We are all guilty of installing a flashy part on our hot rod or engine that doesn’t add any power, but we’re convinced the system “runs better” with it installed. Now imagine installing a part that doesn’t directly build power, but its interaction with the engine’s rotating assembly can release power that may have otherwise been wasted energy in the form of vibration, or maybe the part just reduces premature wear on bearings and associated components. This part–the harmonic damper–already exists on every street-driven engine, and is often overlooked as just a mounting point for accessory pulleys or a location for attaching weights for externally balanced engines. The harmonic damper presents a crucial opportunity to optimize your rotating assembly to insure longer life or to free up wasted energy that could be better applied to the tires.
So is it a balancer, or is it a damper? The two terms are used interchangeably, but technically they have different functions. A balancer adds weight to help (externally) balance the lower rotating assembly of an engine, while a damper, quells the vibrations during crankshaft vibration which occur as a function of the combustion process. Keep in mind, while all harmonic balancers are dampers, all dampers are not balancers. Yet for purposes of bench racing, both terms are often used to mean the same part.
Keeping It Basic
During the combustion process, each piston is forced to move down the cylinder as a result of an explosion contained within the combustion chamber. This stroke imparts a sudden rotational force to the crankshaft. Even though it is a very stout component, a crankshaft is not perfectly rigid. So during these combustion events, the crank will twist slightly in response to each explosion/impact.
This crank twist is analogous to a simple torsion bar with a lever arm at one end. Now assume you hit that lever arm with a hammer. You can imagine there will be some slight twist when you first hit the lever arm, but that would be followed by the arm springing back into place, maybe even vibrating for a short time before coming to rest.
In this example, we have torsional twist followed by torsional vibration (during the spring back event). Torsional twist is a function of part length (inline engines will have a longer crankshaft than V-configurations) and thickness, material shear modulus (think: material stiffness), and Torque (force from combustion x crank throw). Similarly, torsional vibration is a function of part length, torsional stiffness, and polar moment of inertia (think: object’s ability to resist torsion). For a detailed explanation of these phenomena, review Himmelstein’s Technical Memo #8150.
What’s the problem?
Torsional vibration has side effects that are rarely desirable. Because force is transmitted into a crankshaft at discrete points in its rotation (example: every 90 degrees of rotation in a V8 application), the output torque is not continuous, thus creating pulses and torsional vibrations. In the event that these pulses occur around the resonance frequency of a crankshaft, the effects can be multiplied. When this occurs, risk of component failure (broken crank) increases as well as the chance of premature wear of bearings. In some cases those vibrations can be transmitted to other points in the powertrain such as the valvetrain (via timing chain) or down the driveline towards the tires. Either direction isn’t desirable as it can rob power in addition to potentially killing parts.
While a few methods to damping have been invented, the vast majority of automotive OEM and aftermarket companies point towards two solutions for controlling detrimental torsional vibration; elastomeric and viscous damping.
The method for elastomeric harmonic damper construction tends to make them the most cost-effective as an OEM replacement or upgrade. Romac Performance Products specializes in elastomeric harmonic dampers and builds products for mild to full race applications. Construction is rather simple, and consists of four parts: a hub, elastomeric band, outer inertia ring, and snap ring. The hub is constructed to fit over the front of the crankshaft snout with a keyway. The hub will usually have a bolt pattern built into the front face which allows for attachment of accessory pulleys. The backside of the hub may have a cavity built for attaching weights to externally balance the rotating assembly. Applied around the hub is the elastomeric band which is constructed of a rubber polymer. Polymer durometer (hardness) gives Romac the ability to add or subtract damping ability, tune for engine frequency, and long term durability.
Fit to the outside diameter of the elastomer band is the outer inertia ring. This outer inertia ring functions as a small flywheel that wants to remain at constant speed despite the acceleration and deceleration pulses that the crankshaft is experiencing. The elastomeric band between the hub and outer ring allows the outer ring to move slightly independently (or at a more constant speed), while the inner hub runs slightly faster and slower in response to torque pulses. A snap ring is inserted inside of the outer inertia ring in order to keep the band and inertia ring in their respective places.
