The Chevrolet Turbo-Air 6 is a flat-six air-cooled automobile engine developed by General Motors (GM) in the late 1950s for use in the rear-engined Chevrolet Corvair of the 1960s. It was used in the entire Corvair line, as well as a wide variety of other applications.
The engine's use of air cooling made it appealing to aircraft amateur builders, and small-volume engine builders established a cottage industry modifying Corvair engines for aircraft.
History Ed Cole, Chief Engineer for Chevrolet from 1952 to 1956 and Chevrolet General Manager from 1956 to 1961, was the person primarily responsible for getting the Corvair and its engine into production. Cole's experience with rear-engined vehicles began during his time as chief design engineer of light tanks and combat vehicles for Cadillac during World War II. He designed powertrains for the M24 Chaffee light tank and M5 Stuart tank, the latter of which used two rear-mounted Cadillac V8 engines driving through Hydramatic transmissions.
After the war, Cole was promoted to Chief Engineer at Cadillac. In 1946, he began experimenting with rear-engined passenger car prototypes, nicknamed "Cadibacks".
In 1950, Cole was asked to oversee production of the M41 Walker Bulldog tank at Cadillac's Cleveland facility. The M41 was powered by a Continental AOS-895-3 engine. This was a six-cylinder, air-cooled, four-stroke supercharged boxer engine that displaced .
Cole also logged over 300 hours piloting a Beechcraft Bonanza powered by a smaller Continental engine with the same basic layout.
After moving to Chevrolet, Cole instructed engineer Maurice Olley to come up with "something different". Olley and his team assessed both front-engine/front-wheel-drive and rear-engine, rear-wheel-drive layouts and determined that the rear/rear layout would need an engine of aluminum, and that air-cooling would be preferred.
Responsibility for development of the engine fell primarily to Senior Project Engineer Robert P. Benzinger and engine designer Adelbert “Al” Kolbe. The first engine was fired up in the Chevrolet Engineering department in December 1957. For the earliest road tests, a prototype was installed in a Porsche 356. Later development mules were either called LaSalle II or badged as Holdens.
A new casting foundry was built in Massena, New York, at Massena Castings Plant. GM convinced Reynolds Aluminum to build an aluminum reduction plant nearby to supply it. Aluminum parts included the block, heads, flywheel housing, crankcase cover, clutch housing and pistons. of aluminum was used in each engine. New casting and machining techniques had to be developed to produce the light-alloy parts. The aluminum parts were cast with a low-pressure casting technique using machines built and installed by Karl Schmidt GmbH of Neckarsulm, Germany. All of the engines were assembled at GM's Tonawanda Engine plant.
The car and engine were officially introduced on 29 September 1959 and debuted in showrooms on 2 October. Advertising prepared by the Campbell-Ewald agency highlighted the fact that the air-cooled engine did not require anti-freeze, and that much of the engine was made of "aircraft-type" aluminum. The same ad agency gave the engine its official name, the "Turbo-Air 6".
The Turbo-Air 6 engine was used in all Corvair car models in all trim levels, including the 500, 700, 900 Monza, Corsa, and Spyder coupes sedans and convertibles, as well as the Corvair and Lakewood station-wagons. It also powered the Forward-Control 95 series vans, including the Corvan and the Chevrolet Greenbrier van, and both the Loadside and Rampside pickup trucks.
Tuned versions of the engine appeared in some modified Corvairs sold under the customizer's name, such as the Fitch Sprint, the Yenko Stinger, and the Solar Cavalier. Don Yenko claimed as much as from his Stage IV and racing Stingers.
Manufacturing of the Turbo-Air 6 ended with the cessation of Corvair production after 1969.
Technical features The Turbo-Air 6 is a flat-six engine that is primarily air-cooled. The engine's major components include an aluminum crankcase, two three-cylinder aluminum cylinder heads with integral intake manifolds, and six individual iron cylinder barrels. The #1 cylinder is at the right rear with cylinders 1, 3, and 5 on the right, while #2 is the left rear with cylinders 2, 4, and 6 on the left. The firing order is: 1-4-5-2-3-6.
The crankcase is cast as two box-section halves. The assembled crankcase provides for four main bearings. There are four cylinder head studs per cylinder, for a total of twelve on each side. The crankshaft, the earliest versions of which were forged alloy steel, had six throws but no counterweights, permitting a weight-saving of .
Each cylinder head has two overhead valves per cylinder activated through stamped-steel rocker arms and hydraulic tappets by pushrods that run through tubes below each cylinder barrel. The engine developed a reputation for leaking oil past the seals of the pushrod tubes. New seals of Viton solved the problem.
Viewed from the rear, the Corvair engine's crankshaft rotates counter-clockwise; opposite that of most other engines. This allows it to use regular transmission and pinion-gear arrangements when mounted in a rear-engine configuration.
Primary cooling is done by a shrouded cooling fan mounted horizontally on top of the engine. The fans were revised throughout the production run, with early fans made of steel and later ones of magnesium to reduce inertia. The fan is driven by a long V-belt from the back of the engine with an adjustable idler pulley. The belt makes two 90° turns to reach the fan resulting in four 90° twists. An early problem with the fan drive-belt jumping off the pulleys was solved by making the groove in the idler pulley deeper and adding belt guides. A metal bellows thermostat modulated either a ring valve on early engines or a set of damper doors on later ones to regulate the flow of cooling air.
