GM’s Detroit Diesel division’s legendary “Jimmy” two-cycle diesels were a genuine game changer in the transition to diesel engine adoption in trucks and buses starting in 1938-1939. They were powerful and durable, but also expensive. In the early sixties, the GMC Truck and Coach Division released a completely unrelated line of lower-cost diesels, four-stroke engines based on their new 60 degree V6 gas engine. It was called the Toro-Flow, and although it had a lot of promise and hype, it largely failed to live up to its expectations. There’s still huge numbers of DD “Jimmys” in service today; good luck finding a running Toro-Flow anywhere.
Well, I have, and there’s some video if it further down. Before we get to that, let’s take a deep dive in the history, technology and the limitations of the not-so-legendary GMC Toro-Flow.
The 1950’s saw the wide-spread adoption of the diesel engine in long-haul trucking, spurred by the diesel’s efficiency and durability. This coincided with substantial increases in truck freight, fostered by the rapid expansion of the interstate highway network. By the mid-’50s, GM and other diesel makers saw an opening for diesels in medium-duty trucks, but these engines would have to be lighter and most of all cheaper.
Diesel engines had been available in heavy trucks since the late 1930’s and the early adopters found that in long-haul service the substantially higher purchase cost of the diesel over gasoline engines was quickly amortized by better fuel economy, cheaper fuel, and reduced maintenance/repair costs. While this trend towards diesel engines was gaining momentum throughout the 1950’s, truck and engine manufacturers begun to wonder if the diesel’s virtues were applicable to medium duty trucks as well.
Medium duty trucks were (and still are) primarily used for short distance hauling and vocational work at lower gross weights. Due to these factors the diesel’s overall cost-of-ownership advantage over gasoline in medium duty trucks was less apparent, and amortization of the diesel’s higher purchase cost could take much longer. In addition, typical medium duty truck operators were more sensitive to truck purchase price than long distance trucking firms. Nonetheless, a few manufacturers started considering smaller diesel engines for medium duty trucks. The Detroit Diesel Division of General Motors was one of the first manufacturers to offer small diesels for truck use. That they were one of the first came as no surprise, as the ‘modular’ nature of their 2-cycle 71 series diesel allowed 1, 2-, 3-, 4-, and 6-cylinder versions to be easily manufactured.
In 1939, GMC introduced the ADC-500 Series truck powered by the 3-71 3-cylinder diesel. This truck offered a nominal 3.5 ton cargo rating, a whopping 65 h.p., and possibly could be considered GMC’s first medium duty diesel truck although it was still larger and more expensive than typical medium duty gasoline powered trucks in the 1.5-2.5-ton range. Unsurprisingly, the ADC-500 does not appear to have sold in any great numbers.
GMC Development of a Low Cost Diesel
One of the characteristics of General Motors in the 1950’s was that the various divisions acted with a high degree of autonomy. This sometimes resulted in divisions using different technology as Aaron Severson pointed out in automatic transmission development, Hydramatic pursuing fluid coupling/multi speed automatics while Chevrolet and Buick adopted torque converter technology for their automatic transmissions. A similar situation developed between Detroit Diesel and GMC during this time. Detroit stayed with the 2-cycle technology they perfected while GMC Truck and Coach Division began to look toward 4-cycle diesels as both divisions worked towards offering a suitable smaller diesel for medium duty trucks.
GMC knew that Detroit 2-cycle engines used expensive blowers and unit injection systems which put such engines at a cost disadvantage compared to the 4-cycle diesel using a distributor style injection pump that GMC was considering. Another program at GMC Truck and Coach during this period was an effort to develop a new family of division-manufactured gasoline engines. GMC had often relied on other GM divisions for gasoline engines before developing 3 basic families of OHV in-line 6 cylinder engines in the late 1930’s. The smaller ‘Family 1’ engines were somewhat based on Buick designs and powered light and medium duty trucks, Family 2 and Family 3 engines were for heavy duty trucks, the largest being a 503 cubic inch version.
