A short History of Ford’s first IDI and DI diesel pickups
It was the 1970s, and people were reeling from the shock of gas prices that had tripled in just a few short years. The effect was profound. We got a 55mph speed limit, the government began to mandate fuel economy standards and people were scrambling for vehicles with better fuel economy.
Diesel engines had not been much of a presence in the American car and light truck market up to then but were immediately eyed as a fuel economy answer. The collective American eyebrow rose at the idea of noisy, smelly, smoky diesels and the notable scarcity of diesel fuel stations back then. But the higher mpg and low-cost fuel were attractive, especially in the truck world where a torquey gasser meant sub-10mpg fuel economy.
In the light truck world, GM and Dodge were about tied in 1978 for being the first to offer a diesel pickup. The Chevrolet entry was a C10 with the notorious Olds 5.7L V8 diesel making 120 naturally aspirated horsepower. Dodge fielded a 4.0L (243cid) Mitsubishi NA diesel inline six with 100 rip-snorting horsepower in half- and three-quarter-ton 4×2 and 4x4s. Dodge dropped the Mitsu after 1979 and remained diesel-less until 1989. Chevrolet offered the 5.7L in C10s through 1981 and replaced it with the vastly better 6.2L in 1982. The stage was now set for a grand entrance by Ford.
Ford and International Harvester Team Up
IH started development work on a V8 diesel for medium-duty trucks in 1978. The architecture was based on the company’s own 446cid industrial gas V8. “Ack!” you may say. “A conversion!” Not really. The diesel merely adopted similar block dimensions that made tooling up easier. It isn’t exactly clear when Ford Motor Company’s interests were piqued by the IH diesel, but in 1981 a $500 million agreement was signed for IH to develop the engine for Ford light and medium-duty trucks and to supply engines for five years. Because it was a hefty engine, more than 900 pounds, it was soon clear this was going to be a three-quarter ton and up powerplant. When the diesel option emerged for 1983 it cost $2,225.
The Ford diesel light trucks debuted in late 1982 as ’83 models. The lightest duty truck in which the 6.9L (420cid) diesel was available was the F250HD (8,600-pound GVW) and they went all the way up the light line, including the E-Series vans and into the Ford medium duties. The first advertised rating was just 161 hp and 307 lb-ft with a 19.7:1 compression ratio. These were likely preliminary specs because 170 hp and a 20.7:1 compression ratio very soon became the advertised norm.
Cranking out 420 cubic inches from a 4-inch bore and a 4.18-inch stroke, the 6.9L featured oil-cooled pistons, four-bolt mains, a massive forged crank with a 2.2-inch rod and 3.1-inch main journals, valve rotaters, roller tappets, gear-driven cam and injection pump. It was naturally aspirated and indirectly injected using the Ricardo V combustion chamber. Injection came from a Stanadyne (Roosa-Master) DB2 rotary pump and pintle-type injectors that were popped at 2,100 psi. Issues with cold starting came right away, so in ’84 the compression ratio was raised to 21.5:1. Torque increased to 315 lb-ft as a result. That’s where output would stay for the remainder of the 6.9L run that lasted through 1987.
The engine got a makeover for 1988. The bore was increased 0.18-inch and that boosted displacement to 7.3L (444 cid). The heads, head bolts, head gaskets, rocker gear and combustion chambers got a workover and the glow system was completely revised. The injection system also got some tuning alterations. As a result, the engine was boosted to 180 hp at 3,300 rpm and 338 lb-ft of torque (some spec sheets show 345 lb-ft). In mid-year 1992 power output was increased to 185 and torque went up to 360. The 1992 model year also brought a serpentine belt system.
The revisions in the 7.3 were largely successful but there were some stumbles. The overbore and cooling system changes resulted in an increased tendency toward cavitation damage on the cylinder walls in the water jacket. It was manageable using the right anti-corrosive coolant additives (called SCAs, supplemental coolant additives) but it became a well-known problem.
The IDI reached its zenith late in 1993, when the first 7.3L turbo was introduced. It mounted a Garrett wastegated turbo with an A/R of 0.82. Advertised power and torque were 190 hp and 385 lb-ft, but this engine was somewhat underrated. If you look at the nearby power chart, you will see it’s mysteriously cut off at 3,000 rpm vs. the NA engine going to 3,300. The power line looks to still be climbing at 3,000 and extrapolations show the turbo engine was somewhat over
200 hp at 3,300. Speculation within the IDI enthusiast community is that with the new 210hp Power Stroke engine on the horizon Ford marketing wanted to make sure the new engine looked “better” than the old one. The IDI turbo engine was hyped more for its high-altitude performance than its raw power, but tweaking in the intervening years has shown that the IDI turbo is fully capable of 250 hp with only minor tuning.
