Mack Maxidyne Engine Historyby Rob Wagner
Mack Trucks began producing the Maxidyne diesel engine and the Maxitorque transmission in 1966 to meet the demands of modern long-haul driving and to provide a more economical performance. Mack was struggling in 1966 and had severe cash-flow problems, but the truck maker decided to gamble on the Maxidyne as a means to compete against Cummins, which was the leading supplier of diesel engines to big-rig truck builders.
Mack brothers Willie, Jack and Gus founded Mack Trucks in 1900 starting with a 20-passenger bus powered by a 40-horsepower engine. By World War I, the company had gained a reputation for producing durable mechanical workhorses. After World War II, Mack abandoned building trucks under 3 tons and focused on medium- and heavy-duty vehicles. Many of its most common postwar models featured long wheelbases, 306-horsepower engines and the 10-speed Mack Duplex transmission. By 1959, cab-over-engine trucks dominated the highways. Mack produced its trucks during the 1950s and early 1960s with aspirated or turbocharged Cummins or Mack diesels capable of producing up to 335 horsepower.
Even by the truck manufacturing standards of the 1960s, diesel power with a matching 10-speed manual transmission was inefficient. Mack wanted an engine that provided the best use of maximum output at any speed. Mack also needed a transmission to match the engine to reduce shifting. The Maxidyne ENDT-675 solved those problems. Mack introduced the Maxidyne in 1966 for the 1967 model trucks. The Maxidyne developed 237 horsepower with its torque peaking at 1,200 rpm and a powerband capable of cranking horsepower up to 2,100 rpm. The engine provided a low-end torque rise of up to 52 percent for an extra push under heavy loads at low speeds. Mack accomplished this by modifying the diesel engine’s fuel system and adjusting the turbo pressure. The standard of the 1960s was about a 20-percent torque rise, while in 2011, at time of publication, truck makers target 35 percent as the optimum torque rise.
Transmitting Maxidyne Power
While the Maxidyne offered optimum horsepower at any speed, the engine needed a transmission to transmit the power to the wheels without excessive shifting. For Maxidyne-powered trucks, Mack dropped its tough 10-speed transmission for the five-speed Maxitorque TRL 107 Series transmission. It featured a novel lightweight, compact, triple-countershaft design with the highest torque rating in the trucking industry. It was only two-thirds the size of its competitors’ transmissions. The transmission allowed the Maxidyne engine to propel a heavily ladened truck at 50 mph in fifth gear on a 6-percent grade and downshift just one gear. The truck could crest the top of a 6-percent grade at about 25 mph in fourth gear.
Mack upgraded the Maxidyne ENDT-675 with the ENDT-676 in 1973. It generated 285 horsepower and a whopping 1,080 foot-pounds of torque. The Maxidyne Six remained popular among commercial transport haulers and 8X6 cement mixers with a 75-ton maximum capacity. Three-axle dump trucks received the 180-horsepower Mack ENDT-673E Sixes, which was not related to the Maxidyne. By 1987, Mack dropped Maxidynes with the 2,100-rpm powerband capabilities for a lower speed version of up to 1,750 rpm. Contemporary Maxidynes engines have output ratings of 200, 335 and 370 horsepower.
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