These engines served GMC well, but by 1955 to keep up with the competition, GMC starting using Pontiac and Oldsmobile V-8’s in some trucks. GMC begun an ambitious program to develop a single family of gasoline engines that could replace every in-house 6 cylinder and every ‘borrowed’ V-8 in the mid-50’s. This program resulted in the 1960 GMC 60 degree V-6 and V-12 gasoline engines, which could power every GMC truck from a pickup to a tandem axle tractor. Thus, the stage was set within GMC Truck and Coach for the gasoline engine and low-cost diesel engine programs to merge.
When the GMC gasoline V-6 debuted in 1960, it featured a very robust design even for a commercial gasoline engine. The heavy block was cast out of a high nickel iron alloy with deep skirts, the short 6-rod throw crankshaft (cast Armasteel on the smaller versions, forged on the larger) had very large bearings, and the cylinder heads featured 6 head bolts around the perimeter of each cylinder. The heavy-duty design was even over and above what was seen on contemporary large truck gasoline engines like the Ford Super Duty, Reo V-8, and International V series. Interestingly, despite the V-6’s ‘over the top’ build the engine was smaller and slightly lighter than competing engines since it had 2 fewer cylinders. At first glance the design seemed to be quite the case of overkill, but with the introduction of its diesel counterpart the reasoning behind the design became apparent.
Meanwhile, the Detroit Diesel Division had begun to develop a new family of smaller 2-cycle diesels, the 53 series. As the series number denotes the cubic inch displacement of a single cylinder, the 53’s were significantly smaller than but clearly based on the 71 series diesels. For truck use, the 53’s came in 3 and 4 cylinder in-line and V-6 configurations. After extensive testing in the late 50’s, the Detroit 53 series diesels debuted in 1962 in (wait for it…) Chevrolet trucks! Whether or not this was the result of inter-divisional competition or intentionally differentiating Chevy medium trucks from GMC’s no one seems to know.
The 53 series was reasonably successful, proving to be reliable, economical, and somehow managing to be even noisier than a 71 series. It was, not surprisingly, an expensive option in a Chevy medium duty truck. GMC’s competing mid-range diesel program resulted in the introduction of the Toro-Flow V-6 diesel in 1964. Originally available in a 150 h.p. ‘D’ 478 cubic inch version that shared its displacement with the largest gasoline V-6, the Toro-Flow name was in reference to the new diesel’s intake port and combustion chamber design, which created a toroidal air flow into the cylinder. The combustion chamber used the open Hesselman or ‘Mexican Hat’ design in the piston crown together with direct injection (no pre-chamber) and a flat ‘plank’ style cylinder head.
These features contributed to the new diesel’s efficiency and relatively high output. Naturally the Toro-Flow inherited several features of the gasoline V-6, including a 60-degree bank angle and a very oversquare bore/stroke ratio. The oversquare design was unusual for a diesel but not unprecedented, Cummins had recently introduced a series of mid-range oversquare diesels and it was thought that the advantages, such as lower piston speeds, would be applicable to diesels as well. The Toro-Flow shared a few parts with the gasoline V-6 but more importantly it shared tooling and the assembly line, which significantly reduced production costs.
The areas where the Toro-Flow differed from the gasoline V-6 naturally were the cylinder heads. Not only did the diesel heads have to accommodate fuel injectors they were not of crossflow design like the gasoline engine. By directing both the intake and exhaust ports to the outboard side the relocation of the intake manifolds left enough room between the heads for the Bosch PSJ type distributor injection pump and fuel lines. The pump was driven off the cam timing gear and the injectors were also located in the center of the ‘V’ which made for a neat and compact installation. Typically, the air cleaner assembly was mounted above the injection pump with hoses to each intake manifold.