Engine oil is also used as a coolant. To remove heat from the oil the engine used a variety of types and sizes of oil coolers throughout its production run.
Most Turbo-Air 6 engines use two one-barrel Rochester H carburetors; one per cylinder head. A later high-performance engine uses four carburetors; one Rochester HV primary and one Rochester H secondary per head. The secondary carb had no choke plate, idle circuit, accelerator pump, power circuit, or vapor vent.
The arrangement of intake and exhaust valves in the Turbo-Air 6 is considered noteworthy, with the valves arranged as intake/exhaust, intake/exhaust, intake/exhaust down both sides. The use of separate exhaust ports rather than twinned or siamesed ports helps avoid problems with distortion caused by a concentration of heat at these locations.
There is a single cast-iron camshaft located in the crankcase. The shaft has only nine cam lobes on it — the symmetrical arrangement of valves allows three double-width cam lobes to operate all six exhaust valves.
Turbocharging
Chevrolet introduced a turbocharged version of the engine for the 1962 model year. Development of this version was done by engineers James Brafford and Robert Thoreson, under the oversight of Bob Benzinger, who had become chief engine designer for the Corvair in 1959. The turbocharged Corvair was released one month after the turbocharged Turbo-Rocket engine in the Oldsmobile F-85 Jetfire, making it just the second turbocharged car in volume production. This engine was not marketed under the Turbo-Air name, being listed initially as the Super Charged Spyder engine.
Many of the internal components were strengthened or otherwise revised to deal with the stresses of forced induction. The engine received heavy-duty rod and main bearings, chromed upper piston rings, and nickel/chromium alloy exhaust valves. The crankshaft in the turbocharged engine was made of forged 5140 chrome-steel. The compression ratio was reduced to the 8.0:1 of the original 1960 naturally aspirated engine. The multiple-carburetor intake system was replaced with a single side-draft Carter YH carburetor.
The turbocharger was made by the Thompson Valve Division of Thompson-Ramo-Wooldridge Inc., which became TRW in 1965. The model selected weighed . It had a diameter impeller and was capable of spinning at up to 70,000 rpm.
The turbocharged Corvair engine did not use a wastegate to limit boost pressure. Instead, boost was controlled by an exhaust system designed to create back-pressure sufficient to limit the maximum boost. To prevent the engine from running too lean a metering rod and jet were selected that supplied an over-rich mixture when at full throttle. Static timing advance was set to 24° BTDC, with an additional 12° of centrifugal advance coming in above 4000 rpm. To prevent pre-ignition, a diaphragm on the distributor provided a pressure retard function rather than a vacuum advance, and could retard timing by up to 9° at manifold pressures above .
With maximum boost pressure limited to , power output from this engine in 1962 was , a 47% increase over the output of the naturally aspirated engine. Torque was also increased by 58% to .
Production run changes 1960 Engine debuts. Bore and stroke are , for a total displacement of . The compression ratio is 8.0:1, and power and torque outputs are at 4400 rpm and at 2400 rpm. The 1960 engine has two carburetors, but a common air intake and filter with a single integrated automatic choke. Air is fed to the carburetors through two large rubber hoses. The cooling fan is in diameter with 24 blades, and is made of steel. Cooling airflow is controlled by a large-diameter bellows thermostat that drives a ring-shaped air valve in the eye of the blower. The oil is cooled by a folded-fin cooler. The engine does not have a harmonic balancer. A high-output version of the engine called the "Special Camshaft package" is offered this year. This engine uses a different Rochester carburetor, a different camshaft profile with revised timing, and stiffer valve springs as well as a larger diameter exhaust system and a Delco-Remy ignition. Power output is up to .
1961 Bore diameter in increased to , raising engine displacement to . A high performance Super Turbo-Air version is released. This engine includes a revised cylinder head and camshaft, with a compression ratio raised to 9.0:1. Power is up to at 4600 rpm and torque is up to at 2800 rpm. Development was reportedly done by Zora Arkus-Duntov. Engine cooling fan diameter is reduced to with 16 vanes that extend right to the centre of the fan wheel. The common intake with integrated choke is replaced by a shared air intake and manual chokes at each carburetor controlled by a pull-knob on the dashboard. Cooling is now controlled by a new set of damper doors at the lower part of each shroud actuated by small-diameter bellows thermostats.
1962 Naturally aspirated versions this year include a standard engine with an 8.0:1 compression ratio producing at 4400 rpm and , another version with a compression ratio of 9.0:1 for Monzas with Powerglide transmissions that produces and at 2300 rpm, and the Super Turbo-Air engine that now makes at 4400 rpm and at 2800 to 2000 rpm. The carburetors now have automatic chokes. A turbocharged version of the engine is announced on 27 March 1962. Displacement is , and output is at 4400 rpm and at 3200 to 3400 rpm.
1963 Crankcase ventilation becomes standard. The folded-fin oil cooler is replaced with a 3-plate design on most cars and an 8-plate design on Spyders and cars equipped with air-conditioning. Minor changes are made to the voltage regulator, distributor, cam profiles, and valves.
1964 Displacement of naturally aspirated and turbocharged engines grows to by increasing the stroke to . Compression ratios are raised to 8.25:1 for the base and turbocharged engines, and to 9.25:1 for the high-performance engines.