by James Kraus
I have always been a fan of air-cooled engines. I even enjoy the mechanical noises that emanate from their insides, unmuffled by water jacketing and walls of cast metal. It serves as a reminder that the car is powered by an engine containing many precision parts moving in high speed synchronization, akin to an Audemars Piguet running on high octane fuel.
Air-cooling offers advantages in simplicity, light weight and low cost. Automotive air-cooled engines are actually cooled by both air and oil. Basic cooling is provided by generous fins on the cylinders and cylinder heads which expose as much surface area as possible to the cooling air forced across them by an engine-driven fan. Located in the airstream is a heat exchanger to control the temperature of the engine oil. Flow though the cooler is generally controlled by an oil pressure valve that flows oil through the cooler only when the viscosity drops below a pre-determined threshold, indicting that the oil is has reached operating temperature.
A major limitation of air-cooling is that above an engine output of around 100 Kw (135 DIN hp), supplementary external oil cooling in the form of remote heat exchangers is needed. This significantly detracts from the inherent design elegance of air-cooling and diminishes the cost and weight saving advantages. The Porsche 356’s powered by the 4-cam Fuhrmann engine used two such remote oil coolers mounted in front behind the horn grilles. The 911 incorporated a much larger integral cooler mounted on the crankcase; nonetheless, even in original 2.0 litre form, Porsche recommended fitting an additional cooler when using the car for “Sports Purposes.” As the power output of the 911 steadily increased, a front-mounted cooler eventually became standard fitment.
Air-cooling was quite rare before World War II. In the 1940’s only the Czechoslovakian Tatra flat-four and V8 and the VW flat-four were being produced. Following the war, both Tatra and VW restarted manufacturing of their air-cooled engines, the Tatra V8 staying in production though 1975 and the VW flat-four (redesigned in 1960 along the same lines) lasting through 2003.
After the war Citroën and Panhard joined the air-cooled club with the 2CV and Dyna X. The Porsche 356 débuted with a modified version of the VW engine. Later Fiat introduced the Nuova 500 with an air-cooled twin. The Nuova 500 was also produced under license in Austria by Steyr-Puch who developed their own engine for the car; a horizontally-opposed air-cooled twin with hemispherical combustion chambers. The Styer-Puch 650TR, using a high output version of this engine, won the Group 2 European Rally Championship in 1966. In 1959, Chevrolet introduced the Corvair with a horizontally-opposed air-cooled six-cylinder engine.
Then came the final generation of air-cooled powerplants. The decade of the sixties represented the peak of development of air-cooled engine designs, with significant advancements over those created earlier. Most of these engines reflected the latest design currents of the time with overhead camshafts, hemispherical combustion chambers and the ability to rev to 7,000 rpm or higher.
The first genuine 1960’s design to come to market was the NSU Prinz 1000 launched in 1963. This was the world’s first inline four-cylinder to incorporate air-cooling since the ill-fated 1923 Chevrolet, and the first air-cooled automotive engine with an overhead camshaft.
The engine was constructed of an aluminium block with two sets of iron cylinders cast in pairs, and a pair of twin cylinder heads. The cooling fan was built into the flywheel and a single overhead camshaft was driven by a chain from the nose of the crankshaft. 1.0, 1.1 and 1.2 litre versions were built. Uniquely, the rear-mounted engines were mounted transversely, just behind the rear axle.
It was a robust engine with a strong and rigid crankshaft supported by five main bearings. Soon after introduction, the sporting TT variant was launched with twin carburettors and later, the fabled TTS. The TT and TTS versions would happily spin up to 7000 rpm in stock form and were a favourite of sedan racers of the period, facing off against Mini-Coopers and Fiat-Abarths. The NSU’s won many European Touring Car Challenge Division One awards including 1st at Spa in 1967 and 1971 and 2nd at Zandvoort in 1968 and 1970. They also achieved class victories in the 1968 Marathon de la Route and the 1974 German Hillclimb Championship.
Probably the most iconic air-cooled engine among enthusiasts was introduced in the Porsche 911 of 1964. Designed by Paul Hensler and Hans Mezger to supersede both the standard 356 engine and the 4-cam Furhmann engine, the horizontally-opposed six cylinder was originally produced as a 2.0 litre with an aluminium crankcase and aluminium cylinder barrels with cast-iron liners. Each cylinder was topped with its own aluminium cylinder head with a fully machined hemispherical combustion chamber. The single overhead camshafts were chain driven. Cooling was provided by a belt-driven cast magnesium fan surrounding the alternator. The air ducting was moulded from fibreglass-reinforced resin.
