Turboglide

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The Turboglide is a Chevrolet constant torque 3-speed automatic transmission[1] that made its debut as an optional transmission on Chevrolet V8 passenger cars for 1957. It consisted of a concurrently geared (as opposed to sequentially geared) planetary gearbox with a 'switch pitch' dual pitch torque converter stator. Turboglide utilized a die-cast aluminum transmission case, following Packard's Ultramatic of 1956. It was designed to help showcase the engineering features of the '57 Chevy, and was often ordered with the Rochester RamJet Fuel Injection system on the 283 V8. Turboglide cost about $50 more than Powerglide, and was available in all 1957-1961 V8 engine models except the Corvette.

Concept[edit]

Chevrolet Turboglide was modeled along the lines of the Buick Flight Pitch Dynaflow transmission, and operated very similarly to the Buick transmission. Using a five element torque converter, (pump, individual turbines for low, intermediate and direct ranges, and a switch pitch stator) the Turboglide offered the smoothest operation of any automatic in the industry, as no 'shift' occurred between ranges and hence there was no potential to create a disruption in the perfectly smooth flow of power. For extra 'getaway' power at any road speed, the converter stator vanes could be directed to a steeper angle which raised the stall speed of the torque converter and increased torque multiplication, as well as directing the oil flow at the turbine for the next lowest range. Turboglide's selector quadrant was Park, Reverse, Neutral, Drive, Hill Retarder (early 1957 only) P R N D Hr[2] or Grade Retarder (late 1957-1961) P R N D Gr. Some owners of the early 1957 models with the Turboglide transmission marked with HR [Hill Retarder] mistook the HR to mean High Range so the marking was changed early in the model year to Gr [Grade Retarder].

Operation[edit]

The basis of Turboglide's unique abilities was its five-element torque converter. Most automatic transmissions have a three element torque converter, consisting of an impeller, which is an engine driven pump turbine that rotates with the flywheel, a transmission drive turbine that receives the energy of the oil propelled by the impeller, and a stator that redirects the recirculating oil so that it strikes the back side of impeller blades to assist engine rotation and recapture energy from the fast moving oil. The stator is what gives the torque converter its torque multiplication ability — in effect allowing it to reduce ratio to increase torque when the turbine is rotating slower than the pump.

In most automatic transmissions, the drive turbine only functions as an input to the transmission where the power is redirected to gears in its automatic gearbox. Turboglide differs by using three individual drive turbines, each connected to a different ratio inside the gearbox. Oil leaving the pump first strikes the turbine connected to the Low (first gear) ratio. The design of the turbine permits oil to pass through its blades and strike the next turbine in line, the one connected to the Intermediate ratio, which receives some of the energy remaining in the oil flow. After leaving the intermediate turbine, the oil is passed to the direct turbine which is direct drive at drive shaft speed. As the car starts from rest, the low turbine, which has the greatest gear reduction ratio is relatively easy to bring up to match pump (engine) speed and the car starts from rest easily and accelerates quickly. While this occurs, the intermediate turbine receives an ever increasing share of the energy from the pump, as the Low turbine moves faster and oil passes more freely through it. Eventually, the low turbine matches pump speed, and the low turbine freewheels on a one way sprag clutch much like on a bicycle. At this point, the intermediate turbine is carrying the majority of the drive force, and some energy is being sent to the direct turbine from the oil passing through the intermediate turbine as it also begins to match pump speed. Eventually, the intermediate turbine also matches pump speed, and the oil passes freely through low and intermediate turbines to drive the direct turbine alone, in direct drive (1:1 ratio). Because of the overlapping power input in different ratios, there is no 'shift' or sequential gearing, rather there is a concurrent geared drive, in effect the car starts out in 1-2-3, eventually drops 1 to be in 2-3 and eventually drops 2 to be in 3 alone.

The stator element of the torque converter has two blade positions, controlled by the driver via the accelerator pedal to offer a 'passing gear' and extra response at any speed from heavy throttle application. In normal driving the stator blades are arranged at 'cruise' angle which offers improved efficiency and response at light throttle. Flooring the accelerator pedal changes the angle of the stator vanes hydraulically to 'performance angle' which permits the converter to achieve stall about 1000 rpm higher than in 'cruise' as well as redirecting oil to strike the next-lowest drive turbine which effectively lowers the drive ratio of the transmission and allows engine speed to flare to a speed where output is greatest.

Because of its unique operating characteristics, Turboglide did not require large changes in engine RPM even with very large differences in car speed or desired engine output. In fact, accelerating from rest, the engine speed would remain nearly constant(with no movement of the accelerator), even as the vehicle accelerated. The actual RPM would be a function of the pressure applied to the accelerator. One interesting effect during acceleration was the "audio" transition from the first gear turbine (occurring about 2 seconds after takeoff with moderate throttle), then progressing through the remaining 2 turbines which were not so audible. (A similar audio effect occurs with a stepped 3-speed automatic...first gear having a characteristic whine). At full throttle, Turboglide will maintain the engine RPM within a narrow (~400 rpm) span of speed around 3500-4000 rpm where best engine output is maintained, and the car will slingshot to catch up as the ratios drop imperceptibly as the car road speed increases. The effect is very similar to a jet aircraft upon takeoff- the engine maintains a strong high speed, and the car accelerates smoothly and strongly from rest to maximum. In normal light traffic driving, Turboglide automatically selects the highest (lowest numerically) ratio practical and engine speeds run in the 1000-2500 rpm range, saving fuel and improving the throttle response of the car to small pedal motions. Turboglide also offered a 'Grade Retarder' range for engine braking which could absorb close to 200 hp (150 kW) of energy above what was available via engine compression, by counter-rotating the turbines in the converter and soaking up vehicle energy by agitating the oil inside the converter housing. No low range was provided with Turboglide, as in Drive range the ratio was always automatically matched perfectly to the requirements of the operator (all ranges being always engaged, low and intermediate would freewheel on the 'sprag-clutches' as needed), and Grade Retarder made a very efficient engine brake.

According to Chevrolet manual, above 65mph, wide open throttle, the torque converter begun to act as a hydraulic coupling, once third turbine became fully engaged and no multiplication of torque occurred, as it would at lower speeds. So, to all practical purposes, at speeds higher than 65mph, Turboglide acted as if engaged in top gear, while, below that speed, torque multiplication changed imperceptibly, save for mechanical noise and acceleration.

Continuously Variable Transmissions (CVT), which are available in some smaller cars, offer a similar, albeit completely mechanical, feel, but may be geared to multiply torque at any speed.

History[edit]

Turboglide was designed under the supervision of Frank Winchell, Ed Cole and Robert P Benzinger at Chevrolet engineering. Although Turboglide offered remarkable performance and smoothness, the execution of the first 1957 units led to substantial customer complaints and the unit was continuously upgraded to improve its durability until it was discontinued in 1961. The 1959 Turboglide incorporated very significant changes intended to improve its durability, some of which were possible to retrofit to earlier versions.

Notes[edit]

  1. ^ Flory, Jr., J. "Kelly" (2008). American Cars, 1946-1959 Every Model Every Year. McFarland & Company, Inc., Publishers. ISBN 978-0-7864-3229-5. 
  2. ^ http://www.oldcarbrochures.com/static/NA/Chevrolet/1957_Chevrolet/1957_Chevrolet_Owners_Manual/1957%20Chevrolet%20Manual-13.html

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