A performance exhaust system is a characterizing element for any internal combustion conveyance. Defining the acoustic profile and influencing the powerband — exhaust design is a more dynamic science than stringing together a few pipes and tacking on some mufflers. The exhaust system of a car is one of the most commonly modified areas when a gearhead gets ahold of their ride.
We are all seeking that right sound that announces itself like a fight song for our preferred automotive demographic, and those seeking the utmost performance require tuned lengths and forms to achieve the desired power delivery.
Many misconceptions exist surrounding how exhaust systems are tuned and what terms like backpressure and scavenging really mean for performance. Hopefully with this reference you will be better equipped to devise what your specific exhaust system needs are and how to reach that destination.
Let’s Break It Down
An exhaust system is worth more than the sum of it’s parts, and each component has to be tailored to work with the next part down stream, and so on. Starting at the cylinder head — we don’t normally think of the actual exhaust port in the head as a part of the exhaust system — but nonetheless this is where it all begins. Understanding a little about cylinder head intake and exhaust runner design will help visualize what is going on after the burnt gasses leave the engine.
Runners are designed to promote unrestricted flow, while encouraging high velocities. This is the reason porting must be done with care so as not to disrupt the engineered fluid dynamics of the head. When the exhaust valve opens the expanding hot gasses rush out of the exhaust port backed by the upstroke of the piston. In OEM applications this generally means a bank of cylinders dump collectively into an exhaust manifold.
Exhaust manifolds are generally the first line of disappointment when it comes to exhaust routing. Because the cast construction has been designed for ease of production, they are generally heavy, and do not offer desirable mingling of the exhaust pulses. Though some manufacturers have improved on the unequal length manifold, they are often discarded in favor of aftermarket solutions.
The most ubiquitous of which is the “header” — the term headers really refers to the first tubular exhaust manifolds that allow exhaust evacuation from the engine. These tubes are known in the exhaust industry as primaries because they are generally followed by subsequent tubes of varying size.
Primaries run for various lengths and arrangements to achieve different desired effects — ultimately leading to secondaries which are tubes of an enlarged internal diameter such that they form a slip joint over the outside diameter of the primary. Step headers may employ many different sizes of tubing — as many as four or five between the primary and the collector. The theory of this design is to generate a progressive exhaust velocity to optimize scavenging nearest the cylinder while preventing restriction at the outlet. After the individual lengths of tube navigate their way through the engine bay they are often joined together — this fabricated union is known as a collector.
After the collectors an ideally straight length of tubing is allowed to provide some stabilization for the newly mixed swirling gasses before they enter a muffler. The gambit of muffler types, theories, and iterations is as long as the drive home after a noise pollution test failure at the track. We will go over the main competing designs and examine the merits or downfalls of each.
One of the first concerns when building or buying a performance exhaust is sizing. The tube length and diameter directly affect the way in which the final system influences the engine and exhaust note, as in all racing applications form should follow function — and be informed by it. We consulted with Vince Roman of Burns Stainless, authorities on exhaust components for high-end applications. They gave us some insight on how exhaust sizing and design can affect performance.
Tuning an exhaust system to a given application is a case-by-case basis challenge. The displacement, exhaust valve size, induction system, cam profile, exhaust port design and RPM range all factor into deciding what form the exhaust system should take. General rules of thumb are easy to grasp, but applying them correctly is where things get tricky.
A smaller diameter tube will encourage high velocity and high scavenging, translating to good throttle response and low-end through mid-range power. As the tubing diameter increases the velocity may drop depending on the engine configuration, but flow at high RPM operation will improve meaning a high peak power number. The area of a tubing cross section plays an important role in scavenging.
“When you have an exhaust header that does not have a collector, that scavenging wave hits the end of the pipe and comes back there’s an important ratio that comes into affect. The greater the area ratio, the stronger that vacuum wave is. When you have a single pipe the area ratio at the end of the pipe is infinitive because you’re opening it up to the atmosphere,” Roman illustrated.
“When you fire that same tube into a collector the area ratio becomes a finite number, and we reduce the strength of that wave. It sounds counter intuitive, but at the tuned length that wave is going to be plenty strong but when you’re off the rpm we’ve attenuated that wave such that it doesn’t hurt performance,” he continued.
