In the early 1980s, Ford made a bold move in the consumer truck market by partnering with International Harvester to introduce the 6.9L IDI diesel engine in their heavy-duty truck lineup. The engine quickly became a popular option, offering comparable horsepower to Ford's gasoline engine while delivering 6% more torque and significantly better fuel economy.
Over time, the engine evolved to meet the changing needs of the market. In 1988, Ford introduced the 7.3L IDI diesel engine, phasing out the smaller 6.9L IDI. Although the two engines share a similar design, the 7.3L features subtle changes that give it several key advantages, especially in terms of performance.
One major difference between the two engines is their bore diameter. The 7.3L IDI has a larger bore at 4.11 inches compared to the 6.9L's 4.00 inch bore diameter. The 7.3L also features an improved cylinder head design, including a larger prechamber and offset glow plug locations, and 1/2 inch diameter cylinder head bolts compared to the 6.9L's 7/16 inch head bolts.
To combat power loss at altitude, Ford introduced a turbocharged variant of the 7.3L IDI in the 1990s. This engine featured several notable improvements, such as a larger cylinder head gasket fire ring, an increased piston wrist pin bore, and an updated piston ring design. It also had an updated exhaust valve made from a higher-strength alloy, a heavier-duty flywheel and engine damper, and separate injection pump calibration and G code injector nozzles. The glow plug controller was relocated to the passenger side valve cover, and the CDR moved to the driver side valve cover.
Despite its simple design and lack of modern features like direct injection and turbocharging, the 6.9L IDI diesel engine was very impressive in its time period. It was mechanical, used indirect injection, and was naturally aspirated, but its simplicity contributed to its reliability. The 6.9 and 7.3 were used in various power ratings in medium-duty trucks and also found applications in industrial, construction, and agriculture settings.
In its most powerful version, the naturally aspirated 7.3L IDI in the Ford F-Series was rated at 185 horsepower and 358 lb-ft of torque, while the turbocharged version produced up to 190 horsepower and 388 lb-ft of torque. Both engines were well received by consumers and remained in production until they were eventually phased out in favor of newer, more advanced engines.
The evolution of Ford's IDI diesel engine from the 6.9L to the 7.3L is a testament to the importance of constantly innovating to meet the changing needs of the market. While the engines are no longer in production, they remain important milestones in the history of diesel engines and continue to be revered by many truck enthusiasts to this day.