It looks like a round lump of iron sitting on the end of the crankshaft. Some think it's there as a convenient place to check ignition timing. But there's much more to a torsional damper than a place to hang a pulley. This story will deal mainly with small-block Chevy dampers, but the basic information is the same for all engines. So show this to your Ford buddies. Maybe they'll learn something too.
Our first task concerns semantics. Most hot rodders call them harmonic balancers, but if you want to be correct they are called torsional dampers, which is a much more accurate description. During engine operation, cylinder pressure spins the crankshaft, but it simultaneously twists or deflects the crank. Even 4340-steel cranks twist, more so as power and rpm increase. The amount of deflection is based on several variables including compression ratio, cam timing, ignition timing, rod length, stroke, and dozens of other variables. This deflection (you can think of it as a wiggle or wrapup) is constantly changing, but there are specific engine speeds where these harmonics are amplified. The job of the torsional damper is to limit these deflections.
With identical production engines, constructing a torsional damper to limit these vibrations is relatively easy. However, a high-performance custom-built engine with different pistons and a batch of custom-matched pieces makes it difficult to create a damper that will be ideally suited to each combination. So how do you choose a good damper? We'll cover most of the major differences in torsional dampers so you can make an intelligent decision when it comes to your next small-block engine buildup. But the short version is, at best, a guess.
Before we get into the different aftermarket dampers, we need to cover all the production-based dampers. Chevrolet and GM Performance Parts offer several different dampers, but they can be more easily categorized into either internally or externally balanced styles. An internally balanced damper is the more prolific style and will work on any small-block except the 400- or 383-style stroker engine. Internally balanced or neutral-balanced dampers have no offset weight and can be zero-balanced either with the engine or by themselves. Externally balanced dampers are used on small-blocks using a 400-style factory crankshaft where the factory decided to add additional counterweight to the damper instead of the crankshaft throws. This external weight difference is also applied to the flywheel/flexplate assembly.
Beyond this major distinction, small-block Chevy dampers also differ in diameter and width, as well as placement of the top-dead-center (TDC) mark. Chevy has offered dampers ranging from 6 to 8 inches in diameter. The smallest dampers were used on the early 265, 283, and 327 engines. As displacement grew, so did the dampers, increasing the small size to 63/4 inches and employing the largest dampers for the 400 and high-performance 302ci and 350ci engines. What is less well-known is that Chevy moved the location of the top-dead-center mark in 1969. Prior to 1969, the TDC mark was located 2 degrees before the crank keyway. For '69-and-later dampers, the TDC mark moved to 10 degrees before the crank keyway, which is 8 degrees more advanced than earlier dampers.
Most factory and almost all aftermarket dampers use the late-model TDC location, but if you are mixing and matching parts you may want to confirm the location of the TDC mark to make sure the timing numbers are accurate. Early Chevy timing-chain covers used welded-on timing tabs, while later engines used bolt-on tabs. There are also two different Chevrolet bolt-on tabs, depending upon the diameter of the damper used. Both of these bolt-on tabs are to be used with dampers employing the 1969-and-later TDC position.
All production and OEM replacement dampers are designed as elastomeric dampers, where a thin line of synthetic rubber is used to bond the outer damper ring to the hub that is pressed onto the crankshaft snout. The idea is that when the crankshaft twists, the outer damper ring will react to this twist by deflecting the rubber slightly. By using the correct-size damper and a specifically tuned elastic band, this type of damper works very effectively for that particular engine. But if anything is changed, such as lighter pistons or more compression, this tuning is probably no longer ideal. For example, the malleable, ductile iron damper (PN 364709) was originally designed for NASCAR-style 350ci small-blocks running at 8,000 rpm damping critical frequencies for those specific engines.
Most aftermarket dampers also follow the factory lead with an elastomeric design. However, several companies have elected to try and enhance this situation with different ideas. These include the Fluidampr, which uses a free-floating ring inside a sealed case that, according to the manufacturer, can tune out crankshaft deflections over a wide rpm band. The internal damping ring floats in a thick, viscous silicon fluid that prevents the ring from contacting the outer case.
Another idea is the TCI Rattler 2000 damper that employs a series of steel balls that use a centrifugal pendulum theory to offset the torsional inputs of the crankshaft. In both the Fluidampr and Rattler examples, the internal components are allowed to move to compensate for crankshaft deflection.
As engine speeds have continued to climb, various racing sanctioning bodies like the NHRA have outlawed OEM-style dampers in favor of more durable units that must meet stringent SFI 18-1 specs. This is also the case for any engine employing a crank-driven supercharger. Big superchargers like 6-71, 8-71, or the new generation of monster centrifugal superchargers all place a great load on the damper and the crankshaft snout. If you are considering a crank-driven supercharger, investing in a race-quality damper is a must. While we’ve covered the basics of damper operation and applications, there’s much more to this story than we can cover here. The main thing is to pay attention to the details and make sure your damper isn’t suffering from old age or abuse. While you don’t need a killer race-bred piece for the street, you do want to ensure that the damper is right for the job. It never hurts to be careful. That way, you don’t have to learn the consequences of the damper dance.