A new truck with reasonable care back in the 1970s would be good for 350,000 miles before the engine was due for an overhaul. (By comparison, with current engines, 700,000 to 900,000 miles on a new truck before overhaul is not uncommon.)
One of the things that earmarked a particular engine as a good truck engine was that after an overhaul, it would be good for another 350,000 miles. The good old 855-cubic-inch Cummins six-cylinder engine and the Mack 673-cubic-inch “Maxidyne” were two that had this trait.
In the same time-frame, Cummins marketed a family of V-8 diesel engines that did not enjoy the same reputation. While a select few were able to get good service out of the VT-903 engines, most did not. Mack and Caterpillar marketed V-8 diesel truck engines in the same era that were known for lots of power.
This was also the era of the two-cycle Detroit Diesel engines. These engines first found their way into heavy trucks after World War II. Surplus landing craft were the source of four-cylinder Detroit Diesel engines.
One tale told was that a Kenworth truck powered by a surplus 4-71 Detroit engine was pulling out of Los Angeles and pulled into the passing lane and proceeded up the hill, passing almost all of the gasoline- and propane-powered trucks on the hill.
It was after World War II that the diesel engine “took over” as the prominent engine in heavy trucks. Prior to the big diesels the “monster” truck engine had been the Buda (about 1,000 cubic inches of displacement) six-cylinder propane engine.
The family of Detroit Diesel engines most common in heavy trucks were the 6-71 and the 8V-71 engines. Later on the 8V-71 was turbo-charged, resulting in the “350 Detroit” that had some severe problems as first released.
Then followed the “92” series, which debuted with both V-6 and V-8 versions, the “8V-92T” with an advertised 430 horsepower being the monster engine of the day for awhile. The big Caterpillar 3408 V-8 engine was always the engine to beat.
Then Cummins came on the scene with an industrial engine, the KTA 600 with an advertised 600 horsepower. Neither the 3408 nor the KTA 600 stayed available in highway trucks for more than a decade or so; they were too heavy and had emissions problems that could not be overcome.
The emissions monster was also the death knell for the two-cycle Detroit engines. These engines drove differently from one another in a truck that a driver used to one make of engine would take several hours getting used to another brand of engine so he could appear to be competent shifting the gears.
The nature of the two-cycle Detroit engines, for example, revved up quickly, and also dropped off engine speed rapidly when the time came to shift gears.
The old-timers would advise new drivers to smash their thumb with a hammer before getting into a truck with a Detroit engine. The reason was that then the driver would be in pain and “mad” and that was the way the Detroit engines needed to be driven, as though the driver was mad at the engine.
The procedure was to get the truck rolling and wind the engine up until the speed governor stopped it from going faster, then rapidly shift into the next-higher gear, and mash the throttle pedal all the way down until the engine was at the governor again. There was nothing quite like the sound of a two-cycle Detroit being aggressively driven as the driver shifted through the gears.
The two-cycle engines differed from the four-cycle (which constituted everything else) in that each downstroke of a piston was a power stroke; the fresh air charge happened at the bottom of the engine cylinder through ports.
As the piston reached the bottom of the cylinder and exposed the ports, two things happened. The air and diesel fuel mixture that had just burned left through the exhaust valves in the cylinder head into the exhaust manifold as the Roots supercharger forced fresh air into the cylinder from the ports in the bottom of the cylinder.
Every upstroke was a compression stroke, and every downstroke was a power stroke, just like a chainsaw or snowmobile engine. The engine, usually governed at 2100 rpm (revolutions per minute) from the factory and generally turned up in the field to 2300 or 2600 rpm, sounded like twice that.
The 8V-71 Detroit engine, at 71 cubic inches of displacement per cylinder, added up to 568 cubic inches for the engine. But firing twice as often as a four-cycle engine, the effective power displacement per revolution was 1136 cubic inches of displacement per engine revolution.
A four-cycle engine of 855 cubic inches of displacement took two revolutions of the engine to get a power stroke from all of the 855 cubic inches, or 427.5 cubic inches of power stroke per engine revolution.
The merits of the various engine makes and designs were argued with vigor, and the agreements settled either on the uphill side of a highway or occasionally with fisticuffs.
Where the Detroit engines would speed up and drop off engine speed rapidly, usually with a 200 to 300 rpm drop in engine speed to the next gear in an up-shift, the Mack Maxidyne engines were engineered to run up to the 2100 rpm governed speed and then drop back to 1100 rpm before the shift into the next- higher gear.
The Detroit engines were generally mated to a 13- or 16- or 20-speed transmission, or set of transmissions. The Maxidyne engines, designed to lower the average engine speed and thus improve fuel economy, came with five or six forward gears.
A fellow who had been driving the Detroit engines found himself in a Mack truck with the Maxidyne engine and five-speed transmission. After the “fast” shifting Detroit, he described the procedure to shift gears in the Mack.
“You run the engine up to the governed speed of 2100, and then pull the transmission into neutral. Then you pull your bag of tobacco out of your shirt pocket, and pour a little into a cigarette paper.
You lick the edge of the paper and roll it into a smoke, park it in your lips, put the tobacco bag and papers back in your pocket, find a match, strike it on your pants and light the smoke and take a puff.
You then mash the match in your fingers to make sure the fire is out and put it in the ash tray. By then the engine speed has dropped down to 1100 rpm, so you slip it into gear and mash down on the throttle and go on up the road.”
Note that heavy truck transmissions do not have synchronizers so the engine speed must match the road speed of the next gear or the transmission cannot be shifted. Grinding the gears does not help. FG
