The ’70s TV talk show host Tom Snyder was known for asking, “What goes through their minds?” when confronted with inexplicable human behavior. That might apply when pondering why GMC executives directed the development of the Toro-Flow diesel. Given the array of diesels the GMC truck line had to choose from, most notably those built by GM itself, it seems an odd way to spend money. Well, GMC execs saw a low-cost diesel gap in the medium-duty truck market. Yes, the GM two-strokes were widely available, but they were expensive and noisy. GMC was not averse to going outside the company for an engine, but none of the choices quite met the low-cost criteria either. Introduced in 1964, the Toro-Flow was the answer, but it had some baggage.

The development process for a four-stroke, low-cost diesel began in 1953 and was eventually synchronized with the development of a new 60-degree V6 gasoline engine to be introduced for 1960. This is not to say the diesels would be “converted gas engines.” They weren’t. It’s more accurate to say the gas and diesel V6s were concurrent developments to be built with similar architecture and on similar tooling. There was very little actual parts changeover.

Here’s a D478 V6 for a medium-duty application from around 1968. You will have noted by now that the intake and exhaust manifolds are on the same outboard side of the head. It may seem strange on a narrow-vee engine, but was necessary to make room for the injection pump.

The well-regarded GMC V6 gassers were initially offered in 305, 351 and 401 ci, but by ’62 had grown to 478 ci and later 379 and 432 ci versions were offered. They were one of the first V6 engines offered in the United States, and the 305-powered GMC fleet of light trucks, a deliberate snub of the Bowtie Stovebolt inline. But enough gasser drivel!

The D and DH637 V8 Toro-Flows shared the V6 architecture. At 195 and 220 hp respectively, they compared to the bigger V8-265 Cummins of the era (785 ci, 265 hp @ 2,600) and the Caterpillar 1160 V8 was nearly the same displacement and output as the DH637 and, like the Cummins, was seen in the upper end of the medium duty range. Strangely, both the Cat and the Cummins V8s also shared a somewhat tarnished reputation.

The Toro-Flow diesels debuted in 1964 in two displacements, 351 ci (the D351, 130 hp) and 478 ci (D478, 150 hp). A high-output model was also offered, the DH478, cranking out 170 hp. It’s been reported that GMC toyed with the idea of a diesel 305 V6 in the light trucks but it isn’t clear how far the idea went. Probably not very, given the light truck market of the day. It was common enough to see D351s swapped into GMC pickups by the handful of owners motivated to do it. It was a relatively easy swap, sometimes done on new trucks by the dealer.

One of the earliest marine conversion companies to glom onto the Toro-Flow was Crusader Marine. This dolled-up 1964 D478 marine assembly was used in advertising. The Toro-Flows did best in marine pleasure craft applications due to the relatively lowstress environments with unlimited cooling capacity. Toro-Flows were popular in this venue also for their low cost and high fuel economy. General Motors

The D351 was seen in the lightest tonnage models of the medium duty line and the D478 was seen in the heavier 4000 and 5000 lines, with the DH478 as an option. For 1965, certain models in the Chevrolet medium-duty line also appeared with the same Toro- Flow V6 diesels as an option, in addition to the 53 Series Detroit two-strokes. Chevrolet labeled it the Torq-Flow in some 1965 literature but reverted to “Toro” in later publications. It isn’t known if this was an error, a return dig at GMC, or an ill-conceived notion to be different. Chevrolet continued to offer the Toro- Flow in its medium-duty line into 1974, but it was a rare option.

A cutaway DH478 Toro-Flow II from 1969 shows off the rather Detroit Diesel-like green paint as well as some of its internal workings. The four-main-bearing crankshaft was forged and weighed about 100 pounds, with main journal diameters of 3.126 inches and 2.81-inch rod bearing journals. The forged rods had a 7.19-inch center to center length. Oil capacity was 10 quarts. An oil cooler was optional on the D478 and standard on the DH478. The specs book lists the V6 engines as having dual 2.5-inch exhausts. General Motors

For ’66, the D637 Toro-Flow V8 was introduced and rated at 195 hp. Concurrently, a DH637 was introduced with 220 hp. A funny sideline to the D637 V8 is that the ’67-72 637 gasser was, in many ways, a converted Toro-Flow 637 diesel with a lower compression ratio and a carburetor. The combustion chamber was in the piston, like the diesel, and the heads were flat like a diesel.

