TalkingTech: So what is NVH?

By Jordan Cameron Remeljej, Owner, Modern Aircooled

One of the luxuries I have in my shop is being able to see two similar vehicles, in need of a similar repair, with two very different owners and intentions. When advising on repairs I need to know the owner as much as the car to best determine what considerations are most important as far as selecting the right parts.

For example: two 981 Caymans in for a brake service, a daily driven base model, and a track-rubber patina-ed GT4. For the former, nothing will supplant the OE rotors and pads for both safety, and noise / vibration / harshness. The GT4, on the other hand, wants the meanest rotors and pads and considers the banshee howl they make a warning to vehicles lazy on the point-by.

In this situation I use knowledge about the vehicles and the owners to compromise between performance and Noise Vibration and Harshness or (NVH). NVH is defined by Collins dictionary as: “…the study of the noise and vibration levels of a vehicle.” Or more generally, how many squeaks, rattles, and shakes a part produces and how it is felt by the driver. NVH has become one of major focuses of manufacturers, because of its intrinsic link to perceived quality. Consumers praise a solid thunk of a door, but bemoan any interior rattles.

This leads specialist engineers to design components to satisfy stringent NVH requirements, sometimes to the detriment of service life, reliability, performance, and more intangibly feel.

The key places we find significant concessions to NVH is wherever a connection occurs between major systems of the vehicle. This is anywhere the drivetrain bolts to the body, anywhere the suspension is mounted to the chassis, and anything the driver is able to feel while driving.

These points will use some kind of rubber or bushing to isolate NVH producing components (engine / gearbox / tires / suspension / etc.) from NVH sensitive components (Interior trim / glass / cupholders / owners!). These points are energy sinks, and typically wear within the life of the vehicle, I’m sure a lot of readers will nod thinking of all the motor mounts, control arm bushings, strut mounts, squeaky brakes, and shifter cables that have been replaced under their ownership.

Consider the two photos below. Here are a couple of examples of a GT3 that has replaced rubber bushings with monoballs. It gives much more accurate steering and alignments, but greatly increases all aspects of NVH through the wheel to the driver.

Original rubber bushings on a GT3

Replacement monoballs on a GT3

A question I get asked a lot is: “Should I replace my broken OEM part with a stronger aftermarket part to increase lifespan?”. This is especially true with shifter cables and motor mounts. The short answer is always maybe. Porsche employs the best and brightest engineers to deliver a product that is both sporty and luxurious. They build their cars to have the highest performance and perceived quality, this comes at the expense of longevity.

The shifter cables are the best example of this. Many of you will know that the 986’s-7’s, and the 996’s-7’s especially have woven steel shift cables with plastic bushings crimped onto the ends. These ends are notorious for braking and stranding the car stuck in a single gear. Sometimes you get lucky and get to nurse it home in second gear, others have been left gambling between destroying a clutch starting in fourth or a pricey tow bill back from College Station, Texas.

This has opened the door to companies offering reinforced cables with crimped steel ends guaranteed not to break. These cables can greatly improve shift feel, reliability and feedback (read NVH). The increased vibrations into the cabin are acceptable in a track car where performance and reliability are paramount. On daily driven vehicles often the constant (although slight) increase in noise and feel is more annoying that changing out the cables every 5-10 years.

Another common example is motor mounts. The original equipment (OE) mounts are an oil filled rubber / hydraulic combination that is excellent at absorbing the inherent imbalances associated with our engines. These mounts do however allow significant movement of the motor on and off accelerations and during cornering. This movement can lead to awkward handling, and imprecise shifting.

When it comes to aftermarket mounts there are many companies selling a whole range of stiffness from full solid semi-solid to a more OE level of polyurethane. Many sportier drivers claim that switching to a semi solid set of mounts can completely transform the track handling and feel of a 911 especially and they have a good scientific backing for those claims. What these drivers will often do is downplay how much extra noise gets into the cabin and the unintentional massage they sometimes get from the seats!

Another important consideration is less driver focused and more on longevity of the vehicle. As a vehicle drives down the road the engine vibrates, the suspension absorbs bumps and carries vehicle weight, and the rubbers and bushings absorb and dissipate all these forces instead of passing them on. In removing these bushings you can inadvertently pass on forces to components that were never designed to withstand them, thus risking failures.

Years ago on one Boxster in particular a set of semi solid mounts appeared to be a contributory factor in the repeated failure of a weld on a muffler mount. The increased vibrations across the mount cracked the weld and ultimately OE mounts were replaced fixing the issue. Lesson learned!

As brilliantly as Porsche vehicles are engineered there are a myriad of ways we can improve performance or reliability by changing out select components to better suit the demands of the driver. The important thing is to consider that it will almost always be a trade-off, and more often than not the trade-off will result in extra NVH and a loss of comfort and refinement. If Porsche was able to engineer better parts that still satisfied consumer demands, then they would do it. So before jumping on the miracles of an aftermarket modification, or insisting OE is best, consider exactly how you enjoy the vehicle, and what compromises you might be willing to make to increase that enjoyment, and always consider NVH!

2018-10-01T17:01:07+00:00