The noise of a Porsche sports car engine is a good thing to an enthusiast, but in modern times a balance must be achieved between a sonorous exhaust note during sporty driving and a calm, quiet cabin during long freeway drives. Like most automakers, Porsche invests a considerable amount of time and resources on Noise, Vibration, and Harshness (NVH) engineering during the development of a new vehicle’s design, and the tuning of the engine’s induction and exhaust systems play a large role in this process.
All NVH concerns are caused by vibration. Sound, as detected by the human ear and processed by the brain, is caused by vibration of the molecules of a medium such as air or an automobile chassis, and hits the human eardrum as a pressure wave. “Noise” is defined as a sound perceived as unpleasant or disturbing; while what constitutes a noise versus a sound is subjective, anything that causes buzzing, droning, or rattling noises would be considered undesirable by most automotive NVH engineers.
The nature of a given sound or noise is determined by the frequency and the amplitude of the pressure wave(s) created. The frequency is measured in units of hertz (Hz), or cycles per second. The amplitude of a sound pressure wave is measured in decibels (dB), which is a logarithmic scale in that any increases or decrease of the measured dB value represents an exponential change in the actual energy emitted by the source of a sound.
While the decibel level of any noises generated by an automobile is important to NVH engineers, this is often within a set range (the interior sound level inside typical passenger car is about 70-75 dB), the frequency of any noises or vibrations that is often the more closely scrutinized. The absolute hearing range of humans is widely considered to be from about 20 Hz to 20 kHz (or 20,000 Hz), though most of us have an audible range more like 100 Hz to 15 kHz. Low-frequency sounds tend to create deeper, rumbling noises while a screech or a squeal is a high-frequency noise.
The engine of a typical passenger car generates a broad spectrum of sounds at various frequencies due to its many moving parts. The intake and exhaust cycles of an internal combustion engine are the primary noise-generators, though mechanical clatter and timing chain and accessory belt drives can also contribute to the audible range of noises; as mentioned in the November 2014 (#223) tech feature about direct fuel injection, the high-pressure fuel injectors and pumps of modern Porsches are another noise source.
Engine Mechanical Noises
Early, air-cooled Porsche engines generated a mechanical cacophony of valvetrain clatter and whirring cooling fan noise because of cooling requirements; the internal engine components need to be located close to the cooling fins and are thus minimally shrouded. Looming 1990s European Union regulations for drive-by noise emissions for passenger vehicles meant that the lower half of the 1989-1994 964-gen 911’s engine was almost completely enclosed in an attempt to comply.
Sadly, the 964’s sound-deadening under-panels tended to trap heat and cause premature wear of the exhaust valve guides and engine oil seals; after a less restrictive lower panel for the 993-series, Porsche finally switched to water cooling for the 996 of 1999 (besides noise considerations, exhaust emissions and power output were also key determinants for this change). In addition to providing coolant passages, the water jackets surrounding the cylinder bores and combustion chambers of a water-cooled engine help to quell the mechanical noises generated within.
Modern, water-cooled Porsche engines are still capable of generating mechanical noises that could potentially be transmitted into the cabin. Timing chain drives and tensioners, oil scavenge pumps, and accessory drives are all scrutinized to dampen the noises in the frequency ranges that could cause interior items to resonate.
Additional measures are also used on Porsches to reduce the amount of audible mechanical noise inside the cockpit. While early 356s and 911s used horsehair-backed panels for sound insulation, modern Porsches use various grades of foam and other synthetic materials. Front-engined four-door models use additional sound-deadening material on an engine cover panel. This is particularly helpful for engines with plastic intake manifolds, which do not provide as much noise insulation as aluminum manifolds.
As a purveyor of sporting machines with sensory appeal, Porsche spends a considerable amount of time tuning the induction and exhaust noises of its engines. The exhaust sound of an engine is generated by the sudden release of exhaust particles from the combustion chambers as they exit via open exhaust valves in the cylinder head(s).