The length of the primary tubes can be just as important as the tubing diameter. Think of a trombone, and how the pitch of a note changes as the slide is extended or pulled closer in. A longer primary tube will share similar characteristics to the small diameter traits, and a short length tube will be similar to a large diameter tube.
“When the exhaust valve opens you have a pressure wave that begins to travel down the tube, when it reaches the end of the tube it reverses as a vacuum wave and comes back and hits the cylinder. You want to be able to time that wave to hit right around the closing of the exhaust port, what that helps us do is scavenge residuals out of the cylinder and the intake begin to fill,” explained Roman.
It is a common practice to size the first length of primary to as close to the exhaust valve diameter as is reasonably available. This way there is no sudden drop in velocity due to a volume increase from head port to exhaust tube — after a length of at least one-foot it is then common to start stepping up the diameter. In a multi valve application, a happy medium will have to struck to accommodate flow.
A Little Theory First: Backpressure And Scavenging
The term back pressure is far and away the most misused phrase to illustrate the importance of scavenging. Scavenging is the effect generated by harnessing the inertial energy of a high velocity exhaust gas pulse. Bernoulli’s principle was the first to identify this phenomenon, and it has been applied to everything from golf balls to airplanes.
A high velocity pulse of exhaust gas carries with it energy; as the pulse moves through space it displaces the following volume behind it. This generates a low-pressure zone like a weak vacuum. Imagine this, when you are rolling down the freeway a giant semi-truck passes you going much faster — as the truck approaches from behind you are pushed to the outside of your lane by the bow wave of compressed air the truck is generating, and as the truck finally passes the opposite effect is experienced — your car is sucked toward the low-pressure area trailing behind the rig. The same principle is used in drafting across many forms of motorsport.
Scavenging effect is created by implementing an appropriately-sized exhaust system and collector combination. When executed correctly, a low-pressure area is left in the vacated cylinder, ready for the incoming intake charge. When the intake valve opens the air/fuel mixture can cram in, even before the piston begins to travel toward bottom dead center (BDC) — this generates a very mild forced induction effect and ultimately improves horsepower and torque.
To generate the most aggressive scavenging effect is a delicate balance, one heavily driven by camshaft lobe separation (generally narrower equals more because increased valve overlap leaves the exhaust valve slightly open, drawing this vacuum while simultaneously the intake valve is open), and exhaust sizing.
The goal is to generate the highest possible exhaust gas velocities while maintaining best possible flow — these two priorities are opposed from each other when it comes to tubing size, so finding the common ground is a matter of examining one’s priorities.
Back pressure is a term that misleads many into thinking it is a beneficial characteristic, that somehow their engine needs back pressure to operate correctly. The misunderstanding comes into play as we seek to increase exhaust gas velocity by restricting tubing diameter — restriction, i.e. back pressure may be a byproduct or symptom, but is not the goal. A restricted exhaust system is nothing but a hinderance. After all an engine is just an air pump the more air and fuel we can force through it the more power it will produce.
Collector Types And Characteristics
Collectors are the mixing chambers of an exhaust system, this common plenum allows the engine builder to capitalize on the delicate camshaft selection, tactful head porting, and other parameters laid out for the build. Collectors manifest in a multitude of variations and manufacturing techniques. The two main types are formed and merge.
Formed collectors are found more often on budget-minded systems and are comprised of a piece of hydroformed sheetmetal made to accommodate the terminal ends of the primaries. Merge collectors are fabricated from tubing elbows milled from two angles to create a seam along which they can be joined into two, three, four, five, six, or eight tube variations.
The idea of a collector is to allow one cylinder to benefit from the exhaust gas velocity of an adjacent cylinder. Unlike zoomies where each exhaust tube is independent, a collected exhaust has distinct advantages and sound differences. In a properly engineered exhaust system the primary tubes are clocked in the collector to orient the firing order in a circular pattern. The pulse pattern generates a swirling effect which further improves scavenging.
