Vintage Smoke – Ford 6.9L IDI

1983-87 International/Ford 6.9L IDI

Public and governmental outcry after the oil shortages of the early ‘70s forced U.S. auto, light truck and commercial truck makers towards economical diesel power. Inroads had already been made getting more diesels into the Class 3 through 6 commercial medium trucks, and the higher GVWs had good coverage, but Class 3 and 4 were still often powered by little more than beefed-up passenger car engines. To that time, there wasn’t much incentive to develop diesels for Class 3 and 4 because the thrifty buyers of those trucks, mostly in-city short hoppers, were not keen on the usual 20% price premium. Rising fuel costs changed those attitudes but lower cost diesels were needed to push the idea across the finish line.

Fuel economy concerns soon became a factor with Class 1 and 2 light trucks (half to one-ton) as well. Diesels were part of the answer there too but there were few suitable diesels available and the industry addressed it in a short-sighted, haphazard way. Examples of that in the mid-late ‘70s included the 80 hp 3.3L Nissan in International’s Scout SUV and pickup lines, the 100 hp 4.3L Mitsubishi inline in Dodge D/W150-250 trucks and the infamous 120 hp 5.7L Olds diesel in half-ton GM trucks. In the early ‘80s, GM stepped forward a little with the 130 hp 6.2L diesel but you can see the common theme. A high buy-in for the privilege of owning a gutless wonder.

Few knew that International Harvester was working on a new diesel that answered the needs of truck owners from Class 2 through Class 6 (6,000 to 26,000 pound GVW). IH was in the midst of corporate upheavals and red ink but their medium-duty line was a high spot worthy of an investment. A new low-cost diesel was one of those investments. Like most medium-duty lines, the lower GVW end of the IH line was populated with gas V8s, many of those legacy engines from the ‘50s. Beyond the fuel economy angle, they were almost beyond hope of meeting future emissions regulations and being built on tooling that was worn out.

Target Specifications

The engineering target was for a moderately priced V8 diesel in the 400-450 cubic inch range, making 165 net horsepower at 3200-3500 rpm. After engineering and marketing outlined the basics, execs approved the project in 1977. The serious design work started in March of 1978 with a goal of starting production in March of 1983. The development had a good shortcut, the recently developed medium-duty MV404 and MV446 gas V8s intended to eventually replace the old V-345, V-392 and even older V-478. The new diesel borrowed some of the basic MV architecture and allowed the new diesel to be built on the same new tooling. The proposed 420 cubic inch diesel used the same stroke as the MV-446 but had a smaller 4.00 bore (0.125 inches smaller).

In that era, Indirect Injection (IDI) was the easy answer to meeting upcoming diesel emissions requirements and it also delivered better fuel economy than the DI engines of the day. Probably the main reason was low cost, the IDI requiring a much less expensive injection system. In those days, the Ricardo Comet Mark V swirl combustion chamber was the rage. Sir Harry Ricardo had developed the basic design in 1931 and though advances in fuel injection technology would soon tilt the advantage to direct injection, the Ricardo Comet V was a viable answer for the 1980s and was used in many engines of the day.

First Engines

The first prototype engine was completed in August of 1979 and 160 more followed. Early tests highlighted the need for better injectors. The first ones tried were an outward-opening poppet injector that did not deliver anything near the target power level and failed quickly. A temporary breakthrough was achieved by installing a set of two-orifice nozzles from another IH application, getting them through the initial tests and into field testing. It took a while to figure out the right combination of injector, injector targeting, combustion chamber location and compression ratio. The orifice nozzles proved non-optimal from an emissions standpoint, and they were sooting up, so new inward opening pintle injectors were designed and they integrated well with the Stanadyne DB2 rotary pump.

A total of 21,000 dyno test hours were accumulated at 3300 rpm, full load, peak power and some 52,000 test hours total. Starting in September of 1980, prototype engines were installed into trucks and field tested, with over 813,000 test miles accumulated. Once all the bugs were discovered and dealt with, they built 10 preproduction engines on the new line and some were tested a further 1,000 hours at full power on the dyno and then out in the field. Actual production started in August of 1982, beating the original timeline estimate by seven months.

