Chevrolet Corvair engine
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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, with aluminum heads (incorporating integral intake manifolds) 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 (225 kilograms). 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: it was the second production engine ever to be equipped from the factory with a turbocharger, released shortly after the Oldsmobile Jetfire V8.
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
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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).The ultimate performance was found in the Spyder model, which became available with a turbocharged engine rated at 150 hp (112 kW). The turbocharger was mounted on the right side of the firewall behind the rear seat, fed by both exhaust manifolds; a single sidedraft carburetor mounted on the left side of the firewall fed directly into the turbocharger's intake, with a chromed pipe leading from the turbocharger's outlet to what would otherwise be the carburetor mounting pads on the intake manifolds, which were integral parts of the heads. The turbocharged heads received some valve upgrades to improve durability.
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 the high performance normally aspirated engine, while the Turbocharged engine remained rated at 150 hp for this year. This increase in stroke was the maximum the engine could tolerate, to the point that the bottoms of the cylinder barrels had to be notched to clear the big end of the connecting rods, while the bottom piston rings at the lowest point of the piston travel actually passed over these notches. This in turn required that the gaps in the piston rings be aligned so as not to catch onto the edge of the notches, a fact unfortunately not immediately obvious to all beginning Corvair engine rebuilders.For the 1965 model year, all engines had the head gasket area between the cylinder and the head widened, in response to complaints of frequent head gasket failure. A 140 hp (104 kW) version with 4 single barrel 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; in addition, the heads featured higher compression ratio, and the cars received dual exhaust systems. Engines supplied with the automatic transmission were equipped with a camshaft identical to that in the lower end (95 hp) two carburetor engine, while those supplied with manual transmissions got the camshaft out of the higher end (110 hp) two carburetor engine. This engine became optional when the Corsa line was dropped, in 1967. Chevrolet sold 279 of these engines in the 1967 model year, 232 with manual transmissions, and 47 with Powerglide transmissions. Only six were sold with the four carburetor engine and the AIR injection system required by California emissions standards. These figures include 14 Yenko Stingers and 3 Dana Chevrolet variants of the Stinger.
The turbocharged engine now developed 180 hp (134 kW). Contemporary reviews describe a similarity in power between the turbocharged and four-carburetor engines throughout the low and mid rpm range, with the turbocharged engine being superior only on the top end. Enthusiast publications, however, began to describe how to modify the turbocharged engine to deliver more power "off the line".
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. When they were measured on a cold engine, 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 to proper tension, worked well, while other belts even of proper size installed loose or tight would break frequently, giving the engine fan and belt design an undeserved bad reputation. Since failure of the cooling fan on an air-cooled engine leads to immediate overheating much more quickly than in a water-cooled engine (within 15 seconds at the high RPMs when the belts were likely to fail), mechanically inclined owners would routinely carry a spare belt and the 9/16 inch box wrench needed to change the belt, in addition to adding a large and eye catching warning light in parallel with the normally sized factory generator/alternator warning light. Aftermarket manufacturers made available differently sized pulleys which reduced the fan speed to 1.3 or 1.2 times engine speed, rather than the stock 1.5; this reduced the tendency to throw or break a fan belt for engines which spent most of their time at higher RPMs.
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 exposed to the intense heat of the head lost resilience and 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 stock high performance 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 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 and attach the now colinear throttle shafts of the two carburetors on each side together, thereby greatly simplifying the linkage, as well as eliminating the problem of the secondaries becoming clogged from lack of use, mentioned above. 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.
Other owners replaced the four single-barrel carburetors with a single four-barrel carburetor, centrally mounted on a manifold with four long arms that attached to the original carburetor mounting pads on the heads. 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 the air cooled engine 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. The head temperature gauge with which the Corsa was equipped would visibly swing up and down as the car traveled between sunlit and shaded portions of the highway.
Corvair engines swapped into Volkswagens
Initially, the cooling fans were designed with a twist to the vanes, so that they were only efficient when rotating in the correct direction. Early on, however, the vanes on the fan became vertical and radial, so that the fan functioned identically in either rotation. Whatever the reason for this change, one effect was that the engine could easily be configured to run in the direction opposite from stock. This proved useful for those who swapped the engine into Volkswagen Beetles and dune buggies, since the Corvair engine's normal direction of rotation was opposite to that of the Volkswagen (and most other automobiles). Otherwise, the ring gear of the Volkswagen differential had to be flipped over by 180 degrees to allow the transmission's forward and reverse directions to be correct.This swap was fairly common at the time, with the Corvair engine serving to give a power boost to Volkswagen Beetles, dune buggies, and Karmann Ghias. Excessively vigorous use of first gear would break the transaxle (the prudent driver would avoid first gear altogether), and the engine cover of the Karmann Ghia would not close completely with a Corvair engine in place, but otherwise the swap was relatively problem free, as such things go.
Trivia and arcana
Many Corvair engine fans acquired a second life after the demise of their engines, mounted bottom side out on the outside of the wheels of Corvettes involved in road-racing, in order to pull air through the brakes and keep them cool. Lightweight and cheap, they were perfectly sized.The single carburetor on each head of the two carburetor engine was not mounted symmetrically in the center of the intake manifold, where it might be intuitively placed, but offset from the center, between the middle and end cylinders. Although sometimes erroneously cited as an engineering error, this was in fact an example of clever attention to detail; had the carburetor been placed in the center of the manifold, the center cylinder would have received a significantly greater air/fuel charge then either end cylinder. Instead, the carburetor was situated so that the firing order required the air flow to reverse itself when filling either of the nearer cylinders, whereas the airflow to the far cylinder was merely an extension of the airflow to the center cylinder, which was just prior in the firing order. This allowed for a more balanced filling of the three cylinders, and smoother operation.
High performance parts manufacturer Edelbrock made available a set of larger bore aluminum cylinder barrels (with cast iron liners to withstand wear); when combined with their aluminum pushrods, the rate of thermal expansion of all parts of the valve train became compatible, so that solid valve lifters could be used, rather than the hydraulic lifters required by the stock cast iron cylinders. This in turn allowed the engine to run to higher RPMs; in conjunction with the increased torque resulting from the increase in cylinder bore, this resulted in a substantially more powerful engine.
In addition, "stroker" crankshafts with longer stroke were quickly made available for the original engine. When Chevrolet increased the stroke of the stock engine, however, there was no longer room to increase it any further.
Immediately after the car became available with the original two carburetor engine, a number of manufacturers began to sell conversion kits for attachment of four carburetors, with either two stock carburetors, two of the ubiquitous Stromberg 97 carburetors, or a Rochester two barrel carburetor for each bank of cylinders. The means of attachment varied from simple two into one adapters, to machining off the entire top surface of the intake manifold (cast as part of the head), enlarging the internal passages of the manifold, and attaching a new upper surface incorporating the appropriate mounting pads for the new carburetors. Similarly, aftermarket manufacturers provided several means of supercharging the original engine, including belt driven centrifugal, axial flow, or rotary vane type compressors. Chevrolet, seeing the marketing opportunity available in these aftermarket options, of course went on to offer its own four carburetor and turbocharged versions.
See also
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