 compression ratio. Peak output was 156bhp at 5500rpm (up from 150bhp) and 155lb-ft at 3300rpm (up from 147lb-ft at 3000rpm).

Significantly, these US, Canada and Japan-bound 924 Turbos were now fitted with the latest five-speed 016G gearboxes from Audi instead of the superior Getrag units found in Rest of World derivatives.

1529 of these USA specification cars were built for the 1981 model year.

1982 Model Year

Sales of the USA derivative were discontinued in January 1982, at which point the 924 Turbo was replaced by the 944.

The Rest of World variant was dropped for most markets in June 1982.

1982 model year production totalled 1819 units of which 943 were built to Rest of World specification and 876 to USA specification.

1983 Model Year

Although the 924 Turbo had officially been replaced by the 944 by the summer of 1983, an Italian law that heavily penalised vehicles with engines larger than two-litres gave the car a brief stay of execution.

In total, 310 last-of-the-line 924 Turbos were exported to Italy after production had ceased in all other markets.

Of these, 88 were so-called special variants painted Zermatt Silver Metallic with two-tone Anthracite and Burgundy Porsche-script fabric inlays.