The rhythmic thrum of a 356’s flat-four engine and the rasping bark of a 911’s flat six are each the result of a specific dominant frequency imparted by the number of cylinders and their arrangement. The basic determinant of an engine’s exhaust note is the number of cylinders (which determines the amount of exhaust pulses per rotation). In a four-stroke engine, each cylinder fires every other revolution, so the 356’s engine fires two cylinders for every revolution (known as “second order” events in NVH parlance), and the 911’s flat six generates three exhaust pulses per revolution (“third order” events).
The pressure waves and sounds generated by an engine’s exhaust can be converted into hertz (by dividing engine rpm by 60 to get revolutions per second) and multiplying the result by the number of exhaust pulses per revolution. Regardless of the dominant frequency, the sound emitted by the exhaust changes pitch as engine rpm increases; all of this can be expressed mathematically and plotted as perfect sine waves.
Beyond the dominant frequency of the base engine, the specific exhaust tone and pitch of a particular engine configuration are determined by a multitude of factors, including cylinder firing order, crankshaft arrangement, port size, camshaft design, valve timing, and more. These parameters are generally set by the engine designers in order to match expected power output demands and meet exhaust emissions and fuel economy standards. To fine tune the exhaust note of each engine variant, Porsche employs a team of specialists to fettle the final exhaust configuration.
If the raw exhaust note of a racing engine were the only concern of exhaust system designers and NVH engineers, their task would be greatly simplified. However, road-going cars spend a considerable time at the part-throttle and cruising ranges; a poorly-designed exhaust system can exhibit some irritating droning, buzzing, or rattles at certain engine load and rpm ranges, even if it does emit a pleasurable sound a full throttle. Instead of trial and error, Porsche’s exhaust NVH team employs scientific methods of designing exhaust systems and measuring their sound, along with the exhaust system’s effect on other aspects of the car (unintended vibrations, etc.).
Powerful simulation software programs allow Porsche’s NVH engineers to determine possible exhaust notes before any prototype engines have been built. Once such prototype engines and cars are constructed and tested, they are brought into a special acoustic chamber at the Weissach test center to evaluate the exhaust and induction sounds, which are analyzed by both finely tuned human ears and specialized sound testing equipment. Detailed digital plots of the sound spectrum of exhaust systems are pored over by engineers until the optimal balance between a pleasing exhaust note and basic NVH concerns is achieved.
The most basic means of tuning the exhaust note of a road-going car is the muffler. The internal chambers of a muffler allow the exhaust particles to expand, which smooths out their pulsations and vibrations and serves to lower the overall noise level of the exhaust system. A muffler’s effect on the exhaust’s sound waves is determined by its size, shape, the volume and configuration of its internal passage(s) and baffling.
An additional tool for quelling unwanted frequencies of both the exhaust and induction system is the Helmholtz resonator. This is named after the 19th-century German scientist, Hermann von Helmholtz, who first discovered a unique property of sound waves inside a chamber with a calibrated opening at one end.
An oft-cited example of this phenomenon is blowing across the open end of a bottle: the compression of the air molecules in the neck of the bottle causes the air inside the bottle to vibrate at a certain frequency, which emits a low whistling sound. This same property can be harnessed to tune the noise of an exhaust or induction system. When a specifically tuned Helmholtz resonator is applied to the piping of a system, unwanted frequencies are canceled out as the primary mass of intake or exhaust charge passes over the opening to the chamber.
The reintroduction of the flat-four to the 2017 718 (Type 982) series of mid-engine Porsche sports cars caused considerable consternation among Porsche engineers and customers alike. Although this engine configuration had a clear link to Porsche’s storied past, the expectations born from decades of silken flat-six soundtracks were tempered by the prospect of a Subaru-like exhaust note of the turbocharged 718.
