With General Motors celebrating a century of worldwide automotive leadership, it's only proper to examine some of the corporation's many engineering accomplishments in this special installment of Mechanical Marvels. At first this seemed to be a simple matter of identifying a component or engine for a thorough analysis, but it quickly became apparent that we would need to narrow our scope--the manufacturer's history is filled with worthy candidates for examination. Faced with this task, an old adage was applied: If imitation is the highest form of flattery, then in the competitive automotive business world, using a rival's component in your own vehicle speaks volumes about the prowess of that opponent.
General Motors is an engineering and development powerhouse. Auto companies from around the world have put aside their rivalry and admitted that the Warren, Michigan, based research center did many things better than anyone else could. Thus, components invisible to the customer were either sourced from GM or licensed by them and could be found under the hood of a number of cars. In honor of this widespread use by the corporation's domestic competitors and those abroad, we've chosen four distinct areas of technology to represent GM's mark of excellence in engineering. Fuel delivery, climate control, power transfer and electrical charging parts were commonly derived from GM.
Thus, we will examine the Turbo Hydra-Matic 400 transmission that was also used by Rolls-Royce, Jaguar and Ferrari, along with many industrial applications; the Harrison A-6 air conditioner compressor; SI series alternator, and the Rochester Quadrajet carburetor. The Harrison air conditioner was often found under the hood of Ford products and in large agricultural and industrial equipment. The occupants of exquisite British motorcars would be kept cool by this unit, too. In similar fashion, the Rochester four-barrel carburetor was used by both Ford and Chrysler as part of select engine packages, and the SI series alternator became the industry-standard design with its electronic voltage regulation system. This alternator and transmission also became the mainstay of the hot rod and racing markets, and would be adapted to a myriad of engines and applications.
This carburetor, designed and produced by the Rochester Division, first appeared for the 1965 model year, and was a replacement for the 4GC that was used throughout the GM line. It was destined to eliminate the Carter and Holley carburetors that were specific to some of the engine families used through the late 1960s.
It had two stages of operation, a primary and secondary side. This is known as a progressive carburetor, since the two circuits build on each other. The primary, or fuel inlet, side had a small bore with a triple venturi booster equipped with plain tube nozzles. The metering principles were similar to most other carburetors, but the triple design provided more stable and finer fuel control in the idle and economy ranges over a standard single or dual venturi design. The signal provided by the triple booster, along with the placement of the mixture screws in the throttle body, gave the design a reputation for a smooth and slow idle, excellent part-throttle performance and outstanding fuel economy.
The secondary side had two large bores and employed the air valve principle for supplemental fuel control. Using the air valve method, fuel was metered in direct proportion to the air passing through the secondary bores. The fuel reservoir was centrally located to avoid problems of fuel slosh, which could cause engine stalling and delay fuel flow to the carburetor bores. The float system used a single float pontoon for ease of service. A sintered bronze fuel inlet filter was mounted in the fuel inlet casting, which was an integral part of the float bowl and was easily serviced.
The primary side of the carburetor incorporated six systems of operation. They were: float, idle, main metering, power, accelerator pump and choke. The secondary side had one metering circuit, which supplemented the primary main metering system and received fuel from a common float chamber.
The idle circuit was only on the primary side and was evoked not only during curb idle, but also during off-idle operation. The idle system was used during this period because air flow through the venturi was not great enough to create efficient metering from the discharge nozzles.
Each primary bore had a separate and independent idle system. As the primary throttle plates were opened from idle to increase engine speed, additional fuel would be required to match the improved air flow.
The main metering system supplied fuel to the engine from off-idle to wide-open throttle via two bores. The primary bores metered fuel using the venturi principle. The main metering system was in operation at all times when air flow through the venturi was sufficient to maintain flow through the discharge nozzles.
The secondary air valve had an attached assembly that acted as a damper, to prevent oscillation of the valve due to engine pressure pulsation. As the throttle was fully opened and air flow to the engine increased, the air valve would be blown open against spring tension and would lift up the attached secondary metering rods to supply additional fuel.
The Rochester Quadrajet was recognized as a superior four-barrel carburetor for many reasons, but was best known for the extremely strong booster signal created by the triple venturi design, which allowed the carburetor to be sized for large-displacement engines without sacrificing idle and part-throttle fuel atomization. This fact--along with a designed-in tuneability achieved by employing stepped primary metering rods with an adjustable vacuum-operated piston and spring tension--made the Quadrajet a powerful, reliable and fuel-stingy four-barrel carburetor, something that competing carburetor manufacturers couldn't claim.
