revised July 1, 2000
The following is an interesting excerpt from Julius Mackerle's 1963 book "Aircooled Automotive Engines". Mackerle was chief engineer for Tatra motors during the 50's, and wrote the definitive work on aircooled engines. Thanks to Parisian Stefan Belatchev for ordering a copy of one of the last known copies of this book from the Polish National library...
Among air-cooled engines supercharging by turboblower is used in the Chevrolet Corvair engine. Supercharged Corvair engines appeared on the market in 1963 with a performance of 152 b.h.p. at 4000 rev/min, and a torque of 32.1 kgf rn (232 lb ft) at 3200 rev/min. By 1966 these values were increased to 182 b.h.p. and 36.6 kgf ma (264 lb ft) respectively at tile same speeds.
A Thompson turboblower is used in the Corvair. In this case the rotor shaft (12.7 mm in diameter) is mounted in a floating aluminium bush and runs at speeds up to 80 000 rev/min. The diameter of the turbine wheel is 75.5 mm, that of the compressor wheel 76 mm. The maximum admissible imbalance of the rotor is 0.005gin. At the end of the compressor wheel there is a carbon seal preventing escape of oil. At the turbine wheel, sealing is achieved by a piston ring. Heat transfer from the turbine to the casing is reduced by a cast iron plate, with an air gap. The annular sleeve connecting the cast iron turbine casing with the aluminium compressor casing also impedes heat transfer. The supercharging of the Corvair engine begins at full throttle at 2400 rev/min engine speed, and maximum, charging pressure of 0.7 kgf/cm2 (10 lb/in2) is achieved at 3500 rev/min. With further increase in engine speed charging pressure stays constant, and this results in a favourable time -torque curve.
However, increases in performance required some modification of the engine. Compression ratio had to be reduced from the original value of 9 : I to 8 : 1. During long term tests piston cracks were observed, and pistons had to be reinforced. For preventing excessive cylinder wear to the top piston ring was provided with a 0.1 mm thick layer of chromium coating. Connecting rods were also reinforced. To eliminate crankshaft breakages the original SAE 1045 carbon steel was replaced by SAE 5140 chromium steel. After hardening the shaft is tempered Until hardness is reduced from 321 Br to 255 Br followed by low-temperature liquid nitriding.
At maximum performance the temperature of exhaust gases increased by 94 degrees C against an engine without supercharging, and the exhaust valves had to be made from a higher grade material. To the stem made from chromium-silicon steel valve head is -welded from Nimonic 80A composed of 20 % Cr, 5 % Fe, 2.5 0/@ Ti, 2 % Co, traces of Al, Un, Si, C, and the remainder Ni. Cast iron for the valve guides was replaced by aluminium bronze, and arrangements for rotation increased the service life of valves.
An unusual feature of this engine is the positive-pressure timing regulator in the distributor, replacing the more usual negative-pressure regulator. The basic advance is 24' which is increased by the centrifugal regulator by a further 12' within the speed range 2900 to 4500 rev/min. Within the range of charging pressure from 0.07 to O.14 kgf/cm2 the pressure regulator reduces the advance by 9'. The charging pressure 0.01 kgf/cm2 is achieved at 2500 rev/min with the throttle fully open, and the pressure of 0.4 kgf/cm2 at 2750 rev/min. Advance therefore fluctuates between 15 and 36'.
The supercharged Corvair engine has on the left side in head -No. 6 a thermistor installed actuating a red warning light on the instrument board and an acoustic signal when a temperature of 300 'C is reached. However, under normal circumstances this state is never reached even on a motorway.
The performance curve of the engine has a sporting character, and therefore engine speed is mostly above 2500 rev/min at which the engine is already supercharged. Maximum engine speed is 5700 - 5800 rev/min at which there is risk of pump-up of the hydraulic tappets. For this reason the speed range 5300 to 5500 rev/min is already marked in red on the speedometer.
The improved valve gear operation made necessary a now cam shape by which engine noise was also reduced.
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