ALL IN THE FAMILY

The 2-ton truck carried 917 gallons of fuel in the bed. The run lasted through Christmas to December 26, running a total of 13,535 miles nonstop at an average speed of 43.4 mph and on $72 of fuel and 8 gallons of lube oil. Although the truck had several problems— all repaired with the truck still in motion—the engine had only one: A screw from the rear main seal retainer fell out, causing an oil leak. The screw was replaced by slinging a mechanic under the truck by ropes as the truck drove.

Truck Production

On December 28, 1931, Cummins announced that a batch of test engines would be built, installed in trucks and tested in venues around the country. A good number of those would go into Peterbilt trucks, but testing wasn’t limited to the United States (with Citroen testing Cummins engines in trucks). Regular production started for 1933, and it was all hands on deck for the marketing of this new powerplant.

In a move to tempt General Motors with the Model H for its Yellow Coach bus division, Cummins would repower a Mack BK bus. In November 1932, it established a cross-country speed record: 91 hours and 10 minutes between New York and Los Angeles. The big Mack bus averaged 8.9 mpg over 3,220 miles at an average speed of 35 mph, with the engine using 5 quarts of oil. There was absolutely no trouble with the engine, but Cummins was crushed when GM opted to start development of its own two-stroke diesel rather than buy into the Cummins four-stroke. (How much would history have changed had GM gone in the other direction?)

Two-strokes

Two-strokes would play a part in Cummins’ corporate thinking for only a few years in the mid-1930s. It’s been widely asserted that Clessie Cummins was vehemently opposed to the two-stroke diesel idea. Although his “gearhead intuition” kept him focused on four-strokes, he was open-minded enough to test the two-stroke concept several times.

The last test came in April 1934, when two new Cummins-powered Indy race cars were announced— one four-stroke and one two-stroke. Built by Duesenberg, the cars would be powered by raceprepped Model H four-cylinders. To meet the rules, they had to be under 366 cubic inches, so they were destroked from 448 to 364 cubes by reducing the stroke from 6 to 4.875 inches.

During testing and qualification, the two-stroke #5 car proved a little faster, putting in a 105.921 mph qualifying time, with the four-stroke making 102.414 mph. Both times were well under the 119.329 pole qualifier. Come race day, both cars held their own, and while neither car was expected to set records, Cummins hoped they would make up for slower lap times by having to stop only once for fuel. Fuel capacity was limited by the rules, so even the thrifty diesels needed to top off.

Two-stroke vs. Four-stroke

The two-stroke car had problems from the beginning: excessive vibration, a slipping clutch and gradual loss of power. The four-stroke car ran very well, but midway into the race, while accelerating out of the only scheduled pit stop, Dave Evans blew the transmission and put the car out of action. The two-stroke car finished the race, placing 12th.

When the both engines were “autopsied” after the race, the two-stroke was in terrible shape, but the fourstroke was ready for more. Cummins later said the two-stroke had been built in a rush and was not tested extensively. He pointed out that it hadn’t really been a fair test. Nonetheless, Cummins never again gave twostroke diesels much attention.

More Records

The final episode in proving the Model H via competition came in March 1935. The now-engineless #5 car had been stretched at the Duesenberg plant to accept a Model H six, the goal being to run it at the Daytona Beach Speed Trials and set another new record for diesels.

The car reached a peak speed of 144 mph on one leg,

 The Model H was wet-sleeved, and the cylinder heads were cast in pairs and bolted to a block with five or seven main bearings (four or six cylinders, respectively). Note the unusual integrally cast accessory housing. This was a risky and unusual practice proposed by Knudsen that Cummins was initially against. Integral accessory housings would remain unusual, even with Cummins engines, but the Model H and all its descendants share this feature to the current day. Shown here is an HB engine block from the mid-1930s.

 The four-valve NH was a big update in the H series for 1945, along with a big displacement increase. The base naturally aspirated power rose to 200 hp at 2,100 rpm and the NHS to 275 hp at 2,100. This 1949 engine shows the giveaway: the intake manifold on the pump side of the engine. This engine is early enough to still have the older SD pump. It’s also shown with an air compressor.

 The HR had the same 743-cubic-inch displacement as the NH but only two valves per cylinder and a lower base output: 165 hp at 1,800 rpm. This put it between the “old standby” HB (built until 1968, with a few exceptions) and the NH engines. The HR was built in similar configurations as the NH, including a supercharged HRS.

 The first high-output version of the HB came out in 1937, when a belt-driven Switzer-cummins supercharger was added. This HS engine delivered 200 p at 1,800 rpm and 625 lbs-ft at 800 rpm. This became the engine to beat in big trucks.