During operation, the band transmits only part of the vibrational forces from the hub to outer ring, while also converting some of that vibrational energy to heat which is dumped to atmosphere. Romac takes great pride in the fact that all of their harmonic dampers are hand assembled, giving technicians that ability to inspect each assembly for best results.
A good example of viscous harmonic dampers are those offered by Fluidampr. Speaking with Brian Lebarron at Fluidampr, he describes a viscous harmonic damper as “contain[ing] a free rotating inner inertia ring that shears through a thin layer of proprietary silicone. As it shears, destructive vibration is transformed to heat. Heat then radiates through the outer housing to atmosphere.” More simply; the viscous damper includes an outer housing, an inner inertia ring (think free-rotating ring inside the housing), and the housing is filled with a THICK silicone fluid which fills in the gaps between the housing and ring.
Under constant smooth torque applications, the housing and ring spin together. When torque pulses are introduced, the ring wants to continue spinning at a constant speed while the housing is in a constant state of acceleration and deceleration.
The housing and ring speed differences apply a shear force against the silicone fluid between them. The silicone absorbs that shear force in the form of heat which is dumped to atmosphere.
From an OEM perspective, an elastomeric damper covers the vast majority of engines being produced today. The primary reason for this choice is cost of manufacturing as the elastomeric option tends to be constructed of parts which have lower tolerances as well as utilizing raw materials which are more common and don’t demand a premium price. Viscous dampers are often utilized by OEMs in high performance and luxury applications.
Do I Really Need One?
For those familiar with sprint car engines, they know that most sprint competitors don’t utilize a damper at all. Similarly, a minority of drag racers have opted out of a damper with the theory that less rotating mass allows the engine to increase RPM quicker. We’ll keep this discussion at a high level and just point out that sprint and drag engines aren’t intended to see many miles. Careful component selection can help mitigate some overall vibration and ultimate component failures, but the benefits of a damper are undeniable for anybody looking for more than a few quarter-mile passes or a Saturday night main.
Selecting a Damper
Consider the following topics when choosing your next damper:
- Daily Driver to Full Race?
- If a Daily Driver, will stock replacement suffice? Consideration should be given regarding avoiding the previous failure mode.
- If Performance or Full Race, what type of driving or racing (sprint/endurance)? What types of RPM will the engine see?
- Function over fashion
- Always prioritize function. After all, what good is fashion if the chosen damper leads to engine failure? After functional needs are met, feel free to throw all the chrome plating and anodizing as is appropriate.
- Does the engine need crank pulleys or external balance weights? Will the crank pulleys be attached to the damper body or should they be an integral part of the harmonic damper?
- This topic may fall under both Application and Function, but make sure to choose a diameter that meets both functional needs AND will actually fit within the space allotted.
- SFI or not?
- If any type of racing or performance is under consideration, an SFI-approved harmonic damper should be a requirement. Many sanctioning bodies will require an SFI 18.1-approved damper. If not, consider what it would be like if a 6-8 pound metal disc came off the front of the engine at high RPM…yeah, don’t skimp here. To understand more about what it takes to comply with SFI 18.1 guidelines refer to SFI -Crankshaft Hub Harmonic Dampers.
- If you’re rebuilding a daily driver’s engine, that $89 damper from the parts store may be all you need. But if any added performance is your goal, expect to pay according to your intended RPM and speed ranges. That $89 harmonic damper likely will not provide much protection if you’re spinning your engine to 7,000-plus rpm, or hanging a big supercharger on it and doubling or tripling its output. The old saying of “you get what you pay for” is definitely in effect when it comes to harmonic dampers.
- Daily Driver to Full Race?
There is a wide variety of choices on the market when it comes to harmonic dampers; from elastomeric to fluid-based, and each one has its place. The ultimate selection is up to you, and your engine will thank you for taking its health into account during the build process.