In the middle of the 1960s Mack Trucks confronted serious financial strain from cash flow shortfalls, eroding market position, and engine offerings that lagged behind rivals such as Cummins and Detroit Diesel. Executives based in Allentown, Pennsylvania, placed the company's entire future on one bold engineering gamble: the Maxidyne engine fitted inside the all-new Mack R-Model. That single powerplant rescued the firm and rewrote the fundamentals of heavy-duty propulsion.

Conventional diesels of the era produced peak torque inside a narrow band between 1,600 and 1,800 rpm. When road speed fell below that window on a steep grade, output collapsed and drivers had to shift down to avoid lugging. Such behavior explains why many trucks carried 11-, 13-, or even 15-speed gearboxes, with some requiring 18 forward ratios.

The Maxidyne shattered this approach. Originally labeled the 11-liter inline-six ENDT 675, it delivered a modest 237 horsepower yet reached its peak torque of 906 pound-feet at only 1,200 rpm.

The constant-power revolution of the Maxidyne

Beyond its unusually low rpm torque peak, the engine achieved a remarkable 52 percent torque rise, generating 52 percent more torque at low revs than at its rated horsepower peak. Contemporary truck engines typically offered only 15 to 20 percent torque rise. As road load increased and engine speed fell, the exhaust-driven turbocharger together with a mechanical fuel-control system responded by increasing both fuel delivery and air mass per cycle. The outcome was nearly flat horsepower output across a broad operating range from 1,200 to 2,100 rpm, giving the Maxidyne a far wider usable power band than competing designs.

This wide torque delivery let Mack pair the engine with a simple five-speed Maxitorque transmission instead of the 10-, 13-, or 15-speed units common elsewhere. Fully loaded trucks could pull away from a stop in fourth or fifth gear without excessive clutch slip. On a demanding 6 percent grade the driver stayed in fourth, allowed revs to drop to 1,200 rpm, and maintained a steady 25 mph climb rather than downshifting repeatedly and crawling at single-digit speeds.

The Tip Turbine and intercooling breakthrough

Competitors soon recognized the shortcomings of their high-rpm, narrow-torque approach and spent the following decade developing high-torque-rise engines modeled on the Maxidyne concept. Mack continued refining the design to handle the greater thermal and mechanical loads created by high torque at reduced engine speeds.

In 1973 the company introduced the first air-to-air intercooled highway diesel series, the Maxidyne 300. To avoid lengthy intake plumbing, engineers created the Tip Turbine system, first applied to the intercooled ENDT 676 Cool Power engine. Compressed air from the turbocharger was partially diverted through a small auxiliary duct to drive a compact 66-blade turbine. That turbine spun a 10-blade fan that forced ambient air across an intercooler core mounted directly above the intake manifold, shortening the intake tract and improving throttle response.

The arrangement enabled the 11-liter six to produce between 285 and 315 horsepower with up to 1,080 pound-feet of torque, matching the output of Mack's larger 14-liter V8. Later four-valve versions of the 300 series reached 1,425 pound-feet. During the 1980s the family evolved into the E6 and then the 11.9-liter EM7, which entered production in 1989 and remained in manufacture for 15 years. These engines earned a reputation for exceptional durability thanks to their robust mechanical construction.