The Toro-Flow diesel also featured a timing gear driven balance shaft in the left block skirt, a feature not required in the gasoline V-6. Soon after introduction 2 additional Toro-Flow V-6 diesel models were introduced, a 130 h.p. ‘D’ 351 cubic inch version and a high output 170 h.p. ‘DH’ version of the 478. Almost immediately after introduction, the Toro-Flow became a sales success. The Toro-Flow was priced low enough that many medium duty truck users were willing to try it as the combination of low purchase cost and high fuel economy promised rapid amortization in fewer miles. It is also possible that the Toro-Flow benefitted from the gasoline V-6’s excellent reputation for durability. In addition, the TF quickly became a popular marine diesel. Unfortunately, some TF customers ran into problems with their new diesels not long after the engine went on the market, and while it was natural to simply assume the problems were the result of the TF’s low-cost design nature the root causes were a bit more complex.
Teething Troubles
The early Toro-Flow diesels seemed to suffer from 3 common issues. The first and perhaps the most common issue was rapid wear of the Bosch PSJ injector pump. The PSJ pump was used by Mack and some other diesel engine manufacturers, and many experienced wear issues with it. This may have been partially the result of the nature of the design, as the PSJ relied on one pump plunger to feed all the cylinders. Inline and unit pumps had a plunger assembly for each cylinder, so it’s easy to see that in the case of the 6 cylinder TF that one plunger was doing 6 times the work of a plunger in the unit injectors on a Detroit 6V-53.
In addition to the plunger in the PSJ pump there was also the wear-prone rotating distributor block that directed the pressurized fuel to the individual injectors. These wear issues were exasperated by the relatively poor quality and high levels of contamination of diesel fuel available at the time. Over time GMC partially addressed the pump issues by upgrading the fuel filtration system and fleets that were careful about the quality of the fuel they used and frequently replaced the fuel filters were able to get good service out of the PSJ pump.
The other two issues the early TFs suffered from were more serious, however. One was head gasket failure. The head gasket issues must have been a surprise given the number of head bolts and the very robust head and block castings but blown head gaskets were nonetheless not uncommon. Some Toro-Flow operators claimed that periodic retorquing of the head bolts prevented head gasket failures and over the years improvements were made to the gaskets, so in time it did appear that head gasket failures became less common.
The other problem was bottom end failure. The solution to that problem was a little more vexing as the root causes were not necessarily the fault of the Toro-Flow’s design. Before we dive into the issues with TF crankshaft and connecting rod failures, I think it is important to consider how well the TF met the objective of being a low-cost diesel. That’s not necessarily to say the engine was of low quality, but it did encourage misapplication of the engine into trucks that were too heavy for it. To give an idea what a TF cost in the mid-60’s in relation to other engines I looked up the list prices of comparable 1968 GMC 7500 series trucks. By ‘68 the Detroit 6V-53 was no longer exclusive to Chevy trucks but was available in some GMC models so it is a good year to compare. All prices are including standard equipment with air brakes:
- HM-7620 with 195 h.p. 351 V-6 gasoline engine: $5190.00
- HG-7620 with 170 h.p. DH478 V-6 Toro-Flow engine: $7181.00
- HV-7620 with 195 h.p. 6V-53 Detroit Diesel engine: $9200.00
As you can see, while the Toro-Flow option price was not insignificant over the gasoline engine, the Detroit was substantially more expensive. That fact was not lost on GMC salesmen, and in some respects that low price would come back to haunt GMC.
Then as now, many medium duty trucks are purchased by fleets and are bid sales. This is particularly true of government and municipal fleets which are often obligated to purchase the lowest bid. Small private fleets are often price sensitive as their primary business is not trucking and they are looking to control expenses. With the TF, GMC dealers found themselves in a very competitive situation when tendering bids for diesel trucks, and GMC made the TF optional across a wide range of trucks. Perhaps a little too wide, as things turned out.
Even though most advertisements and sales literature for the TF stressed its low price and operating economy particularly in medium duty pickup and delivery applications, GMC did make the engine available in high gross weight single axle and even tandem axle trucks as used in dump, refuse service and concrete mixers. While the gasoline V-6 was more than up to those tasks, the Toro-Flow really had no business in trucks that heavy. But nonetheless, some GMC dealers were winning bids with TF powered trucks that really should have been equipped with the more expensive Detroit Diesels.
