What engine modifications affect torsional vibration?
Two questions we hear are ‘What do engine modifications do?’ and ‘When should I upgrade the harmonic balancer?’
It’s one thing to measure torsional vibration. But like all data it needs context. In our shop we like to tinker. We like to make more power. We like lighter parts. What does all that do to our baseline torsional vibration map? Should I have a concern? The blunt answers are YES and A LOT.
How important is that harmonic balancer?The harmonic balancer is a fundamental engine component to provide durability. The OEMs have already figured out the torsional vibration map. The majority of the time they equip your engine with a tuned, narrow range elastomer harmonic balancer. This reduces it in a key spot for durability, comfort or both. It is a very cost effective part to last the life of the warranty under stock conditions. It is also cheap to manufacture. It does its job. For daily drivers it’s all most of us need. Furthermore, torsional vibration is not generally a concern for OEMs in low torque, small displacement engines. Only a drive pulley with no damping capability is used. Let it be your concern if you do engine modifications. ‘Wait a minute!’ you say. That’s not me. This isn’t another car off the lot. I don’t want that at all! Yeah, us either. Pull a chair up to the workbench and let’s review the basic playbook.
Creating more torqueOur first steps to more power is a cold air intake and performance exhaust. Next you go forced induction or high flow heads to boot. Larger injectors. A pro tune. These engine modifications all give you more air & fuel which of course translates to more torque and horsepower. The cylinder mean effective pressure spike of combustion is what triggers torsional vibration. You’re spiking it with a bigger bang now. Vibration amplitude increases. The stock tuned narrow range elastomer harmonic balancer tries to control it. But it’s only designed for stock power level tolerances. The elastomer layer becomes susceptible to the heat generated by damping. Cracked, bulging or missing rubber are tell-tale signs you’re overworking your stock harmonic balancer. It's time to upgrade. It’s good practice to inspect for this on routine intervals. See Troubleshooting.
During this time the harmonic balancer becomes more and more inefficient. Increased amplitudes accelerate wear on critical engine components. You’ll see this show up on the main bearings. You’re also more likely to see other signs a harmonic balancer is failing. In motorsports, the SFI 18.1 safety standard regulates the design construction of harmonic balancers. This rule is in place because when the stock unit fails and separates it can cause a lot of harm in and out of the engine bay.
Side-by-side video of a five pound solid mass vs a five pound viscous damper on a validation test rig.
When the torsion bar releases it throws each in motion to about 45 degrees.
Rev happy light Weight Engine Modification parts
Lighter is better, right? Lightweight flywheel. Lightweight pistons. Light weight harmonic balancer or solid pulley?
Watch the video above and note the comparison it takes for each to come to a rest. Imagine if this was torsional vibration occurring in your engine. This example shows an extreme amount of twist but it does prove a damper’s comparative ability to rapidly control vibration.
In regards to the harmonic balancer that is the last place to consider cutting weight. It needs optimum inertia weight to control vibration. There’s a very good reason they weigh what they do.
Let’s recap a section from What is torsional vibration. The rotating assembly is a system of masses. Together it has a natural resonance frequency. As you power through the RPM range and torsional vibration frequency aligns with it the vibration amplitude spikes. This can be bad. Real bad. Catastrophic engine failure bad.
When you do engine modifications that change part of the system mass you shift the resonance frequency. Lighter parts will shift it higher. Heavier parts lower. Sometimes you can put the resonance right where you don’t want it. Other times you unintentially discover it by pushing beyond the factory set rev limiter.
The stock tuned narrow range harmonic balancer, and even similarly constructed aftermarket performance replacement dampers, are optimized to function at a known frequency. It will work, just now not ‘in-tune’ to do the most good. Which increases your risks.
When engine modifications are done to rotating assembly components and resonance frequency becomes unkown, the most cost-effective solution for optimum protection is to install a quality broad-range harmonic balancer.