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What Affects the Exhaust Note?
Today, I enumerate all main contributing factors and explain as concisely as possible how each affects the exhaust note; I also discuss how some factors may be modified.
Internal combustion vehicles each have their own exhaust notes. It is almost akin to a fingerprint, because exhaust note is the collective result of the entire power system. Today, I enumerate all main contributing factors and explain as concisely as possible how each affects the exhaust note; I also discuss how some factors may be modified.
Engine
The internal combustion engine is the source of the exhaust sound. It determines the raw acoustic signature that the downstream setup modifies.
- Engine configuration: Cylinder count, grouping, and bank angle determine the firing frequency and the spacing of exhaust pulses, forming the fundamental determinant of exhaust note. It is virtually impossible to replicate the exhaust note of a different engine configuration fully. The higher the cylinder count, the higher the primary exhaust frequency at each RPM.
- Bore and stroke: Per cylinder displacement affects exhaust pulse energy. A higher displacement results in a fuller, deeper sound with stronger pulses. Therefore, a smaller engine of the same configuration will sound thinner. The bore/stroke ratio also affects exahust gas dynamics. Usually, larger bore means larger valves, which release exhaust gas more forcefully, giving it a crisper character that makes exhaust pulses more distinct. Shorter stroke means higher RPM ceiling, which can affect the exhaust note at the high end. Overall, a larger engine with higher bore/stroke ratio will sound richer, slightly more aggressive, while a smaller engine with a lower bore/stroke ratio will sound more tonal but weaker.
- Crankshaft design: Specifically with inline-4, V4, and V8 engines, both flat-plane and cross-plane crankshafts are possible for the same engine configuration. They greatly affect the firing order and the spacing of exhaust pulses at the collector. For V8s, which are the most common, cross-plane will result in uneven per-bank firing that fundamentally change the exhaust pulses at the collectors. It will sound less smooth than a flat-plane V8, with uneven harmonics mixed with even-order harmonics, while flat-plane will sound more tonal, with only even-, higher-order harmonics. This results in higher pitch in the flat-plane V8. If we consider crankshaft design with bore and stroke, it becomes clear that a cross-plane V8 sounds better with a larger displacement, while a flat-plane V8 sounds better with a smaller displacement. In both cases, a higher bore/stroke ratio is beneficial.
- Compression ratio: A higher compression ratio means higher peak cylinder pressure, which results in a more forceful exhaust pulse. The result is richer, deeper sound that is crisper as well. They make individual pulses more distinct, which adds tunability downstream. A higher compression ratio usually results in a sportier, energetic exhaust note, while a lower compression ratio makes the engine sound smoother.
- Valve lift: Higher exhaust valve lift works similarly to higher compression ratio or larger valve diameter. Intake valve lift has similar effects but works indirectly and is less noticeable.
- Valve timing: Valve overlap and exhaust valve timing greatly modify the engine’s exhaust sound. Extra valve overlap helps exhaust scavenging and can increase the velocity of the exhaust gas. Too much overlap can let fresh intake air directly into the exhaust and, in the case of port injection, let unburnt fuel in---these negatively affect the fuel trim and result in extra noises that mask the engine’s exhaust note. Opening the exhaust valve earlier allows exhaust gas to escape closer to peak cylinder pressure, resulting in deeper, crisper sound. This however reduces thermal efficiency and is only used naturally at high RPM. Since VVT is virtually standard in today’s engines, valve timing is a key factor that can be tuned easily to alter a vehicle’s exhaust note.
- Fuel injection: Port injection (PFI) allows air and fuel to mix better and the burn is more progressive, resulting in a smoother exhaust note. Direct injection (DI) is the opposite, resulting in higher-pitched, slightly harsher exhaust note.
- Supercharger: A supercharger increases the effective compression ratio, increases the exhaust volume at each RPM, and makes exhaust gas pass through the exhaust valves more forcefully. It can make an engine sound like one of larger displacement, higher bore/stroke ratio, higher compression ratio, and higher valve lift. Superchargers make a distinct noise themselves that is loved by many, but this is not in the exhaust.
Overall, in my opinion, the sweet spots can be found in (i) 60-deg NA V12, large displacement, high bore/stroke ratio, PFI, moderate--high compression ratio, moderate valve lift---classic Jaguar and Lamborghini V12s, (ii) 90-deg cross-plane NA V8, large displacement, high bore/stroke ratio, PFI, moderate compression ratio, moderate valve lift---many oldies, and (iii) 90-deg flat-plane NA V8, small displacement, high bore/stroke ratio, PFI, high compression ratio, high valve lift---Ferrari Dino. An inline-6 can be made to sound similar to a V12, as an inline-4 to a flat-plane V8, but they are rarely so engineered because power and efficiency are prioritized over exhaust note.
Exhaust Collectors & Crossovers
After the engine produces sound in each cylinder, the sounds are almost always merged in the exhaust system in passenger vehicles. This merge is the biggest factor that modifies the system’s exhaust note. In other words, no amount of resonator or muffler mods can bring as much change as this upstream section.
