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 Critical Speeds & Harmonic Damping Introducing Fluidampr's new Viscous Fluid Vibration Damper for the GM 6.2/6.5 By Jim Bigley Photos by TDP and the manufacturers
Most 6.5L diesel owners don't give a lot of thought to the harmonic damper, but the 6.5 and indeed all diesel engines depend on their harmonic damper to produce a smoother running engine and to reduce the destructive effect of harmonic vibrations in the crankshaft. Whether due to age or accumulated mileage, more 6.5L diesel engines are suffering from crankshaft failure - some of which are due to a deterioration of the factory harmonic damper. Until recently, our only choice was the factory damper. Fluidampr's recent release of their viscous damper for the 6.2/6.5 changes everything. Fluidampr offers a viscous fluid vibration damper for all of the diesel engines used in GM, Ford & Dodge light-trucks from 1982 through 2006. This includes the GM Duramax, GM 6.2L/6.5L, Ford 7.3L/6.0L, as well as the various model year Dodge 5.9L Cummins. 
6.2/6.5 harmonic dampers have been studied and discussed here in The Diesel Page for many years. This photo was taken at the Rendezvous we hosted during the summer of 2000. The damper closest to the camera was used to illustrate deterioration of the rubber elastomer ring, and was compared to a new 6.5 damper you see behind it.
 
Both the 6.2/6.5 and the Duramax 6600 come factory-equipped with what is known as a "Tuned Rubber Torsional Vibration Damper" (sectioned Duramax damper shown). The rubber elastomer is bonded to the hub and the outer inertia ring. At the onset of a harmonic torque spike, as felt by the damper, the inner hub will accelerate ahead of the outer inertia ring. The connecting rubber layer and inertia ring will act to dampen this torsional spike. As the torsional spike passes, the lagging inertia ring will snap back into position, which not only reduces the amount of torsional twist due to the torque spike, but also the rebound as the momentary torque spike suddenly ends. The thickness, durometer (stiffness) and surface area of the rubber layer, as well as the mass of the hub and inertia ring result in a damper that is tuned to a narrow range of harmonic frequencies. In general, due to the limited range of harmonic frequencies a tuned rubber vibration damper can effectively deal with (due to limited deflection of the rubber elastomer), they tend to be designed more for the lower rpm ranges where drivers would be more likely to notice the vibrations and where the engine might be run for extended periods. Factory tuned rubber dampers used on diesel engines live a tough life. The diesel's high compression and high torque at low rpm produce more crankshaft deflection than that seen on an equivalent sized gasoline engine. As a consequence, a harmonic damper used on a diesel pickup engine is larger and heavier, ranging from 18-24 lbs. As a comparison, dampers used on a small block Chevy V-8 might weigh 5.75-14 lbs, depending on year and application. Not only is the inertia ring larger and heavier on diesel dampers, but they also stress the rubber elastomer a lot more. (Note: The weights listed are examples of Fluidampr's cataloged viscous fluid dampers.)  The above photo shows the front side of a defective 6.2L damper. The rubber layer is shown pushed out in some areas and recessed in others, and has lost its bond between the hub and inertia ring. Aside from the impending separation of the inertia ring, a shift in the rubber will also produce a change in the tuning, and can affect the damper's ability to control vibrations in the frequency range it was designed for. Replace any damper showing these sorts of problems.  This photo shows the backside of the same 6.2L damper. Heat, age and flexing fatigue have produced a deterioration of the rubber elastomer. Cracks and missing chunks require replacing the damper.  On the left in the above photo is a factory 6.2L tuned rubber torsional vibration damper. On the right is Fluidampr's new "Viscous fluid vibration damper". Both dampers are approximately 8" in diameter, but the Fluidampr is about 4.4-lbs heavier (~13.5-lbs compared to 17.9-lbs). I was curious to learn whether a heavier damper would produce any positive or negative effects on the 6.5 or harmonic damping in general. To answer my question, Dan Oddy, an engineer with Fluidampr, replied by saying:
 Fluidampr offers just one damper that is designed to work with either a 6.2L or 6.5L diesel. You'll notice in the above photo that the factory 6.2L damper incorporates a longer snout that extends over the crankshaft. The Fluidampr shown on the right was designed for the electronic 6.5L diesel (pn-800141), which requires a shorter snout because of the CKP (crankshaft position sensor) reluctor wheel mounted ahead of the crankshaft timing chain sprocket. Fluidampr pn-800191 comes with a spacer for the 6.2L or 1992-93 non-electronic 6.5L engines. 
This Fluidampr image illustrates the basic construction that defines a viscous fluid vibration damper. The outer housing encloses the inertia ring, which floats in a heavy silicone gel. According to Fluidampr, this gel is 45,000 times more viscous than 30W motor oil. Silicone is extremely stable, even when subjected to wide temperature variations, and is said to produce the same degree of vibration damping whether hot or cold - for the life of the engine. It never wears out or deteriorates because of time, number of hours of service or when exposed to extreme temperatures. According to Fluidampr, the viscous silicone fluid and inertia ring allow their damper to deal with all harmonic frequencies, outperforming a tuned rubber damper - across the entire engine operating range.   This front and rear views of the Fluidampr show that the 6.2 & 6.5 are externally balanced engines. I had heard that Fluidampr was recommending that the stock damper should be used when balancing the engine rotating assembly during a rebuild. Because the 6.2/6.5 Fluidampr incorporates balance weights, I wondered whether that recommendation still applied. Dan Oddy, an engineer with Fluidampr, replied by saying:
Removing the original damper: Remove the serpentine belt, put the truck on a lift or on ramps, and then drop the aluminum skid plate behind the front bumper. You'll have access to the damper/pulley. Alternately, you could remove the top half of the fan shroud and remove the fan & fan-clutch. Place a piece of cardboard over the radiator to help prevent damage. You can remove the damper bolt using an air wrench. A breaker bar will also work, but you'll need to drop the transmission bellhousing cover so you can keep the crank from turning - by wedging a pry bar between the block and a hole in the flexplate. You'll need a puller (like the one shown) to get the old damper off.
A damper installer makes installation a snap. It's easy to strip the threads in the end of the crank if you attempt to run the damper onto the crank using the damper bolt. You can make a damper installer by buying a metric bolt that fits the crankshaft (M16 x 1.5), weld on a piece of all-thread onto that new bolt you bought, then use a nut on the all-thread to get the damper fully seated onto the crank. Once the damper is on, remove your installer, and then torque the damper bolt to 200 ft-lbs. That's a lot... You'll need someone to hold the pry bar in the flexplate while you see what 200 ft-lbs feels like... When re-installing the crank pulley, the 4 pulley bolts are torqued to 35 ft-lbs. Clean off any accumulated dirt & trash so the pulley can sit squarely on the damper. If you do replace the damper, you might as well replace the front crankshaft oil seal. You can make a seal driver out of a piece of appropriately sized PVC pipe.  Sources:
Diesel Performance Parts Inc.
Fluidampr/Horschel Motorsports
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