Turbo Hydra-Matic 400
When the American public began to embrace the idea of automatic shifting, General Motors was at a disadvantage--its self-shifting gearboxes did not have the same reputation for reliability and smooth operation as those produced by Ford and Chrysler. The Turbo Hydra-Matic 400, or TH-400, was meant to be a replacement for the company's unconventional automatic transmissions, such as the Buick Dynaflow Special, Roto Hydra-Matic, Chevrolet Turbo-glide and the very heavy Hydra-Matic 315 controlled-coupling four-speed transmission. Thus, the TH-400 was meant not only to be used in heavy vehicles with 500-lbs.ft. of torque engines, but, more importantly, to enhance GM's reputation in the field of automatic shifting.
The TH-400 first appeared in the 1964 Buicks and Cadillacs, then in the 1965 Chevrolets, Oldsmobiles and Pontiacs. By 1966, the auto industry recognized how good this transmission was, and some AMC Jeep models and all Rolls-Royce cars were factory-equipped with a TH-400.
This was the first transmission built by the Hydra-Matic Division to employ a torque converter. The previous designs all used a fluid coupling that offered no torque multiplication from a standing start. Chevrolet did use a torque converter in its cast-iron case Powerglide, and Buick in its Dynaflow, but those gearboxes were built by the car division and not by Hydra-Matic.
From 1965 to 1967, a variable-pitch torque converter version of the TH-400 was produced. This design was offered only in Buick, Oldsmobile, Rolls-Royce and some Cadillac models. It featured a torque converter with a variable pitch stator. The stator angle could be varied between two positions dictated by throttle angle. Engineers and performance enthusiasts welcomed this design, but most drivers felt it had no value. After the 1967 model year, the Variable Pitch 400 was discontinued due to higher manufacturing costs and customer complaints about a high-pitched "whirring" noise. Since 1968, all TH-400s have used a fixed-pitch design.
Other versions of the TH-400 were created as well, including the 375 and 475 models. The 375 was used in 1972 to 1976 passenger cars and featured a unique design that employed a high stall rate torque converter for performance and a low stall speed for fuel economy and durability. The 375 was used with smaller displacement engines and used fewer clutch plates. The 475 was used in medium-duty trucks and school buses, motor homes and delivery vans. The major difference between a passenger car TH-400 and a 475 was the use of straight-cut spur gears in the planetary instead of the normal helical-cut pinions.
All TH-400 transmission designs employed a planetary gear assembly and the gear ranges were automatically selected and hydraulically actuated. The distinguishing feature of a planetary gear assembly is that output shaft speed changes can be made without disengaging the driveline. Gear changes are made by holding one member of the planetary assembly, or stopping it from rotating, or releasing one member so it is free to rotate.
The TH-400 also used multiple disc clutch packs, bands, one-way clutches, rollers and sprags to move the power from the engine to the driveshaft. A vacuum modulator monitored engine load and worked in conjunction with a governor to control upshifts; the governor was responsible for controlling the engine speed at full throttle and dictating when the transmission would shift to the next gear.
By virtue of its design and manufacturing quality, the TH-400 is a smooth-shifting, bulletproof transmission that often outlasts the car's engine with only minimal maintenance.
Delco SI Series Alternator
Delco Radio began a development program in 1963 to produce an integrated circuit (IC) voltage regulator to replace the then-used mechanical (vibrating) voltage regulation circuit. The program was coordinated with GM's Delco-Remy Division in order to make the design compatible with Delcotron alternators.
A conventional mechanical voltage regulator is mounted separately from the alternator in a convenient location; it senses air temperature and is influenced by the air flow in that part of the engine compartment. Engine compartment operating temperature is the predominant factor in determining the temperature of the battery mounted under the hood, at least after any significant period of driving. Changes in electrical load affect the operating temperature of the charging components.
The advent of the alternator as the primary electrical generating equipment for automobiles brought with it a modified version of the electromechancial regulator, long used with DC generators. The typical design used with an alternator was smaller, lighter and more dependable than the DC generator regulator, but still was subjected to similar problems and limitations.