- Exhaust manifold: Most vehicles today use exhaust manifolds that progressively merge all cylinders front to back. This merge disregards the engine’s firing order and disturbs exhaust scavenging. It creates uneven exhaust gas temperatures across cylinders. Since most manifolds use thick iron casting, they muddy the exhaust note, too. Exhaust manifolds therefore greatly reduces the engine’s character, which many manufacturers want to hide.
- Header: Headers address the problems of exhaust manifolds by improving pulse merging and exhaust scavenging. Equal-length (EL) headers and unequal-length (UEL) alter the timing and spacing of exhaust pulses as they merge into the collector. EL produces a smoother, more refined, higher-pitched exhaust note, while UEL sounds deeper, more aggressive. Long-tube headers optimize exhaust scavenging and make the sound more aggressive and mechanical, while short headers offer a middle ground between long headers and exhaust manifold.
- Multi-stage collectors: In larger engines such as V12s, manufacturers often use a two-stage merge, especially in combination with catalytic converters in-between the collectors. Unfortunately, this can make the V12 sound like 4 inline-3s if not done well. This is the main reason why the German V12 sedans cannot reproduce the exhaust of Italian exotics.
- Crossovers: In engines with more than one collector, there is usually some type of crossover downstream. They can equalize pressures, facilitate exhaust scavenging, and filter out harshness. An X-pipe lets the exhausts blend smoothly and creates a scavenging effect. It usually sounds smoother but slightly higher-pitched. An H-pipe equalizes pressures between the two banks but allows for less blending. It usually reduces drone. A Y-pipe is akin to introducing another collector downstream. True dual exhausts have no crossover and are more raw and mechanical.
Overall, the best setup can be found in (i) V12 with equal-length, long-tube headers in a 6--to--1 merge with a single X-pipe---Ferrari F140, Lamborghini. (ii) Cross-plane V8 with UEL, short headers with a single H-pipe---muscle car.
Exhaust Restrictions
While the engine produces exhaust sound, what the exhaust pulses must pass through afterwards greatly modify the acoustic characters of the system. Exhaust restrictions muffle the sound in general, but the effects differ.
- Turbocharger: Turbocharger are broad-spectrum mufflers that greatly reduce the exhaust energy, resulting in a muddier, weaker, lazier exhaust sound that is very much quieter. In some modern vehicles, even straight-piping the exhaust system after the turbocharger does not produce abnoxious levels of noise. At the same time, turbochargers introduce their own distinct turbo noises. In some custom setups, this can be very obnoxious.
- Catalytic converter: Catalytic converters are also broad-spectrum mufflers that reduce the exhaust energy, but less significant than turbochargers. The tiny honeycombs inside the catalytic converters can also filter out high-pitched noises by breaking them up, making the exhaust note slightly smoother and less mechanical.
Overall, in my opinion, the sweet spot is without exhaust restrictions purely in terms of exhaust sound engineering. This is why modern vehicles cannot reproduce the sounds of the classics I listed above.
Downstream Exhaust System
Most of what is usually considered an ‘exhaust system’ sits way back and can only modify what is already there. They can filter out unwanted frequencies and reduce the exhaust volume, but there is little that can be done to introduce pleasant sounds. It is possible to introduce reverberations and resonances, but those are usually disliked. The items below include also items that live more upstream.
- Pipe sizing: Wider pipes slightly favor lower frequencies than narrower ones, making the exhaust sound deeper and smoother. It works in conjunction with thicker pipes, though weight must be a consideration.
- Pipe material: Different materials reflect and absorb sound differently. Although this is intimately linked to the shape and dimensions. Stainless steel is usually a good balance that dampen high-pitched noises and provide a deeper, smoother noise. Titanium sounds raspy and mechanical and may not suit everyone. Aluminum and steel each have their own characters, too.
- Bends and shapes: Unusual shapes and sharp bends tint the exhaust sound by introducing resonances. They are not ideal.
- Resonator: Resonators selectively filter out unwanted frequencies. They can reduce high-frequency rasp and sometimes drone to make the exhaust sound smoother, deeper, and more refined. It is usually unwise to delete resonators.
- Muffler: Different types of mufflers work differently, but they usually do not muffle all frequencies to the same extent. They usually cannot eliminate unwanted noises that already exist, and when used incorrectly they can introduce drone and rasp (but will sound quieter).
Overall, usual shapes and sharp bends should be avoided. Stainless steel is often sufficient. Properly sizing the exhaust piping is important, but excessive sizing adds little. The exact routing, resonator, and muffler setup needs engineering and experience; there is no general rule of good-or-bad. Unrefined, loud, and thin noises can be obnoxious and usually does not bring the attention that the owner hopes for. Modifying the exhaust system by deleting the resonator is often counterproductive.
Gallery: Exhaust Note
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