The crankcase had a dry sump, with a single dual-chamber pump handling both pressure and scavenging functions. An eight-litre oil reservoir and full-flow filter were located behind the right-rear wheel. In 1970, oil spray jets were added to cool the underside of the pistons, a feature which became a common feature of turbocharged engines in the years following.
The 911 also featured a new and unique fuel system. Conventional carburettors relied on a very accurate fuel level in the float chamber to control the final fuel mixture. This worked very well in a stationary vehicle, but once subject to the centrifugal and inertial forces of cornering, acceleration and braking, the sloshing fuel caused the mixture to vary considerably. This is a concern in any engine but especially critical in an air-cooled application since running lean can cause operating temperatures to soar to dangerous levels in short order.
To address this issue, Porsche and Solex engineers developed a radically new fuel delivery arrangement for the new 911. Each bank of the engine was equipped with three single-throat Solex “Spill-Tube” carburettors which shared a single fuel reservoir mounted below the carburettor base.
An electric pump delivered fuel from the tank to the reservoirs. An engine-driven double fuel pump (one chamber for each bank) continuously recirculated fuel from the reservoirs to the carburettors. The fuel chambers in the carburettors were kept 100% full at all times, the excess fuel exiting through the spill-tubes and back into the reservoirs. It was a brilliant concept, but unfortunately Solex were never able to perfect the system and Porsche had to switch to conventional carburetion.
Porsche immediately took this new engine to the track, installing tuned versions in the 904/6 in 1965 and the 906 the following year. In 1967 a near-identical version to the 906-spec powerplant was made available in the 911R that developed 210 DIN hp at 8000 rpm, an output that would not again be available to the public in a 911 until the Carrera 2.7 RS of 1972.
In the fall of 1968, E and S versions incorporated mechanical fuel injection and electronic ignition. Over the ensuing years the engine grew in steady increments from the original 2.0 litres to 3.6 litres and was developed in both normally aspirated and turbocharged form. It went on to win almost every major race in the world in which it was entered including the Monte-Carlo Rallye in 1968, 1969, 1970 and 1978, the Tour de France in 1970, the Targa Florio in 1966 and 1973, and Le Mans in 1979.
1968 saw the introduction of the VW 411 with a newly-designed 1.7 litre air-cooled flat-four. This was actually the second new VW engine of the sixties. The first was introduced in the fall of 1960 in 1.2 litre form in the Beetle, Bus and Karmann-Ghia, and later powered the 1500/1600 Series.
Unlike all other air-cooled engines debuting during the decade, VW retained pushrod-activation for the valves. The new 1.7 litre engine, initially equipped with twin single-throat carburettors, would later be offered with Bosch D-Jetronic and L-Jetronic fuel injection.
There were some refinements over the smaller VW motor. The crankcase was cast in slightly heavier but more robust 319 aluminium-silicon alloy, and the larger-capacity lubrication system featured VW’s first full-flow oil filter. The cylinder heads incorporated a full cross-flow design for the first time, with exhaust ports exiting at the bottom rather than out from the sides.
Besides powering the 411 and 412, the new engine found its way into the VW-Porsche 914, the VW Bus, and finally, in its most prestigious application, the Porsche 912E. In 2.0 litre high-output form, Porsche-designed cylinder heads boosted power to as much as 100 hp DIN; the pinnacle for Volkswagen air-cooled engines.
One of the most legendary racing cars of all time, the Porsche 917 was powered by a air-cooled horizontally-opposed 12-cylinder DOHC engine of between 4.5 and 5.6 litres, in both normally aspirated and turbocharged form. Another Paul Hensler and Hans Mezger design, the basic architecture was largely based on Paul’s eight-cylinder Porsche Formula One engine of 1962.
To reduce torsional stresses on the long 12-cylinder crankshaft, all takeoffs for power and ancillary drives were taken from the centre of the crank. In 911 fashion, each cylinder had its own individual aluminium head. These were topped with a common camshaft carrier, one per bank. The dual overhead camshafts, four in all, were gear driven.
