Chevrolet Turbo-Air 6 engine


The Chevrolet Turbo-Air 6 is an air-cooled automobile engine developed by General Motors in the late 1950s for use in the rear-engined Chevrolet Corvair of 1960. The engine has six cylinders arranged in a horizontally-opposed boxer configuration. The Turbo-Air 6 was used in the Corvair and all models derived from it, 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

, 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. 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 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 for the earliest versions was of forged alloy steel and 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. The fan is driven by a long V-belt from the back of the engine with an adjustable idler gear. The belt makes two 90° turns to reach the fan. An early problem with the fan drive-belt jumping off the pulley was solved by making the groove in the idler pulley deeper. 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 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 only has 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 running changes

1960

Modular construction

In January 1960 Frank Winchell, who had a hand in adapting the Powerglide transmission to the Corvair, was made head of Chevrolet Engineering's Research and Development group. In summer 1961 this group was working on two projects: the development of a counterpart to Ford's proposed Cardinal small car design and development of a second generation engine to succeed the Turbo-Air 6. Among the goals for the new engine were increased horsepower, and elimination of some of the problems encountered with the original design, such as head-gasket failures and oil leaks.
Winchell first built an engine with displacement increased to, but this only made the existing head-gasket problems worse. Winchell then proposed casting individual cylinder barrels and cylinder heads as a single piece, eliminating the head-gasket completely. A team was assembled that was led by Al Kolbe, who was responsible for the design of the original Turbo-Air 6. Heading up design for the new engine was Joe Bertsch, who was joined by engineers Len Kutkus and Jerry Mrlik. The engine they designed and developed kept the Turbo-Air 6 engine's boxer configuration and use of air cooling, and became known as the modular engine.
As with Winchell's earlier engine, bore and stroke were increased to and respectively. The combined barrel-head was to be die-cast in aluminum with cast-iron cylinder liners. Each of these castings also included a channel for the pushrod path, doing away with the previous design's pushrod tubes. Valve covers were held in place by quick-release clips. Rather than use four long bolts per cylinder, the bottom of each cylinder barrel had a heavy flange which was bolted to the crankcase. The single horizontal cooling fan was replaced by three vertical fans on a common shaft.
Problems with heat-distortion of the early alloy cylinder barrel/head units led to a subsequent redesign that included cooling fins angled at 45° to eliminate cutouts needed for access to the bolts holding the barrels to the crankcase, and the reintroduction of push-rod tubes.
The engine was "modular" in that the individual cylinder/head units allowed Chevrolet to design engines with 2, 4, 6, 8, 10, and 12 cylinders, of which versions with 2, 4, 6 and 10 cylinders were built. The 6 cylinder version produced about and was tested in a Corvair, while 2 and 4 cylinder engines were installed in a Renault Caravelle and two Alfa Romeo Giuliettas. The 10 cylinder version was called P-10 and was installed in a 1962 Chevrolet Impala converted to front-wheel drive. This engine produced. Design project XP-790 was originally meant to be the basis for a front-wheel drive replacement for GM's E-body cars, and incorporate flat-10 engines based on the P-10. Project XP-787 was split off from XP-790 to allow further development, while XP-790 was returned to the Research Studio, where it became the basis for the Firebird IV concept car. Project XP-787 was cancelled. Engines with 8 and 12 cylinders were designed but not built.
A private Corvair owner bought a collection of engines and parts and installed a running modular engine in his personal car.

Overhead camshafts

Chevrolet developed a prototype of the Turbo-Air 6 with a single overhead camshaft in each cylinder head. This was during the time from 1964 to 1966, with the SOHC project lagging the modular engine by approximately a year.
The camshafts were driven off the crankshaft by a timing belt. The valves were operated through rocker arms. The engine used three cooling fans, each directing air to one pair of opposed cylinders. The air/fuel intake used one Chevrolet-designed three-barrel carburetor per side. The original stroke was retained while the bore was increased to, giving a total displacement of. The pistons were a pent-roof type, and the combustion chamber shape approximated a hemisphere. The compression ratio was 10.5:1. While claimed output was as high as at 7200 rpm, none of the three prototype engines developed more than at 5700 rpm and at 5200 rpm.
In the Chevrolet Final Report on the engine written 22 February 1966, the need for improvements in cooling was highlighted. Power losses to the cooling fan was reported to be at 6000 rpm and was expected to nearly double at the engine's redline, bringing useful power down to the vicinity of the naturally aspirated engine.
One of the SOHC engines was displayed alongside the Astro I concept car, although it is reported that it was ever installed in the car. All three prototype engines are believed to have been destroyed.

Fuel injection

General Motors began investigating the use of fuel injection on the Turbo-Air engine on 11 August 1962. A mechanical injection system made by the Marvel Schebler division of BorgWarner was installed on a pre-production engine. The engine received Bill Thomas 4X1 cylinder heads with larger valves; intake and exhaust. A camshaft from Iskendarian provided high-lift and longer duration. After the initial feasibility study serious development started on 12 February 1963.
Extensive testing of intake systems was done. Eventually the team settled on diameter ram tubes each long and a central plenum. This configuration was tested against both a 1963 and 1964 turbo engine as well as engines with six individual carburetors and two three-barrel Webers. The injected engine's output of was higher than both turbos but lower than either carbureted engine. Road testing of the injected engine began on 19 April 1963.
In May 1963 a new injection system designed by Rochester was installed that produced nearly the same output as the Marvel-Schebler system. Performance of the most recent development engine was compared with a 4X1 engine on 5 November 1963; the injected engine produced approximately more. Another extensive road-test evaluation began on 15 November 1963. During October and November 1964 four more injected engines were built, one of which underwent road testing. By 2 February 1965 the injected engine was producing gross and net. On 24 February 1965 a final lab comparison was run, that concluded the 30-month development program. The extra cost of fuel injection could not be justified based on the power gains achieved.
Some versions of the Yenko Stinger were available with fuel injection. This system was based on GM's work.

Water-cooling

While not a Chevrolet project, at least one water-cooled Turbo-Air 6 was built by independent engine designer Lloyd Taylor.

Other applications

Apart from the production Corvair models, the Turbo-Air 6 engine was used in a variety of other applications.

Prototypes and styling exercises

Both General Motors and some major carrozzeria have used the engine in several Corvair-based concept and show cars.
Some companies modified stock Corvairs to create vehicles that offered improved performance or individualized appearance that were sold under the customizers' or parts suppliers' names.
Private hot rodders and a few small companies built one-off cars, some intended for series production that never materialized, that used the Turbo-Air 6 engine.
Some smaller manufacturers used the engine in limited-production cars, some with heavily modified Corvair chassis and some with fully custom frames.
The Turbo-Air 6 powered several cars of different types that were purpose-built to be raced on pavement.
Several Corvair-powered motorcycles have been built by individual fabricators and bike shops. Some of the most well-known are listed below.
Several options existed for adapting the Turbo-Air 6 engine to the transaxle in Volkswagen-based cars, or to fit a complete Corvair power-train into a modified VW chassis.
The air-cooled Corvair engine has been widely used in homebuilt aircraft. Some aircraft, such as the Pro-Composites Personal Cruiser have been specifically designed for them. The defunct American company Hegy Propellers, which was based in Marfa, Texas, produced propellers specifically for Corvair engines.
A variation on the six-cylinder engine is an opposed-twin version based on the Corvair pancake six. Some individuals have also experimented with inline-triple configurations based on half of a Turbo-Air six.

Aircraft applications

The following codes identify the year, size, power, and transmission of the engine
;Terms