It is easy to see how misapplication of the Toro-Flow in demanding high gross weight trucks led to a rash of often spectacular bottom-end failures. Lugging a TF in a high-GVW truck up a steep grade with the accelerator floored would literally pound the bearings right out of it if it didn’t bust the crankshaft first. In addition, there was another factor that caused bottom end trouble in the TF, and that was inexperienced drivers. Keeping in mind that most medium duty trucks were powered by gasoline engines, drivers of these trucks typically relied on downshifting and engine braking to slow down on grades. The TF, like any diesel in the days before engine and exhaust brakes, didn’t offer a lot of braking when downshifted. Consequently, when going down a steep grade with a TF in too low a gear it was easy to spin the engine well past maximum rpm as the governor in the injection pump would have no effect on engine speed. Even though the Toro-Flow was rated for a high (for a diesel) 3,400 rpm maximum speed, turning it over 4,000 rpm was another recipe for bottom-end disaster.
Further Developments
In 1966, GMC brought to market a heavy duty addition to the Toro-Flow family, the D and DH 637 V-8’s. There was also a gasoline version of the new V-8 which debuted at the same time as a replacement for the 702 cubic inch gasoline ‘Twin-Six’ V-12. These new V-8’s were 60-degree engines based on the V-6 architecture and shared a bore and stroke with the 478’s. Initially available in the GMC 9500 series trucks, the 637 Toro-Flows were offered in 195 hp ‘D’ and 220 hp ‘DH’ versions, the new V-8’s promised to extend Toro-Flow economy to the heavy duty field, which it did at least as far as purchase price was concerned.
The 637 Toro-Flow was a bargain at only $1773.00 more than a 637 gasoline V-8 in a 9500 series truck, while the Detroit 6-71 was an additional $3721. In this case however, the 637 TF seems to have not met the same sales success in heavy trucks that its smaller V-6 brother had met in the medium duty field. It’s quite possible that the less than stellar reputation of the TF was keeping the customers away, or the very conservative nature of long-distance trucking fleets kept them buying reliable old favorites like the Detroit 6-71 and Cummins NH-220. Either way the 637 TF V-8 was quite the rare engine, even though the 637 gasoline V-8 proved moderately popular in fire apparatus.
GMC Toro-Flow DT478 V-6 twin turbo
Around the same time the 637’s came out an even rarer member of the Toro-Flow family was introduced, the twin-turbo DT478 V-6. This engine was something of an enigma, while turbocharged TFs were common in marine use the DT478 was only offered in the DSPA5019 rear-engine bus chassis, supplied exclusively to the U.S. Army for use with a 45 passenger Superior body. Over 2,200 were built.
In 1968, the low power, slow selling D351 V-6 was discontinued. The D351 always had limited availability in low-GVW 3500-4500 series trucks, but for some strange reason was never offered in the Step-Van chassis which seemed like a natural application for the small TF. Diesel Step-Vans were only offered with the 3-cylinder Detroit 3-53.
Toroflow D351 installed in Chevy pickup
There were rumors that GMC was very close to making the D351 TF a factory option in GMC 2500 series pickups around 1967 but this never came to pass, though a few GMC dealers did install D351’s in new pickups for customers during this period. It was an easy swap if the truck was equipped with a gasoline V-6, practically ‘bolt-in’ as the same transmission could often be used.
In 1969, GMC made significant upgrades to the Toro-Flow aimed at increasing reliability (and improving the reputation!). In addition to the D351 being dropped, the ‘standard’ D478’s and D637’s were also discontinued, leaving only the DH478 and DH637. The ‘new’ engines were referred to as the ‘Toro-Flow II’ and GMC even went so far as to change the color the engines were painted, from a golden-yellow to ‘Alpine Green’, that familiar color used on Detroit Diesels. Was this an attempt to insinuate that the TF was now comparable to the Detroit in reliability? Probably. Were they? Probably not, but at least they were much improved.
