Managing Torsional Vibrations

At high rpms, the Holden crank suffers from a fair bit of torsional vibration of the crankshaft, though it's not as severe as in some other straight sixes. For those not familiar with the phenomenon a quick summary goes like this: the crank is being continually subjected to impulsive forces from combustion and compression pressures as well as inertial loadings from accelerating and deccelerating the reciprocating bits. These forces vary in direction and magnitude and tend to make the crank motion somewhat jerky rather than spinning at a constant speed. Now, the crank isn't perfectly rigid and is somewhat restrained at one end by the flywheel and the load but is relatively free at the front. Because of this there is some relative twisting forward and back between the ends of the crank. Providing this isn't excessive it's not a problem (say not much more than a degree or so). The thing is though the crank (because of its springiness) has its own natural resonance or frequency that it wants to vibrate at, a bit like a guitar string. And if the frequency of the impulses fed into the crank match the natural resonating frequency of the crank (or a multiple thereof) then things can get ugly. If left uncontrolled the amplitude of the torsional vibrations will jump dramatically. This isn't just a gentle buzz either, the vibrations can be violent enough to break the crank, or shear the flywheel bolts or shake the rim off the balancer. At milder levels it can badly affect spark timing. Incidentally, it's quite common for straight six crankshafts to resonate at a frequency that corresponds with 6000 - 6500 rpm. Provided you can stay above or below these critical speeds then vibration is usually negligible or at least manageable.

There are basically four things you can do to minimise torsional vibration problems:
1.Use lighter reciprocating components
2.Use a flywheel and balancer weight that shifts the crankshafts resonant frequency above or below the engines rev range
3.Use an effective harmonic balancer/damper
4.Use devices such as heavy main girdles and grout filling to minimise any sympathetic vibration of the block

Most of these are fairly self explanatory but it might pay to take a closer look at the second one. As we said earlier the crank will have a natural resonant frequency, and this can be shifted up (by using a lighter flywheel and balancer) or down (by using heavier components). It might seem counter-intuitive, but it's quite likely that a light flywheel and balancer will be best on a relatively mild engine - it will help raise the troublesome rpm range above the engines normal operating range. Conversely, heavy flywheels and balancers can help with a very high revving engine by shifting the resonant range below the engines powerband - providing the engine can accelerate quickly through the resonant patch then vibration shouldn't be much of a problem.

I've recently spent a fair bit of time studying published information regarding torsional vibration and different types of harmonic balancers. The idea was to gain an understanding that would help me select a suitable harmonic balancer for a somewhat oddball Holden six. Unfortunately after many hours of research I'm really no further ahead; while the physics of torsional vibration are well understood, there is little in the way of reliable data related to the hardware needed to control it. Manufacturers data often seems to be deliberately incomplete or misleading, and much of the information related to practical control of TV is contradictory. For what it's worth (and it's not worth much) here are a few notes on different styles of dampers and balancers:

Rubber bonded dampers (like the OEM style) are by far the most popular. They consist of a heavy outer ring attached to a crank mounted hub by a thin layer of rubber. They have a natural resonant frequency that depends on the mass of the ring and the characteristics of the rubber. Manufacturers claim that they are carefully tuned to match the particular engine but this is not strictly accurate. All that's really important is that the resonance of the balancer doesn't match the resonance of the crank. Detractors claim bonded balancers are only effective at a certain rev range but in reality they are fairly effective over a wide range of speeds - excluding of course the speed that matches the balancers own resonating frequency. Manufacturers publish graphs showing that these types outperform other styles and as far as I'm aware these are the only type of balancer available off-the-shelf for the little Holden.

Fluid damped units (eg. Fluidampr) again use a heavy ring, but this time it's in a closely fitting steel shell that holds a heavy viscous silicone fluid along with the ring. Viscous shear provides the damping action. These units have no natural resonance of their own so I guess they would eliminate the possibility of inadvertantly operating them in the "wrong" speed range. They seem to be mildly effective over the entire range but perhaps less effective than the other types at very high frequencies. Commonly fitted as OEM on low speed/high amplitude applications such as large diesels where they seem to perform exceptionally well. Again, manufacturers publish graphs showing their product outperforming the other types.

Pendulum type balancers have been used extensively on aircraft engines for years, and an automotive unit using roughly the same priciple is available in the TCI Rattler. Strictly speaking these are actually a puck type of balancer, though TCI refer to them as pendulum balancers. The feature that distinguishes these units from the others is that unlike the dampers, they don't actually absorb any energy or create heat. Theoretically they should have a slight power advantage but whether it's enough to be measurable is unclear. This design uses a solid wheel which has had several (usually nine) holes drilled through, close to the periphery. Steel rollers fit into each hole with a certain amount of clearance and as the crank vibrates the rollers are displaced within the holes to a different position. The mass of the rollers looks quite small compared to the pendulums in the aircraft engines though having said that the Rattler does seem to enjoy a fairly good reputation. There is no natural resonance with these balancers and they are said to be effective over the entire range. TCI publishes graphs showing (surprise, surprise) the Rattler outperforming the other types.

TCI Rattler
Pointless picture of TCIs "Rattler". You can't see the rollers in this shot but just look at that cool rattlesnake!

If you plan on frequently running high revs - say 6000rpm plus - then it's important to get a suitable balancer on the front of the crank, and this will very likely be bigger and heavier than the stocker. Romac make some fairly big competition balancers for the six - as to their effectiveness I don't know for certain. Fluid filled dampers from Perkins diesels have been used very successfully in the past though I doubt if the original designers of these ever intended them to see very high speeds. Another alternative is to adapt steel competition dampers made for larger engines, eg. Chev V8s. The damper rim/rubber ring combination on these style units is supposedly tuned to suit specific applications, but the important thing I think is getting a unit with sufficient mass. And you could probably argue that a Holden 6 at 8000rpm would be producing torsional vibrations at a similar frequency to a Chev V8 at 6000rpm anyway. Adapting a fluid or pendulum type balancer from another engine would sidestep the potential tuning problem and may be a safer option. Finding the space to accomodate a big balancer might not be easy, but if you're turning big revs then you really have no choice.

A steel flywheel is also a necessity if you'll be running higher rpms, say 6000 plus. You might get away with the cast wheel but for the price of a steel one it's just not worth the risk. As we mentioned earlier, flywheel mass can be used to shift the cranks resonant frequency up or down, and if vibration is a problem then this should take precedence over other considerations. If torsional vibration isn't excessive though, flywheel weight can be used as a tuning tool. Cars with a very high power to weight ratio will benefit from the lightest possible flywheel, while at the other end of the scale it could pay to use plenty of flywheel weight with a heavy, modestly powered car. Full bodied sedans built for drags that aren't traction-limited (and that would be most n/a sixes) will usually run quicker with a lot of flywheel mass. The heavy wheel will help get the car off the line and may more than make up for the slight drop in acceleration. Torsional vibration also manifests itself at the flywheel end, most commonly by continually loosening the flywheel bolts. The later engines used a dowel to help stop the flywheel from walking on the crank flange. As a minimum on a competition engine you should use two hardened dowels and a set very high strength bolts (the ARP kit for a BMC Mini fits). The mating faces must be perfectly clean, flat and dry before assembly. If there is any evidence of the flywheel moving (galling etc.) or if there is any measurable runout of the flywheel face then you should get both faces trued up.