The most common collector arrangements are commonly referred to as four-into-one, and tri-Y. Utilized most commonly on four- and eight-cylinder engines, these layouts offer distinctly different traits. You may hear the power delivery of an engine described as peaky, exponential, or linear and flat — these identities are influenced by collector choice. To visualize the difference keep in the back of your mind the concept of a vacuum signal.
“The merge angle of collectors is something that comes up quite a bit, our standard collector is 15 degrees — we’ve found through our research and testing that anywhere from 7-15 degree merge angle gives you about the same performance. Less than seven degrees and collectors become too long and you get drag losses, steeper than 15 degrees and you start to turn the flow too much,” Roman concluded.
The exhaust primaries are arranged in consecutive firing order around the collector such that one pulse is strengthened by the previous and so on. This power delivery can be akin to a two-stroke where the engine needs to be kept ‘on the pipe,’ or in the narrow powerband to be driven effectively.
A tri-Y collector arrangement utilizes three simple two-into-one collectors pairing down to one outlet. Since these collectors are paired off, the whole bank of cylinders does not see the pulse signals from other cylinders at the same time — instead the scavenging pulses are weaker, but sent more frequently. This generates a flatter power delivery and generally lower peak power but a more useable delivery.
Selecting what collector arrangement is best suited to your engine combination can be confusing and a moving target when it comes to tuning.
“Dealing with 4-cylinder, and flat plane crank V8s, we find that because the pulses are relatively far apart in the collector a four-into-one and a tri-y are about the same, we can optimize both. When we’re talking about cross-plane crank engines where you’ve got two consecutive cylinders firing on one side, we find that theoretically a tri-y can result in better performance because we can separate those pulses out at the first collector,” Roman recalled.
There are countless variations on these themes, from 180-headers made famous by the GT40, to six- and eight-into-one collectors that generate howling exhaust notes from otherwise rough and tumble domestic powerplants.
Mufflers are a necessary evil in the eyes of most speed freaks, but they need not be a hinderance to performance. OEM style mufflers generally reduce noise by forcing the exhaust gases to navigate a maze of chambers, perforated tubing, and tight bends relying mostly on the slowing of exhaust gases to achieve their goal.
The aftermarket for mufflers has exploded over the last generation of enthusiasts who demand options and choices like no previous consumer. The easiest way to categorize muffler designs is by the methodology employed to dampen sound. The most commonly found schools of thought are packed mufflers, chambered mufflers, reflective sound technology, and tunable disc.
In the heyday of hot rodding there was only one option between stock exhaust and wide open, which was glass pack. Packed mufflers are generally a straight-through design — an inner perforated or louvered tube is wrapped with a synthetic material like fiberglass matt and then encased by an outer can. These mufflers rely on the insulative quality of the packing to dampen out the noise, but are known to deteriorate over time.
Chambered mufflers are rarely straight-through, and usually direct the exhaust gases around baffles or plates welded internally to the muffler can. The classic ’70s musclecar sound is characterized by the chambered muffler, and they are available in a multitude of sizes and noise output levels.
Reflective sound technology mufflers are some of the newest on the market. Borrowing noise-canceling technology like you might have in your headphones, these mufflers are tuned for a specific engine and pit the specific wavelength and amplitude of the undesirable sound waves against each other. This cancellation effect means that the internals of the muffler offer the least restriction to the exhaust gas flow, effectively a straight pipe with few holes.
The last muffler design is one of the most simple and unique, rarely finding it’s way outside racing circles the tunable disc type muffler has been pioneered by one company — Supertrapp. These systems are modular and employ a stack or steel discs formed to nest with each other but allow gasses to escape between them. Discs can be added or subtracted from the stack to change the noise level and influence the mixture.
Most often found in applications where some noise attenuation is required but no maximum decibel level is established, these disc-type spark arrestors are common throughout the off-road community, and even among top road racing teams like Flying Lizard, as seen on their Audi R8.
If there is one thing taken away from this brief surface introduction to fluid dynamics and the exhaust system is that it’s all about balancing velocity against flow. Maximizing both will help your carefully selected package perform at its best. Assemble a system that meets your needs, and is appropriate for your powerplant. Huge diameter tubes might look cool, but anyone who knows the ins and outs of exhaust design will call you out on the choice.