The biggest improvements were to the lubrication system. Engine oil coolers, which had been optional previously were now of higher capacity and standard equipment along with substantially higher volume oil pans. Larger radiators were also included. In addition, there was one additional improvement that was nothing more than an adjustment to the injector pump but was probably as significant as the other upgrades: The maximum rpm was reduced from 3,400 to 2,800 rpm. (which also resulted in slightly less hp.). GMC further hedged their bets on the Toro-Flow by restricting its availability in heavier trucks. The 637 TF was removed from the option list of the heavy duty 9500 models and became an option in the 7500 ‘light-heavy’ trucks. By 1970 the 478 TF was restricted to the 6500 (Chevy 60) and below medium duty models. Despite the issues, every year up to this point since introduction GMC was able to make the claim that the Tor-Flow was the bestselling diesel in the medium duty field.
The Competition, and a Lawsuit
GMC Truck and Coach Division was not the only truck manufacturer to explore the medium duty diesel market in the 1960’s. As previously mentioned, Chevrolet Division was in the game with several models powered by various Detroit Diesel 53 series diesels and met with some success with these reliable but relatively expensive engines. Interestingly, not only did GMC offer the 53 series diesels in certain models by 1965, but Chevy also added the Toro-Flow to some of their trucks as well. Initially Chevy named the engines ‘Torq-Flow’ diesels, but they were identical to the GMC TF in all respects.
Mack offered the Swedish Scania diesel in the lighter R and MB series trucks. During the 60’s Dodge was a major manufacturer of medium duty trucks, and at the time Chrysler owned a large share of British diesel engine manufacturer Perkins. The 354 cubic inch Perkins 6 cylinder was a natural for medium duty truck service, and Dodge began offering the diesel in 1962 in several of their 500 and 600 model trucks. It was also available in the W300 4×4 1 ton pickup (the first production diesel pickup) and the D400. The Perkins was reliable but only produced around 131 hp, which was adequate for city pickup and delivery work.
International Harvester tested the medium duty diesel market by offering the 354 Perkins in some trucks along with their own D-301. The D-301 was an in-line 6 cylinder 4 cycle that had its origins as an agricultural/construction engine. It was unsuccessful as it was severely underpowered and very noisy but nonetheless was a special-order option until 1968. Beginning in 1968 International introduced their own in-house direct response to the Toro-Flow, the DV-462 and DV-550 V-8 diesels. These engines were basically straight diesel conversions of IH’s V series 461 and 549 gasoline V-8’s.
Ford was also able to offer small diesels in some of their lighter medium duty trucks, relying on their own British-sourced Dagenham diesels. The 4 cylinder was used in P-350 van chassis while the 6 cylinder Dagenham was an option in F series medium trucks. These engines, like the IH D-301, were not well suited for domestic medium duty truck service either, so Ford first turned to Cummins for a true diesel V-6 competitor to the 478 Toro-Flow.
Cummins Engine Company had also reached many of the same conclusions that GMC did and developed a family of mid-range high speed V-6 and V-8 diesels for medium duty trucks in the early 1960’s. These diesels were of 90-degree V oversquare design and initially ranged in displacement from a 352 cubic inch V-6 to a 470 cubic inch V-8. Like the Toro-Flow, the Cummins engines were direct injection but instead of using a distributor type injection pump they used a version of the Cummins PT fuel system, which used unit type injectors actuated by the camshaft.
Other than that feature, the specifications and output of the Cummins V-6 and V-8 closely resembled the various TF models. So close, in fact, Cummins sued General Motors for patent infringement. The lawsuit focused on the oversquare bore/stroke design the Cummins engine featured which they contended was unique in the diesel field, and that it facilitated their engine’s efficiency, compact size, and light weight. Cummins also claimed that while they were not the first to use an open type Hesselman combustion chamber their application of such in an oversquare diesel engine was also unique.
The case came to trial in 1969, and Cummins was unsuccessful. General Motors was able to prove that there was nothing novel about oversquare diesel engines as they had been experimenting with such since 1953. In addition, Ford’s agricultural division had been manufacturing an oversquare 4 cylinder diesel that also featured a Hesselman combustion chamber. Ford may have been the first to market such a diesel but did not seem to have ever patented the design.