The narrow 60-degree vee is apparent here and illustrates why the manifolds are outboard. It’s a pretty common-sense design that allows a compact package size for installation. Note also the combustion chamber in the piston, but unlike most direct-injected diesels the injector is canted. The whole thing was designed to induce a swirl effect to the intake flow for a more thorough mixing of the air and fuel. Whatever faults the Toro-Flow had, it delivered on the promise of fuel economy. This engine is shown with the optional oil-to-water cooler. Compression ratio was 17.5:1.

The Toro-Flow II debuted for 1969 but it was little more than a few reliability updates and a new engine color, a Detroit-like green replacing the earlier GMC orange or Cat-like yellow. The D351 was eliminated, as well as the standard power D478 and D637, making them all “DH” models. For 1972, GMC renamed them again, adopting the “Turbium” moniker and this remained until the end of production in ’74.

The mighty DH637 Toro-Flow II V8 shows a little cheesecake in this dolled-up show display. In this era, turbocharged marine version of the DH637 were seen with both single turbos and twins. It’s doubtful GMC had much to do with those, but they did develop a turbocharged DHT478 for bus applications.

The Toro-Flow became a relatively popular marine diesel in pleasure craft due to its low cost. Crusader Marine, American Marine, Barr Marine Products and Daytona Marine offered converted V6 and V8 Toro- Flows. There were twin-turbocharged marine versions that never saw use on the road. A marine DHT478 cranked out 220 hp and the turbocharged DHT637 V8 made 300 hp. A turbocharged DH478 V6 made the road in the late ’60s GMC bus lines, but finding one is rare. The available books don’t show a turbocharged Toro-Flow V8 in trucks, though some unconfirmed sources claim they existed.

The “Toro-Flow” name was derived from the engine’s toroidal combustion chamber action, designed to induce a swirl to better mix the air and fuel. The Toro-Flow engines were all narrow 60-degree V-type engines and could be built with both right- and left-hand rotation. Both the V6 and V8 Toro-Flows used balance shafts. All the Toro-Flows were over-square, with the D351 having a bore of 4.56 inches and a stroke of 3.56 inches. All the other engines shared a 5.125-inch bore and 3.86-inch stroke.

The injection system came from American Bosch in the form of the PSJ rotary pump. That’s not one many people remember but it was used on a few engines in that era and is similar to the better-known PSB. It fed Bosch injectors that popped at 3,000 psi. Max full-load rpm was 3,200 for the V6 engines, 2,600 rpm for the D637, and 2,800 for the DH637.

There isn’t a whole lot to dislike here, at least on paper: fourbolt mains, a chrome-nickel alloy block and six bolts per cylinder to tie the head down. With a 950-pound long block assembly weight (D478, the D351 was 10 pounds less), it seems like there was plenty of beef to work with.

The Toro-Flow has a rather notorious reputation and is known for two main problems, the American Bosch PSJ injection pump and head gasket failures. The word from back in the day was that regular head re-torquing eliminated most problems relating to head gaskets. The reputed PSJ pump issues are not clear. Various failures are also attributed to continuous hard use and high revving. Overall, Toro-Flows did better in the lighter duty applications than they did in the higher GVW trucks. They fared better in marine applications because those were mostly in pleasure craft. In either case, many parts are incredibly difficult to find, although there seems to be no shortage of remaining engines.

Circa 1964, here are the power and torque graphs for the D351, D478 and DH478. In this era, the engines were run out to 3,200 rpm for rated peak power. Later, rated rpm was dropped to 2,800 rpm for engine longevity. Though it doesn’t have the highest peak number, the small D351 has the flattest torque curve.

The Toro-Flow’s bad rap was rooted in reality but also needs to be viewed in context. They were some of the first diesels designed and built specifically for the medium-duty truck market of the ’60s, which at the time was a market still dominated by very well-proven gasoline engines. Strange as it sounds, that market was skeptical of diesels and not 100-percent ready for the change to diesel. Could GMC have done better, even on the tight budget they set for themselves? Sure. Let’s call just call the Toro-Flow an evolutionary step with some stumbles and give it a little respect for being first. DW

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