Originally Posted by 4 Jaw Chuck
Crane Cams Article-Valve Spring Resonance: What it is and how it can be controlled!
Best description I could find that describes the function and application of the valve spring coil dampener. Here is the only advantage of the flat wound spring dampener as described in the article;
"One real plus of a damper used with a (single) coil spring is that if the spring should break, the damper acts as a safety to keep pressure on the retainer/lock package to prevent dropping the valve into the cylinder."
Kind of explains why they started using them on very early engines with poor quality spring wire, they would occasionally break and this was a safety device. Most engines today (except for heavy duty diesels) don't use them as the engine speed is never constant and loads on the engine isn't high enough which could cause spring surge situations.
People give lots of goofy reasons why engineers do things, in this case the flat wound damper is there for one purpose as its name implies.
It is a damper, it uses friction between itself and the spring, which is why it always has physical contact with the spring wire. It works in exactly the same way as friction shock absorber did back when they were used on suspension systems rather than contemporary hydraulic tubes. The friction between the elements disrupts the harmonic response of the spring which has nothing to do with being made from poorer or better materials. All springs have a harmonic response or beat frequency. For the valve train the beat frequency or some resonant of it would be hit when the actuation rate based on engine speed happens to coincide the springs natural frequency. The damper coil is put there to change the springs natural frequency, which was usually found by test but today more commonly by digital analysis, to be within the operating range of the engine. Such frequency responses in the spring are simply the result of high RPM operation of the engine, these frequencies and their resonant responses can occur anywhere within an engine's operating RPM range. The masses of the valve, keeper, locks, rocker, pushrod, and lifter also affect the harmonic response of the spring. So does the dynamics of the cam lobe, its shape, timing, and lift.
So the flat wound damper is a simple device that changes the spring's natural frequency so that it doesn't develop harmonic responses within the anticipated operating range of the engine's valve train. Changes to the afore mentioned components and the engine's operating RPM range will change the installed springs harmonic response.
For the average engine builder, this extent of engineering analysis required to understand spring harmonic response is simply beyond their capabilities, if they even understand this stuff exists. We, therefore, are dependent upon the cam manufacturer's recommendations as to which springs to use. Unfortunately, many people choose this as a place to save money when buying a cam which often gets them into a lot of trouble. Needless to say the professional race engine builders spend a lot of time on this as the average solution used for street and occasional race engines of stiffer and stiffer springs is an insufficient solution for guys who earn their living racing.
I suppose dampers could be a last ditch device to prevent a valve from dropping too far if a spring broke, but usually when a coil breaks the valve only drops the distance of the coil to coil clearance of one coil, which usually is not far enough into the cylinder to interfere with the piston. Although I'll give it that in really high strung engine's I've seen springs practically disintegrate, but on the street or with the occasional racer such incidents are rather rare.