Supercharged Cummins

By the late 1930s, the diesel engine was rising to meet the needs of a market growing ever more friendly. The support infrastructure was improving and diesels had made leaps and bounds in output, power density, reliability, and user-friendliness. One of those improved diesels came from the Cummins Engine Company.

Supercharged power! On October 6, 1942, this HBIS was built and sold to General Electric in Erie, Pennsylvania. The build card doesn’t list its intended use, but it did show a lot of additions and omissions. Five pages worth, in fact. Not long after this engine was built, the drive mechanism for the Schwitzer-Cummins supercharger was changed. It went from the triple belt setup you see here to a gear drive setup. This industrial engine differs in many ways from the automotive engine shown nearby. The exhaust is directed up, for one thing, and the intake manifold is considerably different. The bell housing and mounting feet here are more suitable for a stationary application but engine mountings were variable. The engine is rated for a maximum of 200 horsepower at 1800 rpm. However, the continuous rating is 140 horses at 1800. The naturally aspirated continuous rating for the 150 maximum horsepower HBI-600 was only 102 horsepower at 1800. A stationary operator would pay more attention to the Cummins continuous power rating. A trucker would be most interested in that maximum number.

Cummins Power Curve

The engine that would put Cummins into the big leagues began production in 1933 and was designated the Model H. This diesel was so successful that it became the cornerstone for a corporate history that has passed the century mark. Still, it shows no signs of fading away. The Model H started as a good design and improvements of all types would follow. In the April and May 2019 issues of Diesel World, there’s a more detailed two-part story of its long history.

For contrast, here is an automotive application HBS from 1942. Note where the exhaust manifold exits and the lower profile of the engine. Also, the cooling fins on the intake manifold help cool the boosted charge of air. The automotive installer (the truck factory) usually designs and builds the mounting system for the chassis. Then, the engine is mounted in it.

One update that yielded a significant boost in power came for 1937 in the form of a supercharger. They knew then, as we know now, there is no good excuse for a naturally aspirated diesel. Implementation of practical forced induction had held things up until a transplanted Austrian engineer came along. He developed a practical, affordable supercharger that helped move Cummins way up the industry power curve. Even better it came from a company based in Indianapolis, less than 50 miles away from Cummins’ Columbus, Indiana, home.

Alphabet Soup

Cummins had a somewhat incomprehensible engine nomenclature system in the early days. We need to outline it here before we go further. Cummins historians have dubbed it “alphabet soup.” It started with a basic one letter engine model series identifier, such as “H.” To that could be added more designators to further define the engine’s intended purpose.

In the earliest days of the H series diesels, adding a “B” indicated an automotive application, e.g. “HB.” The HB standard build-out often included automotive-style mounting points, water pumps, flywheels, a generator, bell housing, and possibly a fan… all subject to the customer’s needs, of course. The “BI” (“I” for Industrial) was the so-called “mobile industrial” engine (e.g. “HBI”). They were intended for a variety of uses, including heavy off-highway such as graders, cranes, or crawlers. They could also be used in stationary jobs. Some might include a few automotive-like features but most likely would be built to match a customer’s specification.

Just an “I” designation (e.g. “HI”) indicated a stationary industrial powerplant. “IP” (e.g. “HIP”) indicated an industrial engine on a base with a radiator and fan and possibly a clutch and power take-off. “HM” was the marine version. HGA was a generator setup. An “S,” for supercharged, was another letter that could be added to any engine so-equipped, such as the HBIS in this story. Model H engines also had a numeric designator, 400 for the four-cylinder and 600 for the six-cylinder. These numbers happened to roughly coincide with the displacement range. The 400 being 448 cubic inches and the 600 at 672 cubic inches.

Made Sense At The Time

On the pump side, we can see the single-disc Cummins injection pump. This was a very reliable unit, though perhaps a little imprecise. The 1949 DD (Double-Disc) pump was intended to be an evolution of it. Unfortunately, it took a couple of steps back in terms of reliability and reputation. The last of the HB engines saw the introduction of the PT (Pressure-Time) system in 1954. It was a home-run system that carried Cummins through decades of engine evolution.

The alphabet soup was served into the early ‘50s and then gradually began to fade away. It contained many contradictions and could change without warning. An example of that was when larger main bearings were introduced in 1935. This was followed by all the H-line being designated the HB series. The earlier engines became “HA” by default. A plain “HB” was still an automotive engine to which all the other letters could be added. Then we have the HBI and the other letters to stir around in the soup. While the sales and service literature is not always consistent, it’s relatively easy to figure things out.

The Super H

By 1937, the H-Series had undergone many small improvements that made the supercharger possible. One of the more significant was that previously mentioned upgrade to the stronger crankshaft in ’35. The main bearings were increased in diameter from 3.250 to 4.50-inches. The HA was originally rated for a maximum of 125 horsepower at 1800 rpm and 420 lb-ft at 500 rpm. The HB updates brought it up to 150 horsepower at 1800 rpm and 500 lb-ft at 800 rpm. The addition of the supercharger gave the HBS engine 200 horsepower at 1800 rpm and 625 lb-ft at 800 rpm.

From the rear of the blower side on this HBIS, you can see the generator. It drives off the end of the blower. We’re not exactly sure where the starter would have attached in this application.