To this end, Porsche’s engine designers and acoustics experts collaborated to develop an asymmetrical exhaust manifold, which not only enhanced exhaust flow to the single turbocharger, but it also quelled the most objectionable second-order noises of the four-cylinder engine. After passing through the turbine and catalytic converter, the 718 exhaust system splits into two branches before re-converging at the muffler. The longer branch contains a Helmholtz resonator to further refine the exhaust note. Additionally, the opening of the turbocharger’s wastegate is manipulated during certain conditions to alter the sound emitted from the tailpipes!
A trick employed across much of the Porsche range during the last 15 or so years is the installation of switchable bypass flaps in the exhaust mufflers of optional sports exhaust systems. This allows a quiet exhaust note at idle and cruising rpm, and when a predetermined rpm is reached, the engine/DME control unit signals a solenoid to open a vacuum port to the exhaust flaps, which open to enable increased exhaust flow and sound. The exhaust flaps can also be operated independently via a console-mounted switch, and are also opened when “Sport” or “Sport Plus” mode is engaged depending on the model.
Airboxes, Sound Symposers
The induction system of a four-stroke engine also contributes to noise emissions. The manic gulping sounds generated by the velocity stacks of Weber carburetors mounted directly behind the head of the driver of a modified Porsche 914-6 is perhaps the high-water mark of the induction noise experience to a Porsche enthusiast, while the unrefined, vacuum cleaner-like noises of a four-cylinder 944 with an open conical air filter element might be less desirable.
The opening and closing of the engine’s intake valves cause air to reverberate back and forth inside the intake manifold. This is because air has mass, and its inertia keeps the column of air filling any cylinder(s) on the intake stroke even after the piston has reached bottom-dead-center (BDC). When the intake valve(s) close, the air continues to stack against the closed valves until it achieves maximum compression, at which point the air deflects and goes back up the intake runner, hits the wall of the intake plenum and bounces back towards the intake ports.
As with exhaust systems, Porsche engineers spend a great deal of time and effort on the tuning of the intake system for both engine output and acoustic reasons. Engine designers calculate the effects of the pressure waves within the intake manifold and tune intake runner size and configuration to take advantage of the pressure waves to improve cylinder filling. These same pressure waves are the source of intake noise, which is often desirable to enthusiasts, but not as much to consumers of luxury cars. As the Porsche brand has expanded more into the mainstream, the company has devised clever ways to showcase the race-bred pedigree of its engines during certain conditions.
The 991-series 911 of 2012 introduced a “sound symposer,” which piped intake noise into the cabin during certain conditions. A tube is connected between the engine firewall and the intake pipe just downstream of the air filter box. Mounted in the tube is a special Helmholtz resonator chamber with a tuned rubber diaphragm inside. When a vacuum-operated switching valve is opened (via console-mounted “Sport” mode switch), intake noise is piped into the cabin, with the diaphragm amplifying the vibrations much as the human eardrum does. An additional Helmholtz resonator chamber is mounted on the 991 air filter box to amplify the intake noise between 4,500 and 6,000 rpm (a flap opens to allow airflow through this resonator).
Turbochargers alter the intake sound of an engine much as they change the exhaust note. With the development of the turbocharged 2017 991.2 Carrera engines, Porsche engineers sought to retain as much of the character of the traditional, naturally-aspirated flat six as possible by subduing the whistling noise of the turbochargers. The intact tracts to the turbocharger inlets act as dampers to reduce the vibrations of the intake air and reduce unwanted noises. A pair of sound symposers is present to pipe intake air into the cabin; only one symposer is active above 1,800 rpm in Normal mode, while Sport or Sport Plus modes activate the second sound symposer.
The aforementioned exhaust system of the 718 series did not silence many of the critics who pined for the predecessor 981’s flat-six powerplant. For the latest GTS versions of the 718 Boxster and Cayman, the exhaust system is largely unaltered, but the intake manifold was redesigned for both increased engine output and better sound. The runner size was actually reduced, but the air is allowed to pass between adjacent runners through calibrated orifices, which enables the resonant frequency of the air in the manifolds to match the intake cycles of the engine at maximum engine load and rpm.