Into the 70’s and the End of the Line
As the new decade began, there were several mid-range diesels available in the medium duty truck market. Not terribly satisfied with the small Cummins V-6 diesel (often referred to as the ‘hummingbird’ by drivers), Ford commissioned Caterpillar to design and build a new 4 cycle 90 degree V-8 diesel of 636 cubic inches. The new engine was referred to as the V636 and painted ‘Ford Blue’ in Ford trucks but was also marketed to other truck manufacturers as the Caterpillar 1100 series (naturally in Cat’s familiar yellow paint).
Ironically, however, none of these mid-range diesels (#1 selling Toro-Flow included) sold particularly well. I think it was likely that low gasoline prices and higher purchase prices of diesel-powered medium trucks, even with these new relatively low-cost diesels, prevented the diesel from achieving the wide acceptance it enjoyed in the heavy truck field. In addition, none of these new mid-range diesels, except for the Detroit 53’s and the Scania, proved to be particularly reliable. Though the older small IH D-301 and Perkins 354 were quite durable, they were of limited application due to their modest output. The Cummins V-6 and V-8 suffered from many of the same issues the Toro-Flow did, save for a more reliable fuel system. The International DV was known for blowing head gaskets, cracked heads, and valvetrain issues. Even the relatively late-to-the-party Caterpillar V-8 initially had issues, most notably a wear-prone 2-ring piston.
In 1972 GMC renamed the DH478 the ‘Turbium 7.8’, though it does not appear that any improvements were made to the engine at that time and availability was still restricted to GMC 6500 and Chevy 60 series models. The Turbium was finally dropped mid-1974 and some of the last engines may have been built with Bosch in-line injection pumps instead of the PSJ distributor type. This would have been a great improvement had it been implemented six or so years earlier, but I suspect the reason for the new pump at this time was Bosch discontinued the PSJ pump. The DH637 V-8 had been dropped as a regular option in the 7500 series in 1972 but appears to have stayed in production until 1974 as a special order in trucks and for marine applications.
Aftermath and a Belated Replacement
The rise in fuel costs during the mid-70’s spurred a renewed interest in diesel engines for medium duty trucks. With the Toro-Flow/Turbium diesel discontinued and the DD 4-53 dropped from Chevy medium duty trucks by 1975, GMC (and Chevy) were only offering the DD 6V-53 along with the new Caterpillar 3208 in the 7500/70 series trucks and no diesels at all in the 6500/60’s. The 3208 was an improved version of the 1100 series V-8 diesel. In 1978 GM reintroduced the (still expensive) DD 4-53 in certain medium duty conventional trucks but Detroit Diesel was at work on an all-new low-cost diesel engine, the 8.2L ‘Fuel Pincher’. Similar in concept to the Toro-Flow, the 8.2L made it to market for the 1980 model year but in too many was a repeat of the Toro-Flow story!
The Fuel Pincher was eventually offered in several GMC, Chevy, and non-G.M. medium duty trucks. By the end of the 70’s diesel engines were taking a larger share of the medium duty market as their operating economies and overall cost-of-ownership became much improved in relation to large gasoline engines. The Detroit 8.2L, Caterpillar 3208, and International’s excellent DT-466 sold in large numbers and became very common throughout the 80’s. International continued to offer the DV-550 as the ‘new’ 9.0L into the 80’s but it was never a popular engine, particularly after the DT-466 was introduced. The Cummins V-6 faded away from truck use early on, but the V-8 grew in displacement and eventually was turbocharged, becoming more of a heavy truck engine known as the VT-225.
A “Deadly Sin” of a Diesel?
The Toro-Flow/Turbium remained in production for 10 years, and for many of those years it was the best-selling mid-range diesel on the market. Under normal circumstances that alone may have qualified the engine as a success, but despite the popularity the Toro-Flow did not develop the best of reputations. While the engine did return very good fuel economy, it wasn’t very durable and was particularly susceptible to misapplication in too large a truck, poor fuel quality, and driver abuse. Some fleets after repeated TF engine failures simply replaced the diesel with a new GMC gasoline V-6, an easy ‘bolt-in’ swap in most cases. Seeing a GMC or Chevy with ‘Toro-Flow’ emblems on the hood and a V-6 gas engine under the hood was not uncommon back in the day.
