Every time the cylinders in your engine fire torque is imparted to the crankshaft. The energy that is transferred from the piston to the crankshaft can induce as much as 2 degrees of twist in the crankshaft. A harmonic balancer is mounted on the end of the crankshaft to help dampen this twisting.
Harmonic balancers are typically constructed in three pieces. The first piece is often called a hub. It bolts to directly to the crankshaft. This hub is then wrapped in rubber. The outside, or third layer is a heavy metal. This outer layer generally has grooves on it for the drive belts.
The pliable rubber middle ring is what dampens the crankshaft vibrations. As the crankshaft accelerates this rubber will twist and allow the mass of the outer ring to lag slightly behind center of the damper. When the crankshaft snaps back to it’s original configuration the mass of the harmonic balancer cancels the harmonic. By calculating the precise characteristics of the engine and sizing the harmonic balancer accordingly engineers can control torsional distortion. This helps to prevent critical harmonics that might break the crankshaft.
A few years back GM ran a series of test using a 427-cubic-inch small block. From 3,000 rpm to approximately 6,500 rpm all of the dampers tested limited crankshaft twist to no more than 0.6 degrees. Above 6,500 rpm things got interesting. At 7,800 rpm one damper allowed a rather considerable 2.0 degrees of crank twist. The second damper they tested allowed 1.7 degrees of twist, and the third had allowed 1.25 degrees of twist. And these were all brand new units.
You’re in even bigger trouble if you use a 20-year-old damper. Over time the rubber layer deteriorates causing the outer part of the damper to shift. The outer balance ring actually shifts out of position over time.
Even if the outer ring doesn’t shift the rubber can change consistency over the years and that will change the frequency of the harmonic balancer. Frequency range is determined by the hardness of the rubber. After ten or twenty years there’s been a lot of change.
Given what we know about crankshaft twist there’s been an effort to try different solutions. We have three basic types of dampers.
Elastomer: This is the basic stock damper found on most cars. Elastomer is simply a fancy word for rubber. What makes elastomers special is that they can be stretched to many times their original length, and can bounce back into their original shape without permanent deformation.
When it comes to vintage racing cars you might want to think about replacing the original with a new stock unit. The harmonic damper in your car may be old and that’s a problem. The old rubber may be hard and lacking elasticity.
The real advantage of the elastomer damper is that you won’t have to get involved in a tech line discussion. The high performance replacements are pretty obvious. Some sanctioning groups are starting to actually enforce vintage rules. Ask your tech chairman before you run out and buy a super trick damper.
Viscous: Viscous dampers remove vibration energy from the system by moving a free rotating inertia ring back and forth through a highly viscous silicone fluid. This converts the vibration energy into heat, which is easily dissipated through the sealed inertia ring housing.
A free rotating inertia ring adds the extra advantage of controlling torsional vibration across a broad frequency range and provides protection as engine modifications are made. Fluidampr is the largest producer of these viscous dampers.
The issue with Fluidamper is whether or not the silicone used in the damper changes over time. The actual evidence here is rather sparse. Fluidamper says it's simply not an issue. I've seen no real documentation on the issue - from either side. It may simply be another internet legend. A number of race teams though treat the Fluidamper as a replaceable item when refreshing an engine. They treat it the same way they treat rod bearings. Just change them.
The issue with Fluidamper is whether or not the silicone used in the damper changes over time. The actual evidence here is rather sparse. Fluidamper says it's simply not an issue. I've seen no real documentation on the issue - from either side. It may simply be another internet legend. A number of race teams though treat the Fluidamper as a replaceable item when refreshing an engine. They treat it the same way they treat rod bearings. Just change them.
Rotating Pendulum: This technology has been used in aircraft engines since the 1930’s. In recent years TCI automotive has introduced it to the automotive market. They refer to it as the Rattler Series.
Their ability to absorb, rather than dampen, is the key to their success. Rotating pendulum dampers are effective for the entire rpm range. Their design does not utilize viscous fluids and they require virtually no maintenance. You can always depend on the timing marks to be correct because they're etched onto the body and cannot move relative to the crankshaft centerline.
The problem with this style of damper is that it was designed for aircraft engines which run at a steady rpm. A road race or drag race engine runs through the entire rpm range very quickly. A road racing engine is constantly varying the rpm of the engine. In other words we're asking the rotating pendulum damper to do something it was never designed to do. Scat is rather adamant that you should not use this type of damper.
SFI Rated Dampers This is something to consider if you run extremely high rpms. The inner hub and outer ring are manufactured from high quality billet steel forgings, with elastomer injected between the two component parts.
I recently came across an article dedicated to the Porsche 911 engine. If you're working on a 911 you really need to read this article.
This article was originally published in Vintage Motorsport
I recently came across an article dedicated to the Porsche 911 engine. If you're working on a 911 you really need to read this article.
