BEGINNING with descriptions of some past and present transmission designs, this paper goes on to describe the twin turbine Dynaflow transmission used in the 1953 Buick models. This design uses the torque multiplying charac- teristics of a planetary gear set and a hydro- kinetic torque converter while retaining the smooth, uninterrupted power flow typical of a fluid torque converter. A unique feature of this Dynaflow torque con- verter is that all the power transferred is through the gear set and first turbine at low speeds and gradually and smoothly diminishes as the power transferred through the second turbine increases until it does all the work at higher speeds. Several vector diagrams analyzing the driving and reaction torques and forces aid in explaining the operation of the twin turbine Dynaflow. The Author R. J. GORSKY (M '54) is staff engineer in charge of Buick's Transmission Division in Flint, Michigan. He came to Buick in 1931 as a cooperative student at General Motors Institute. After completing the college graduate training course, he joined the Transmission Division. From 1941 to 1945, he rep- resented Buick at Pratt & Whitney to expedite the manu- facture of the aircraft engine Buick was building. Upon returning to Flint, he began work on the development of the Dynaflow. TRANSTRANSMISSIONS are as "old as the hills," and MISSIONS one reviews some of the many designs ex- perimented with and those used in production, he concludes that there are about as many varieties. The use of an automotive transmission has al- ways been, and still is required to compensate for the shortcomings of an internal-combustion engine. The internal-combustion engine, as we know it to- day, cannot run below approximately 400 rpm with- out stalling; it develops relatively little torque at low speed, where the torque demand is the greatest to obtain good car acceleration; and it cannot be made to run counterclockwise readily to reverse the motion of the vehicle. Therefore, the primary purposes of a transmis- sion (or a transmission and clutch combination) are to prevent the engine from stalling when the car speed is low or zero; to enable the engine to operate at a higher speed where it develops more power while the transmission multiplies the engine torque for better acceleration; and to reverse the rotation of the driveshaft, using a gear train to provide a means of reversing the vehicle motion. These requirements can be obtained in many waysBuick's Twin as one realizes when he sees the many types of transmissions which have been used in the past 50 years. There are many secondary requirements which are expected of the transmissions of today, such as, ease of operation, smoothness, maximum car performance, best fuel economy, cost of manufac- ture, and weight. Everyone evaluates these items in a slightly different manner, and no one combina- tion has been arrived at which has the best of all items. Consequently, compromises must be made which result in different transmission designs, and the perpetual argument as to which is the best goes on. A few of the many transmission designs which have been used are as follows : The friction drive, which has a wide range of torque ratios, infinitely variable, was used in some very early vehicles. It has smoothness of ratio change and could be easily disengaged for neutral and shifted to obtain reverse. However, its short- coming was durability. Also, for the high-powered engines of today, materials have not been discov- ered which can endure the pressures required to prevent slippage. This type of drive is used today on some machines which do not require the trans- mission of much power and which can utilize the desirable characteristics of this design. Inertia drives have been designed and seem to be more of a novelty than a utility. They require a ratchet mechanism and consequently "free wheel." The public more or less rejected "free wheeling" cars back in the 1930's, and a reliable ratchet mech