Theories And Practice: Fluidampr And The 5.9-Liter Cummins


Since its invention in the late 19th century, diesel engines have been making the world a better place. Companies have built their entire foundations upon these icons of machinery, with their tight tolerances and compression-based quirks and attributes.

Anthony Reams' truck relies on Fluidampr to make to the finish line every time.

Anthony Reams’ truck relies on Fluidampr to make to the finish line every time.

One such company is Cummins, which has been in the business of diesel since 1919 and had tremendous success partnering with Chrysler since the late 1980s. The two reshaped what it meant to have a diesel engine inside of a regular, everyday truck when, in 1989, Mopar trucks rolled off the line with a B Series Cummins motor churning happily underhood.

The B Series gave the world two versions: a 3.9-liter inline four, and the 5.9-liter inline six. The latter engine has since become legendary and respected in its own right and is the focus of this article. We talked to our good friends at Fluidampr about the importance of keeping these powerplants running well.

Background Of A Diesel Icon


Two versions of the 5.9-liter for Dodge Rams were produced over the course of two decades. The first, designated 6BT by Cummins and first used in agriculture and construction equipment, went from 1989-1998. It is commonly referred to as the”12-valve” owing to the dozen valves (two per cylinder) used in the motor. Among diesel enthusiasts, this motor is highly treasured for its relative simplicity and great potential for power upgrades without much hassle.

This motor was turbocharged, naturally. According to Allpar, the motor had a 4.02-inch bore and a 4.72-inch stroke, a compression ratio of 17:1, and made 160 hp and 400 lb-ft of torque. It had an iron block, aluminum intake manifold, forged connecting rods, and was manufactured out of North Carolina by the Consolidated Diesel Company. On its own, the 6BT had (and still has) all the hallmarks of a great design which came at the right time and place to show Ford and GM what a real diesel motor could do.

The 6BT engine as it appeared in 1989. Photo credit:

Following the 6BT was the ISB (Interact System B), which popped up in the middle of 1998. Changes to the motor included doubling the number of valves, from 12 to 24, as well as an electronically controlled Bosch fuel system replacing the previous mechanical setup. More than anything, these changes were made to help the engine meet stricter emissions standards, but they did also effect an increase in horsepower and torque; 235 hp and 460 lb-ft, respectively. Four years later, the introduction of common-rail fuel systems came about.

Throughout it all, the 5.9-liter had proven itself as a very adaptable and versatile platform. It was able to adjust to the market’s ebb and flow, as well as overreaching governmental constraints placed on it, and still provided consumers with a tough, powerful, and efficient mode of transportation. But the engine was not perfect.

Enter Fluidampr


As resilient as the 5.9-liter Cummins is, it still has its weak spots in areas like the harmonic balancer. Fluidampr, with decades of experience in the field of engine harmonics, has the know-how and quality to match the smarts of the Cummins straight-six. We spoke with Brian LeBarron to get a better sense of what these products do, how they are made, and the expanding importance of these products as the years go by.

The Cummins 5.9-liter is a fantastic foundation for many types of builds, but it's not without its faults.

Regarding diesel engines as a whole, LeBarron affirmed that the concerns over excessive vibration are warranted. “Diesel motors generate higher torque amplitudes than gas motors do,” he said. “We’ve seen cylinder pressure in excess of 17,000 psi on a light-duty truck diesel engine, compared to about 1,100 psi on a modern performance gasoline engine.”


The Performance Diesel Damper for 5.9-liter Cummins motors (PN 960311).

With the higher vibration comes a higher risk for damage, so a viscous damper is an absolute must on these motors, including the 5.9-liter Cummins. “A viscous damper protects the engine from destructive crankshaft torsional vibration,” offered LeBarron. “Torsional vibration is the end-to-end twisting and rebounding of the crankshaft created from internal combustion. If left uncontrolled, it can lead to early catastrophic engine failure.”

The viscous damper for a diesel differs from a gasoline version in that the damper requires more inertia mass (weight) and a larger diameter to control things. “Heat dissipation also factors in,” added LeBarron.

Composition Of A Fluidampr Viscous Damper


Fluidampr uses a high-grade steel alloy to make the outer housing and inner inertia ring on the viscous damper. Computer balancing also factors in, making the dampers safe to use at high speed.

Evolution touches all aspects of our universe, and viscous dampers are no different. “They have come a long way, and our involvement has spurred many advances,” said LeBarron, referencing the company’s past in diesel motorsports development.

Steel is a term that speaks to every motorhead out there, and Fluidampr uses a high-grade version of the alloy to make its outer housing and inner inertia ring on the viscous damper. The housing and ring are also computer-balanced at a high speed during manufacturing.


“A small amount of specialized silicone between the inner inertia ring and outer housing provides shear or a damping medium,” explained LeBarron. “The silicone is about 45,000 times thicker than 30-weight oil and is used because it holds its viscosity tolerance across a broad temperature range, from -40 to 300 degrees Fahrenheit. Last but not least, corrosion resistance is achieved through a black zinc chromate finish.”

Torture testing will speak to whether a part can or cannot pass muster, and Fluidampr exercises this with its viscous dampers. “Our Performance Diesel dampers are SFI 18.1 certified for professional competition,” said LeBarron. “To meet that standard, a damper has to be spun at 12,500 rpm for an hour without self-destructing; no small feat. Some of our dampers for the 5.9-liter measure 9.75 inches in diameter and weigh 24 pounds.”

When, Why, And How To Upgrade To Fluidampr


Running without a damper is obviously crazy, but running with a Fluidampr damper just makes sense, as LeBarron explained. “The 5.9-liter Cummins came stock with an elastomer style damper,” he said. “We recommend customers check their owner’s manual. Some years had a 30,000-mile inspection interval. The main thing to look for is if the rubber between the hub and outer inertia ring shows signs of cracking, bulging or has pieces missing. If it does, upgrade to a Fluidampr performance damper. At this point, the stock elastomer damper is no longer adequately protecting the engine.


Things to look for in a damper are if the rubber between the hub and outer inertia ring shows signs of cracking, bulging or has pieces missing. If it does, it’s time for an upgrade.

“Additionally, if you’ve done any performance modifications it is also highly recommended to upgrade,” continued LeBarron. “Although the later 6.7-liter Cummins came stock with a viscous damper, it should be noted that it is not intended for performance or race applications.”

With four part numbers (960311, 960341, 960301, and 920301) currently offered for the Cummins straight-six motors, customers are sure to find something to suits their needs. “Different dampers are offered for different year ranges to accommodate for tach pickup, belt alignment, etcetera,” said LeBarron. “The exception is part number 960341, which is our competition series that features no pulley and is often used on professional pulling and drag trucks.”

Having a 5.9-liter Cummins motor run cleanly and efficiently may just seem like a natural thing to many Cummins fans. The fact is, there is no perfect guarantee that these engines will last on their stock parts for 50,000 or 500,000 miles. Trusting in Fluidampr to keep your Cummins humming is not only sensible but popular as well, as LeBarron divulged, “Today, we sell more dampers for the 5.9-liter Cummins than we do for the Chevy small-block.”

So do yourself a favor and check out how Fluidampr can make a difference in your application. This also stretches to the Power Stroke and Duramax fans out there, too, as Fluidampr covers all of the Big Three diesel platforms on the market. See more on Fluidampr’s website and Facebook page.


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About the author

David Chick

David Chick comes to us ready for adventure. With passions that span clean and fast Corvettes all the way to down and dirty off-road vehicles (just ask him about his dream Jurassic Park Explorer), David's eclectic tastes lend well to his multiple automotive writing passions.
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