Low weight, both in terms of the complete engine, and the rotating masses within, was a key priority. The crankcase was aluminium-magnesium alloy and the cam carriers and cam covers were magnesium. Cylinder barrels were aluminium with Nikasil liners. The connecting rods, rod bolts, fan drive shaft, auxiliary and output shafts and other miscellaneous hardware were made of titanium. The fan shrouding, cooling fan and intake stacks were fibreglass. The cooling fan displaced up to 148 cubic meters of air per minute.
Like the 911, the 917 employed a dry sump oiling system. This one utilized no less than seven pumps. A triple unit in the sump provided pressure and scavenging of the front and rear of the crankcase, and four small pumps located at each end of the exhaust camshafts allowed for scavenging oil from the cylinder heads. The system held 30 litres of oil. Each cylinder had dual spark plugs, ignited by two separate distributors. Fuel was supplied by Bosch mechanical injection. The initial batch of 4.5 litre versions produced 520-580 hp at 8500 rpm, the turbocharged versions generated up to 1580 hp on full boost.
The 917 won Le Mans and the World Sportscar Championship title in 1970 and 1971, the Interserie Championship from 1970-1973 and the Can-Am Championship in 1972 and 1973. During the 1973 season, it won every single race. In 1975 a 917 set a closed course speed record of 356 kph/220 mph at Talladega Speedway, hitting over 400 kph/250 mph on the straight sections.
Finally, the last mass-produced automobile introduced with a clean-sheet air-cooled engine design; the Citroën GS. The GS went into production in 1970 and garnered the European Car of the Year award in 1971. In accordance with traditional small-Citroën practice, it made use of an opposed engine driving the front wheels, this time with four cylinders. Initially just 1.0 litre in displacement, it was ultimately enlarged to 1.3 litres. The crankcase and heads were cast of aluminium and the cylinder barrels were cast iron. The cooling fan mounted directly to the nose of the crankshaft in the manner of the earlier Citroën twins. Following another practice dating back to the original 2VC, the connecting rods were one-piece and installed on a built-up crankshaft.
Like the NSU and Porsche 911 engines, the GS employed single overhead camshafts, but toothed belts rather than chains drove them. Another revver, the engine produced its maximum power at 6750 rpm.
A distinctive feature of the engine was that the crankcase incorporated a double oil pump; an internal section for the engine oil and an external one to supply fluid pressure for the GS’s hydropneumatic suspension system. The engine’s compact layout allowed for the spare wheel to be stored in the engine bay, a Citroën tradition.
While largely renowned as an economical family sedan, the GS also enjoyed a career in rallying, finishing 6th overall at Caledonia in 1973, 4th at the Rally Torre del Oro of Spain in 1975 and 3rd at Cyprus in 1977.
There are two I left out as they sold only in small numbers but are worthy of mention: the Honda air-cooled twin of the 360/600 (1967-1972) and the air-cooled in-line four of the short-lived but technically intriguing Honda 1300 and 1300/9 Coupe (1969-1973) developed under the direction of none other than the majordomo himself, Soichiro Honda. The 1300 engine was an inline-four with a flywheel fan in the style of the NSU 1000. What made it unique was that unlike other air-cooled engines that utilized sheet metal or fibreglass ducting to contain and direct the flow of cooling air over the engine, the cooling passages of the 1300 were cast into the block and head in the manner of a liquid-cooled design. This served to considerably reduce engine noise from the level normally associated with air-cooling. Installed in the Honda 1300/9, the quad-carb dry-sump unit produced 110 DIN hp at 7300 rpm.
The Citroën GS motor would prove to be the last automotive air-cooled engine design. With the increasing emphasis on quiet operation, low emissions, fuel efficiency, larger displacements and heat producing ancillaries; air-cooling was no longer an attractive option.
Air-cooled engines traditionally ran slightly rich to reduce combustion temperatures. Unfortunately, this both reduces fuel efficiency and increases hydrocarbon emissions. In addition, the cooling system of most modern cars has to cope not only with engine heat, but the heat generated by air-conditioning condensers, forced induction intercoolers and transmission fluid coolers. These additional loads tip the balance well in favour of a liquid cooling system. Finally, it is quite difficult to design a multi-valve cylinder head for an air-cooled engine as space quickly runs out for adequate fin sizing and airflow.