Nonetheless, there were success stories. A large restaurant supply company in Los Angeles, S.E. Rycoff & Company, operated a fleet of GMC’s with Toro-Flow diesels and got good service out of them. Rycoff had an extensive preventative maintenance program and was capable of overhauling the PSJ injection pumps in their own shop. The Toro-Flow also performed well in buses, and I remember seeing a number of TF powered GMC school buses in the Los Angeles Unified School District fleet up into the early 1980’s.
In addition, the Toro-Flow was used in GM’s smaller transit buses, the ‘Old Look’ TDH-3501 and ‘New Look’ TDH-3301 where coupled to an Allison automatic transmission in a relatively light vehicle gave good results. The TF also met with success in marine applications, in fact about the only source for TF engines and usable parts these days is out of classic boats that have been restored and repowered, as marine Toro-Flows seem to have lasted decades longer than those in trucks. Whether or not the Toro-Flow can be considered a success overall is open to debate. I think it’s safe to say it was a pioneer in the medium duty diesel field although the market for such engines had not fully developed for some years after the Toro-Flow was discontinued.
The Toro-Flow met its design objectives in that it was priced low, offered outstanding fuel economy compared to gasoline engines, was reasonably powerful, and was compact and light in weight. What it lacked was durability, but I think it’s important to note that it wasn’t significantly worse than contemporary low-cost medium duty diesels. GM to their credit made an effort to address the durability issue, and probably the most effective change they made to the Toro-Flow was simply limiting the size of truck they offered the engine in. The funniest thing GM did regarding the Toro-Flow was to include a chapter on engine part failure analysis in the engine shop manual!
Remembering the Toroflow
Where are they now? The Toro-Flow disappeared from the trucking scene with stunning rapidity. Very few were left even as the 1980’s dawned, which wasn’t terribly surprising as few medium duty trucks are treated to an engine overhaul or replacement during their service life. Older TF-powered trucks were typically junked after engine failure, though some did soldier on with GMC gasoline V-6’s as this was the cheapest and easiest way to keep one on the road. The few running TF-powered trucks that made it to the wrecking yard often gave up their engines to pickup truck diesel swaps, I remember seeing a few in the late 70’s. But regardless, TF powered GMC and Chevy trucks were once a common sight, and sound too for that matter. At idle they had a somewhat rough mechanical sound not unlike a G.M. 6.2L light truck diesel but under way the Toro-Flow made a smooth deep sound, certainly different than Detroit but really unlike any other diesel as well.
Many of these engines were equipped with dual exhaust which likely contributed to its pleasing exhaust note. Hearing one today is an exceedingly rare occurrence as there are very few running examples left and aside from an occasional marine Toroflow for sale on Ebay there are almost no repair parts available.
The Southern California Railway Museum of Perris, CA., has in their collection a running 1967 G.M.C. TDH-3501 that is used on occasion for demonstration rides around the museum grounds. The former Modesto CA. bus was in service well into the 1980’s and at some point during its long career was treated to a new DH478 ‘Toro-Flow II’ replacement engine. It smokes a bit at idle but runs well and is usually easy to start.
It is the only running Toro-Flow I personally know of at present.
I think in retrospect I would consider the Toro-Flow as something of a flawed pioneer. It was one of the first successful mid-range diesel engines, it demonstrated the importance of durability in the application of diesel engines in medium duty trucks, and its employment of a high-speed oversquare design was technically a road to nowhere. The competition at the time for the most part was not any better, so let’s just say they had to start somewhere!
Related CC Reading
Curbside Classic: 1963 GMC Pickup – The Very Model Of A Modern V6 Truck Engine by PN
The GMC Twin Six V12: 702 Cubes, 275 HP At 2400 RPM, 630 Ft. Lbs. At 1600 RPM by PN
What a great Monday morning read about an engine I knew little about. I do have a mid 70 Chilton truck manual that covers them. Thank you!