In 1964, Delco-Remy declared to the semiconductor manufacturers the need and requirements for IC voltage regulation. The first working samples were completed around February 1965, and test quantities of regulators were available throughout 1965. Comprehensive testing increased GM's confidence to the point that the company began a program in July 1966 to launch an integrated circuit regulator on a 1968-model passenger car. The testing pace increased and the first production charging systems with integrated circuit regulator were used on the 1968 Pontiac Grand Prix.
There are many unique features that make the Delco SI (systems integrated) alternator so special. Not only does it feature the integrated voltage regulator, but it is adjustment-free, lasts more than 100,000 miles, is not affected by vibration, is fully protected against detrimental voltage transients, and is impervious to all environmental conditions, including dirt, temperature extremes, oil, road splash, anti-freeze, detergents, etc.
The Delcotron SI alternator raised the bar for amperage output with a small external case and was so successful that, by the early 1970s, all GM cars were fitted with one. The internal IC voltage regulator produced smoother current output at low engine speeds and aided the installation process by eliminating the amount of wiring required. The integral regulator has become the industry standard.
Harrison A-6 A/C Compressor
The Harrison Division, located in Lockport, New York, was General Motors' thermal management branch. Responsible for heat exchangers, the division also worked diligently to keep GM ahead of the curve with in-vehicle climate control and air conditioning.
An automotive air-conditioner compressor required much engineering and design due to the environment it was required to perform in and the load it placed on the engine's crankshaft. The ideal placement of the compressor was as close as possible to the crankshaft drive pulley and on the same plane. This was often not feasible due to packaging and service concerns, so a smooth and efficient compressor was of great importance. At Chrysler, the York-style two-cylinder compressor was known for good cooling, but was noisy and, more importantly, very rough running--not something GM wanted for its customers.
Debuting in 1961 for the 1962 model year was the Harrison A-6 compressor. This unit revolutionized automotive air conditioning with smooth, quiet and efficient operation. The new compressor displaced 12.6 cubic inches per revolution and weighed only 33.2 pounds. Inspired by an engineer's visit to the United States Air Force Museum in Dayton, Ohio, the Harrison compressor was a six-cylinder axial design that was modeled from an aircraft engine.
The A-6 (for axial, six-cylinder compressor) was mounted to the engine and used an electromechanical clutch which was belt driven from the crankshaft and allowed the compressor to be turned on and off and to cycle during operation to control pressures. The Harrison design relied on the rotation of a tilted disc to oscillate the pistons. This manner of operation was once widely used in specialized internal combustion engines of the "axial" type, which were known for their inherently good balance and smooth operation.
The Harrison A-6 compressor was quickly recognized for having an exceptional ability to produce cooler air conditioner discharge vent temperatures while also consuming less power. The component was also known for smooth pressure rise and, due to the lower power it took to operate the compressor, it had less of an impact on the engine's coolant temperature from the accessory load.
By the mid-1970s, the A-6 compressor became part of a Harrison air conditioner system known as VIR, for "valve in receiver." This design had a serviceable receiver dryer that was located on the suction or low-pressure side and resided near the evaporator. This was in contrast to the common placement of the dessicant in the high pressure or liquid side of the system. The large capacity of the A-6 compressor allowed it to be used with excellent results in the largest full-size GM cars that ended production for the 1976 model year. These vehicles had an extremely large greenhouse area (a great amount of glass), which placed a higher demand on the climate control system. The A-6 met this challenge and was known to produce "ice cold" vent discharge temperatures even when the VIR system was low on Freon and needed to be serviced.
The Tradition of Excellence Continues
Today, General Motors, though bombarded with lots of unfair negative press, is still a world leader in engineering and design. Unbeknownst to many, the corporation has a long list of modern accomplishments in the realm of mechanical marvels. Some of these are:
A joint venture six-speed transaxle with Ford
Supplying 3.1-liter V-6 engines to Honda and Isuzu for the Passport and Rodeo SUV
Dual-mode hybrid technology
The Cadillac Northstar engine, capable of driving 60 miles with no coolant
On-Star satellite communications
The upcoming Chevy Volt continuous-range electric car
On its 100th anniversary, we salute GM for its reputation for excellence. We are proud of their American heritage and wish them another 100 years of success.
This article originally appeared in the March, 2008 issue of Hemmings Classic Car.
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