Internally, the IDI turbo was given improved head gaskets with a heavier fire ring. The pistons were given keystone rings and anodized crowns. The wrist pin diameter was increased from 1.110 inches to 1.308 inches. Inconel exhaust valves were added and the oil cooler bundle was increased from 24 fpi (fins per inch) to 30. The injection pump calibration was altered and new injectors were used. Minimum boost was 5 psi but most developed around 8-10 psi in service.
The Turbo IDI and the Power Stroke shared the stage in ’94 and sales overlapped a little after the PSD’s mid-year intro. In reality, the Power Stroke is very much the IDI engine’s progeny. Especially in the first-gen PSDs, the IDI heritage is pretty clear and there are even a few interchangeable parts.
The IDI Today
Nearly 1.5 million 6.9L and 7.3L IDI engines were produced by International Harvester and its offspring, Navistar International. They are truly one of the cornerstones of the growth of diesel power in pickups. The IDI is an old-school engine and can’t hold a candle to the modern electronic engines in the power department, yet it has a large following. There are still gobs of them on the road, some still even in commercial livery. And they just go on and on. One IDI diesel fan has been heard to say, “This thing will have parts availability until the sun runs out of hydrogen!” That may be an exaggeration, but the IDI Ford is nowhere near the end of the road.
The Ford Package: 1983-1986—The Bullnose Era
Ford had debuted a new line of trucks in 1980 and the diesel entered the game right in the middle of that ’80-86 generation. Today, this era of Ford trucks is often called the “Bullnose” for its broad snout. It came in a Standard (base) trim, XL and the full-boat XLT. Body styles ranged from regular-cab longbed, SuperCab longbed and, in some years, a crew-cab longbed. “Lariat” was added to the XLT designation in ’85.
In this era you had the choice of two transmissions, the Warner T-19 four-speed or the legendary C-6 automatic three-speed. If you ordered four-wheel drive, it came with a BorgWarner 1345 transfer case. Axles varied by year. To mid-’85 the rear axle was a Dana 61 or 70 axle. In mid-year ’85 the 10.25-inch ring gear Ford Sterling debuted in semi or full-float versions (full-float only on the diesels). F-350 DRWs most often had Dana 70 HD axles but there are DRW Sterling axles out there too. Up front, the Twin I-beam non-driving axles carried the load for 4x2s. The F-250HD 4×4 had a twin traction beam (TTB) Dana 44 as standard with a Dana 50 TTB optional. The F-350 came standard with the stronger Dana 50 TTB. In ’85 the Dana 60 solid front axle was introduced and most F-350s had it. Only two axle ratios were offered for the diesels, 3.55 and 4.10:1. This was to remain the practice all the way through the IDI era.
1987-1991—The Flatnose Era
In 1987 Ford undertook some restyling without changing the basic body shape all that much. To IDI diesel nuts these trucks are known as the “Flatnose” or “Bricknose.” Flattening the nose and using flush headlights achieved a more aerodynamic look. Fender well shapes changed as well as the interior. A small “Diesel” badge appeared under the driver’s headlight. Trim levels changed a little, the base level being called “Custom,” but the things included in each of those levels also changed, generally moving upmarket.
The 6.9L engine soldiered on for ’87 but was replaced by the 7.3L IDI for ’88. A big innovation that occurred later in ’87 was the ZF five-speed manual trans option. At first it was a choice between the standard T-19 four-speed, the five-speed and the C-6 automatic. Later (by ’88) the T-19 was dropped and the five-speed became the only manual option. Like the T-19, it had a taller first gear than the gas engine version.
For ’89 the E4OD overdrive automatic was offered in the diesel line. That was a big innovation, offering the mpg improvements of overdrive to those desiring automatics, but it had taken Ford a while to build one stout enough for use in the higher GVW light trucks. There was a mix of three-speed C-6 and E4OD automatics that year and it remained that way through 1994, though in later years the C-6 was available only in the commercial cab-and-chassis lines. Suspension and axles stayed the same as the previous trucks, but in this era the Dana 44 TTB was dropped and the F-250HD inherited the Dana 50 as the standard front end for 4x4s. With a few exceptions, F-350s still used the Dana 60.
1992-1994—The OBS Era
The front wrap changed again for ’92 but, again, the basic body shape didn’t change much. This time the front end was more rounded. For some reason, Ford diesel fans are calling this generation IDI the “OBS” for “old body style.” We suppose that’s because the Power Stroke also used this body well into the ’90s. The interior got a big makeover again. The powertrain options changed little.
The big news came in ’93, when the turbocharged version of the IDI diesel became an option. The NA IDI continued as the baseline diesel option but the output had increased to 185 hp and 360 lb-ft, the highest power and torque levels the NA IDI would achieve in factory trim. The Power Stroke was a mid-year entrant for ’94 and the IDI trucks were soon in the “Oh, by the way” category.