Robert, this has been a superlative treat! I’ve always wanted to know more about this engine which I used to see quite a bit when I was a kid, but then it seemed to disappear so quickly. I knew it had some issues, and now I am enlightened.
The whole history of diesel engines is of particular interest to me, and this chapter of these smaller, lighter ones has been rather obscure. It’s not like one is likely to see them at a truck show or such.
I distinctly remember that fleet of pea-green Rycoff GMC trucks in SoCal in the ’70s; they were everywhere. I just found a picture of them and added it to the post. I mostly remember the subsequent generation of their GMC trucks.
In 1969, our high school in Towson got a bunch of new GMC school buses and they had Toro-Flows and automatics in them. A distinctive sound, and diesel school buses were not at all common back then.
Fascinating read. In 2005 as a newly minted fleet manager for a building supply company, I was surprised to find 5 Detroit Diesel V8s in a sea container at our home office shop. Pulled out of units years ago, my new mechanic said they were junk. Not one hit on eBay when I offered them for sale. Our DT466 equipped 1983 International box truck was still running when I sold it in 2010…guys loved it. 2006 International CF500 with the CT275 V6 was pure junk though..
I’ve worked on a Toro Flow, very interesting stuffed into a 1 ton pickup. It sounded amazing. Work had a couple of Bird buses with Cummins 504 v8s, pretty doggy, leaky and deafening loud mechanical noise. It It was under 70f they needed a sniff of ether to start. Work also had a pair of Birds with 1 1160 and 1 3208 Cat. Super smoky on cold start and super noisy and oil burners as Cat decided that 2 ring pistons were adequate. The last old bird had an 8.2 Fuel pincher conversion. It ran ok but very sluggish and unimpressive but at least it wasn’t as deafening as the Cat or Cummins. All had Allison MT 640 series transmissions. I happen to know where 2 lonely D 478s are languishing if anyone is interested.
The more you learn, the more you realize you don’t know. This is awesome, Robert. If I had another go around, I’d want to collect nothing but diesels so I could learn all about them.
Excellent article on the Tor-Flow! As an old (rather old) former truck salesman, i enjoy this type of information very much. Truck manufacturers had to learn that one cannot “Dieselize” a gasoline block. One great hit was when International discovered a farm tractor engine that became a hit for their trucks- the DT-466. Another interesting Diesel is the 8.2 Liter. It was designed as a “throw-away” block. In use in proper application, it was a honey of an engine. I emphasize in the proper application. I sold it accordingly. In the medium-duty market where heavier service was required, I used the Cat 3208. Once again, thanks for a terrific essay.
A very interesting article about an engine that I knew little about. I’d seen a few in old trucks we inherited when my employer acquired a Ready Mix company and was always curious to learn more.
In my days as a mechanic, then shop foreman, then fleet manager I worked on and later spec’d and purchased many medium duty diesel trucks so I’m familiar with many of the engines mentioned here. In my opinion, the best of the crop was the DT 466, after about 1984. Before that they had a lot of problems that usually manifested themselves after the warranty was up and negated any financial benefits that diesel offered when you figured total cost of ownership. Distant second was the Cat 3208, an excellent engine in marine and stationary applications but not really happy in vocational trucks.
Cummins and IH V-8 diesels were time bombs, at least in truck applications.
And then there was the 8.2 Fuel Pincher. We had lots of these in both F-800 Fords and GMC 6500 trucks. Somewhere I still have the dial indicator tune up kit, and the booklet for “Base circle timing” them after you’ve had to remove the cylinder heads. And you will have to…. We used to call these disposable diesel. Re and Re (and you will have to…) from the F-800 was not that bad once you figured it out but the GMC required a lot of front end bodywork removal. I always thought that the guys who came up with the Olds 5.7 engine must have had a hand in this one as well. Gutless, unreliable, noisy, smoky…
Classic GM from the period.
There were also some Japanese mediums, like Hino, on the market in the ’70s and they had their problems as well so it seems the solution to an affordable, reliable medium duty diesel was a long time coming.
Again, great article that I’ll come back to when I have time to pour a cold one and really digest it!