Chevrolet Corvair engine

The Chevrolet Corvair engine was a flat-6 piston engine used exclusively in the 1960s Chevrolet Corvair automobile. It was a highly unusual engine for General Motors: It was air-cooled, used a flat design, and with aluminum heads and crankcase, and individual iron cylinder barrels. The heads were modeled after the (excellent) standard Chevrolet overhead valve design, with large valves operated by rocker arms, actuated by pushrods run off a camshaft in the crankcase operating hydraulic valve lifters (which eliminated low temperature valve clatter otherwise seen with that much aluminum in the engine, due to its high degree of thermal expansion).

The flat horizontally opposed ("flat engine") air-cooled engine design, previously used by Volkswagen and Porsche as well as Lycoming aircraft engines, offered many advantages. Unlike inline or V designs, the horizontally opposed design made the engine inherently mechanically balanced, so that counterweights on the crankshaft were not necessary, reducing the weight greatly. Eliminating a water-cooling system further reduced the weight, and the use of aluminum for the heads and crankcase capitalized on this weight reduction; so that with the use of aluminum for the transaxle case, the entire engine/transaxle assembly weighed under 500 pounds. In addition, the elimination of water-cooling eliminated several points of maintenance and possible failure, reducing them all to a single point; the fan belt. As with the Volkswagen and Porsche designs, the low weight and compact but wide packaging made the engine ideal for mounting in the rear of the car, eliminating the weight and space of a conventional driveshaft.

Two years after its 1960 debut, the Corvair engine gained another unusual attribute; along with the Oldsmobile Jetfire V8, it was the first Detroit production engine ever to be equipped from the factory with a turbocharger.

Aircraft hobbyists and small volume builders, perhaps seeing the Corvair engine's similarity to Lycoming aircraft engines, very quickly began a cottage industry of modifying Corvair engines for aircraft use, which continues to this day. The Corvair engine also became a favorite for installation into modified Volkswagens and Porsches, as well as dune buggies and homemade sports and race cars.

140

The initial Corvair engine displaced 140 in³ (2.3 L) and produced 80 hp (60 kW). The high performance "Turbo air" version produced 95 hp (70 kW).

145

In 1961, the engine received its first increases in size via a larger bore. The engine was now 145 in³ and the base engine was said to produce the same 80 hp (60 kW), but 84 hp (63 kW) if equipped with the optional automatic transmission. The new high performance engine was rated at 98 hp (73 kW). In 1962 the high performance engine was rated at 102 hp (76 kW) and the Spyder model became available featuring a turbocharged engine rated at 150 hp (112 kW).

164

The engine was stroked out to 164 in³ (2.7 L) for 1964 . Power output was boosted to 95 hp (70 kW) for the base model and 110 hp (80 kW) in high performance N/A engine. The Turbocharged engine remained rated at 150 hp.

A 140 hp (104 kW) version with 4 linked carburetors was introduced in 1965; because this became the base engine for the Corsa model, it was unofficially known as the Corsa engine. The carburetors consisted of a single barrel primary and a single barrel secondary on each head, connected by a progressive linkage. The turbocharged engine now developed 180 hp (134 kW).

Problems

The Corvair engine design was so unique that good dealer service and maintenance was spotty. Mechanics, unused to the aluminum head and crankcase, would frequently overtighten threaded fasteners and spark plugs, stripping the threads out of the aluminum, requiring extensive repair.

Due to the greater thermal expansion of aluminum, valve clearances had to be measured at operating temperature. As a result, engines frequently and incorrectly were diagnosed as needing "valve jobs" early in their life. In fact, the valve train in most engines usually functioned perfectly for well over 100,000 miles.

Early engines were subject to frequent failures of the head gasket, between the heads and the cylinder barrels; this was addressed in later models by increasing the width of the sealing area.

The large cooling fan located on top of the engine required the fan belt to bend from the vertical plane of the crankshaft to the horizontal plane of the fan, causing additional stress. Chevrolet engineers designed a unique fan belt, which many owners and dealers replaced with an inappropriate design. The correct fan belt, properly installed on the loose side, worked well, while other belts, installed loose or tight, would break frequently, giving the engine fan and belt design an undeserved bad reputation.

The pushrods were located below the cylinders, each in a separate metal tube between the crankcase and the head; these tubes also served to return oil from the head to the crankcase, and were fitted with neoprene O-rings at each end. After a short time, the neoprene developed a tendency to leak oil which became characteristic of Corvairs; unfortunately, since engine cooling air was diverted to the interior heater, this caused an unpleasant odor. Improved elastomer O-rings with much greater durability became available from aftermarket suppliers.

In the original four carburetor engine, only the two primary carburetors were equipped with idle circuits, for simplicity in tuning; however, with the progressive nature of the linkage, if the engine did not spend much time near full throttle there would be no fuel flowing through the secondary carburetors; as a result, the stagnant gasoline in their float bowls would continuously evaporate and leave a residue which built up, eventually completely destroying the carburetor. In the later versions of the engine, idle circuits were included in all four carburetors to prevent this problem, and there was a great deal of retrofitting and replacement of the older secondaries by owners. However, this in turn required that all four idle adjustments be balanced for a tuneup, which could be a problem in itself.

The progressive throttle linkage of the four barrel model was complex, with the axis of rotation changing planes. Unfortunately, this linkage was constructed simply and cheaply with rods simply passing through holes in bell cranks, rather than Heim joints; as a result it rapidly became imprecise and grew even worse with wear, making tuning the engine an exercise in futility. Some owners laboriously modified the linkage with precision components, but more purchased an aftermarket kit to rotate the carburetors through ninety degrees, thereby greatly simplifying the linkage. However, this also eliminated the progressive feature of the stock carburetor linkage, so that performance could not be optimized both at low to midrange rpm and at high rpm. However, it did also eliminate the problem of the secondaries becoming clogged from lack of use, mentioned above.

Other owners replaced the four single-barrel carburetors with a single four-barrel carburetor, centrally mounted. While this caused the carburetor and manifold to be slow to warm up to operating temperature and therefore caused problems with flooding and cold temperature operation, it eliminated linkage problems, simplified tuning the carburetor, and provided access to the large variety of four-barrel carburetors available on the market.

A factor which would have, in itself, led to the demise of this design was the rapid and relatively large temperature variation of the air-cooled engine with variations in load and rpm; this would have made meeting the upcoming emissions requirements of the 1970s difficult.