The compression ratio dropped on the blown engine from 17:1 to 14:1. Fuel delivery at 1800 rpm increased from 28 cc’s/500 revolutions with 0.020” orifices to 41 cc’s. Yes, that is an arcane way to measure delivery. According to Keith Baylor, that is how the Single Disc pump test stand measured it; 500 revolutions of the pump, and end result should be the specified amount of fuel. Fuel rail pressures increased from 120 psi at governed speed to 135 psi. Also, yes, the Cummins single-disc system was an early form of a common rail system. We didn’t find boost pressure listed in any publications specific to the HBS line. In some other applications, the Schwitzer-Cummins supercharger delivered 8-10 psi and the HBS line most likely fell into this range.

Starting in 1945, the HBS line gave way to the 743 cubic inch model NHS 4-valves. Alongside them was the NR, 2-valves with the same displacement. The 672 cubic inch HB line continued in some form or other until 1970. Although it wasn’t a “mainline” engine after about 1956. The HBS supercharged coexisted with the 743 cubic inch, 275 horsepower NHBS supercharged from ‘45 into 1954. By then, Cummins was in the midst of their troubles with the Double Disc injection pumps and moving towards the PT system. Also, the superchargers were giving way to turbochargers. From what we can see, the last of the supercharged HB and NH diesels were gone after 1956. In the 1960s, the C180 engine was supercharged, again using a Schwitzer blower.

Schwitzer-Cummins Superchargers

The Switzer-Cummins supercharger was a positive displacement pump invented by Louis Schwitzer (1880-1967). It was similar to what we know as a Roots blower (as used on the GM 2-strokes). It was invented in the 1860s for supplying air to blast furnaces in steel manufacture. Schwitzer is often credited with building the first positive displacement supercharger in the early 1920s. However, Gottlieb Daimler used Roots blowers on engines as early as 1900. We don’t know all the contextual details to make the call on who was first and with what. We do know that Schwitzer had a supercharger on the market by 1930. Schwitzer was also a pioneer in developing centrifugal blowers and turbochargers. Everything we’ve mentioned here is just the “tip-o-the-iceberg” on a long career list of technological innovations he made.

The company he formed with Laurence G. Cummins in 1918 began in Indianapolis, Indiana. Initially, it was more to manufacture liquid pumps and air-moving machinery than it was to build superchargers. Just to settle the obvious question, Laurence Cummins had no connection to Clessie Cummins and the engine company. Although, because of the name there must have been a distant familial thread at least. We consulted Clessie Cummins’ son, Lyle, for this story and he backs up the lack of a direct connection. He said Clessie Cummins and Louis Schwitzer are acquainted professionally.

1942 was the last year the supercharger was belt-driven. From 1943, the blower was driven via gears inside the front cover.

Schwitzer’s Speedway Days

Schwitzer was an engine nut. He even won the first race at the Indianapolis Motor Speedway in 1909 (not the 500). He remained on the Speedway Technical Committee until 1945. Not only that but he is also considered a founding father of the Indy 500 race. Before he was the President and Chief engineer of Schwitzer-Cummins, he worked for Pierce-Arrow and Marmon. He had a hand in designing some famous technology for those companies. In his later years, Schwitzer became an Indianapolis philanthropist. Today, you can see his name on buildings and institutions all over that city.

The Schwitzer-Cummins supercharger was an easy choice for the Duesenberg-built, Cummins-powered race cars. Especially in their legendary first try at the Indy 500 in 1934. These cars used highly modified Model H four-cylinder engines. That’s probably where the connection was made to add blowers to the production HB in 1937. Schwitzer was also already developing centrifugal superchargers. These were fitted to the Lycoming engines used in Auburn and Cord automobiles. Schwitzer-Cummins blowers also ended up on Cat, Mack, and International diesels in the ensuing years.

Taking A Turbocharged Turn

This is one of the early style Switzer-Cummins blowers used on the 1934 Cummins Indy cars. The 1935 Daytona Beach Speed Trial car is one of the Indy cars repurposed with a six-cylinder HA installed. The blower produced an 8 psi boost on the HA six-cylinder installed into the car. It delivered 300 horsepower and a speed of 144 mph on the beach. The blower is very similar to the one used on the HBS. However, it’s driven at crankshaft speed off the front of the engine. This was shot in 2018 at the Cummins restoration facility while the race car was under restoration.

Schwitzer-Cummins became Schwitzer Corporation in 1957 and had already begun producing lightweight turbochargers. Schwitzer licensed Holset to produce some of the earliest turbochargers in Europe. Eventually, Cummins Engine acquired Holset, so there’s one small connection at least. Schwitzer lasted into the early 1990s, still in Indy, and then the core of its turbocharger operation became a part of Borg-Warner.

Cummins Context

Those 200 horsepowers put the Cummins HBS at the top of the power heap in the late 1930s. In its displacement and package size range, nothing could beat it. We’re not sure exactly how many trucks the HBS powered. We are sure that they would have been the trucking industry’s barn burners. Cummins Applications Engineer and Cummins historian Keith Baylor says, “I’ll bet it was not much different than later years, where the fleets ran base models and wildcatters drove the hotrods.”



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