Enter Ford

Once the concept was proven and testing well underway, IH began actively shopping the engine around. International wasn’t interested in supplying other medium-duty manufacturers but light-duty was another matter since they had discontinued their own light line by then. Ford Motor Company was on the hook almost from the start. Being only slightly larger and heavier than their existing 460 bigblock V8, it was a sure fit in Ford light trucks. Ford opted to offer it only in light trucks with an 8,500 pound GVW, or more, meaning F250HD and F350 models. That included the Ford E250 and E350 vans but not Ford’s medium-duties. It began a business relationship that lasted 30 years, though it ended badly.

Ford started installing 6.9L engines in August of 1982, first at the Twin Cities Assembly Plant and at all the relevant truck plants shortly thereafter. Until about March of 1983, you could only get it with a manual trans until the tooling was ready to mount the C-6 behind it. Ford advertised 161 horsepower at 3300 rpm and 307 lbs-ft at 2000 with a 19.7:1 compression ratio. These specs come from Ford Data Books, brochures and Ford Facts Books dated August and September of 1982. The early magazine tests listed these specs as well. International documents from the same era show the engine with a 20.7:1 compression ratio and it’s very likely the 19.7:1 ratio listed is either a preliminary number that was changed or incorrect. As it was, the 20.7:1 ratio delivered cold starting issues and it was increased to 21.5:1 for 1984 with piston and cylinder head changes.

Impact on History

In the medium-duty market, the 6.9L added an entry-level diesel into International’s 1600, 1700 and 1800 lines, both busses and trucks, with GVWs up to 26,000 pounds. That was good for International’s bottom line but it didn’t rock the medium-duty world and other manufacturers followed suit with similar engines. It sounded the death knell International’s medium-duty gas engines, including the relatively new MV series. Production of gassers at IH (later Navistar) dwindled into the hundreds of engines by the mid ‘80s and had ended completely by 1988.

The 6.9L was replaced by the 7.3L IDI in 1988, which acquired turbocharging in it’s final year of production, 1994. A total of 436,868 6.9L engine were built from August of 1982 to August of 1987. The HEUI diesel emerged in ‘94, called the T444E by International, and Powerstroke by Ford. By then, International Harvester had been broken up. The Motor Truck Division, including the engine manufacturing component, became Navistar International.

The 6.9L had more significance in the light-duty market and talking about it forces us to risk inciting GM versus Ford rivalry. GM beat Ford to the punch with the intro of the 6.2L diesel for 1982. It was a far better engine than the 5.7L it replaced but was still low on power. The 1982 6.2L GM diesel emerged with 130 horses and 240 lbs-ft. (135 hp in HD emissions) and stayed at those ratings until later in the ‘80s. The 6.9L emerged for ‘83 at 161 horsepower and 307 lbs-ft but was up to 170 horses and 315 lbs-ft by 1984. The 6.2L was available in all GM trucks into the C/K 30 realm and did pretty well in the half-ton lines. It was less stellar mounted in hard-working 3/4 and 1-tons where low power delivered mediocre loaded performance and some significant degradation in reliability.

The buy-in was still pretty high for both engines (vs a big block which was around a $500 for both FOrd and GM) so recovering those costs from fuel and maintenance savings could take a while. The 1983 GM 6.2L diesel option was $2375 ($1334 engine plus $1041 for the required diesel equipment package). For ‘83, the complete Ford 6.9L option at  $2225 for the 4×4 and $2543 for the 4×2. Considering the power difference, which was the better deal? Not to disparage the 6.2L, but in looking back, and using boxing metaphors, you could say the 6.2L was a lightweight or welterweight in it’s output and construction. The 6.9L was a solid middleweight or maybe a light heavyweight. The GM 6.2L was a good and economical choice in a half-ton but if you towed or hauled, the Ford 6.9L was the better choice. Today, most would sneer at a 170 hp NA diesel with 315 lbs-ft, but in the early ‘80s those numbers gave you some braggin’ rights.