An Exhaust Education
11-21-2010, 09:03 PM
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An Exhaust Education
So how do you pick the right exhaust for your ride? To answer that question, we talked to two industry-leading exhaust system and muffler manufacturers – Cherry Bomb and Flowmaster. Both companies have a unique take on the science and technology of sound suppression and exhaust design, but there are lessons you can take away regardless of who’s system you’re considering.
Size Matters
The first question you need to ask is, “What diameter tubing should I look for?” If bigger was always better, then the answer would be simple – just cram in the biggest tube that will fit between the floorboard and the pavement. But it’s not that easy, and there are several things to consider. While a larger pipe has a greater ultimate flow potential, size isn’t always an advantage.
“When it comes to exhaust, you want velocity,” explains Cherry Bomb’s Matt Graves. “If you get too big, the air gets lazy and turbulent – this will create restriction.” Dale Dotson, Motorsports Manager for Flowmaster, adds, “Running too large a diameter of exhaust tubing allows the exhaust gas pulses to expand, losing the valuable velocity that assists in carrying the spent gases out through the system. The remaining exhaust gases in the system can then contaminate the fresh, incoming fuel air charge that the engine needs for peak performance. This especially impacts torque levels at low to mid range RPM, which can be very noticeable because after all, torque is what moves the vehicle. Other negative effects will be a loss of fuel mileage due to torque losses and increased and excessive noise levels, both inside and outside the vehicle.”
It helps to picture the way a river flows – fast where it’s narrow, slow and meandering with stagnant pools where it widens out. Exhaust gas has mass and inertia, and a properly-sized exhaust system takes advantage of that fact to keep the energy of the stream high. “A common misconception is when people say you need some backpressure,” says Graves. “This is absolutely not true. It’s all about scavenging and velocity.”
It’s a horrible, overused cliché to say that an engine is an air pump, but there’s a grain of truth in that when it comes to sizing an exhaust system. Running boost or nitrous oxide will require more exhaust flow, just like more naturally-aspirated horsepower will, but since you’ve got positive pressure on the intake side, it’s not a linear relationship. “A belt driven supercharger pressurizes the cylinders, and helps force the exhaust out of the cylinders during overlap of the valves (the time both valves are open at the same time),” Dotson explains. “A turbocharged engine is a little more susceptible to exhaust backpressure at lower RPM than a supercharged engine. Larger tubing sizes will allow the turbo to “spool up” faster; however, at higher RPM the turbo still pressurizes the cylinders to help force exhaust out during overlap, more so than a naturally aspirated engine.”
Tubing Tips:
“I recommend larger tubing for anything that has forced induction,” says Cherry Bomb’s Graves. “You are exponentially adding air to the engine, and you need the plumbing to get it out, especially on turbos.” Exhaust sizing is particularly important for turbo applications, because the efficiency of the turbine section is directly related to the pressure differential between the inlet and outlet. It’s one situation where it’s hard to go “too big” – the turbocharger has already extracted a lot of energy from the exhaust flow, so the goal should be to provide an easy exit route for large volumes of somewhat lazy spent gasses
With that special case in mind, it is possible to make some general recommendations, though. “We base it on engine size and power,” Graves explains. “A good rule of thumb is a 2.25-inch dual for 150 hp up to 250 hp (200-250 cid), 2.5-inch dual for 250 hp to 400 hp (250 cid to 400 cid), 3-inch dual for 400 hp to 500 hp (450 cid to 500 cid), and 3.5-inch dual for 550 hp to 700 hp (500 cid-plus). Start small and work your way up in size. You may have to experiment to find the right balance.” It’s also worth pointing out that tubing diameter has an effect on the sound of the system, regardless of the mufflers used. Dotson explains, “The larger the internal volume, i.e. tubing size, the more sound frequencies can pass through the exhaust system, making it louder. Generally, larger tubing diameter also emits a lower, deeper tone. Think of it as the difference in yelling in a tunnel versus though a hose.”
X (or H) Marks the Spot
Another hot topic in exhaust system design is the debate between H and X style crossovers. For the uninitiated, a crossover links the two sides of a dual exhaust system. The firing order of most V8’s puts sequential pressure pulses on the same side of the car, and a crossover will generally help even out the flow. This is advantageous not only for making power, but also preventing your exhaust from sounding like a UPS truck. “Both H and X crossovers have multiple benefits, and should be considered a mandatory feature of a properly designed exhaust system,” Dotson states. In theory, an H-pipe merely balances pressure, with pulses traveling back and forth across the crossover without much actual flow between sides, while an X merges the exhaust stream from each side before diverging again.
“An X pipe will generally shows gains over an H pipe on smaller cubic inch engines, or engines with restricted intake inlets,” says Dotson . “As the exhaust pulses travel past the common area of the X, it creates a vacuum on the opposite side of tubing and actually helps scavenge the exhaust through the X with both sides feeding off each the other. At the same time, when the sound frequencies meet at the X, there’s a cancellation of sound frequencies (generally the lower frequencies) and it gives it a higher pitched tone while helping reduce the overall sound level.”
“A traditional H pipe or balance tube also relieves the pressures that build up,” Dotson continues. “An H pipe will generally provide an increase of horsepower and torque on most engines regardless of cubic inches. The exhaust gases don’t flow through the H pipe, but the sound frequencies do, and the cancellation that takes place creates a deeper, smoother tone than the X design, and also eliminates back rap on deceleration.”
In practice, it’s harder to quantify whether an X is really “better” than an H. “Here is my theory,” expounds Graves. “It’s mostly installation preference. If your pipes are too far away to merge into an X pipe, run an H-pipe. As far as any power differences, I have never seen one – at least not anything really measurable. The main thing these do is balance the pulses so it sounds better. It can take away the deceleration pop in some cases.”
When it comes to locating the H or X in the system, Graves explains, “You want the crossover to be as close to the engine as possible. When the exhaust is hot, it expands really fast and that’s when you want to get those banks together. When it cools down its expansion rate has slowed.” To determine the right place, Graves has a clever technique. “A good trick when you have unbalanced pipes on a car and you want to install a crossover is to take a crayon or spray paint and start at the header, making a line back toward the muffler. Run the car to warm it up. Where the flow slows down the most, you’ll see the mark stop melting or burning – that’s the best spot for the crossover.”
Silence is Golden – Choosing the Right Muffler
An open exhaust is fun in small doses, but unless it’s a track-only car, your ride is going to need a muffler of some sort as part of the exhaust system. In some cases, even dedicated race cars need mufflers to meet class rules or track decibel limits. Whatever the case, flow, tone, size, and weight are all considerations. Though clever design and materials have made smaller, quieter mufflers possible, Graves advises, “The longer or bigger the body is on the muffler, the quieter it will be. Most muffler sound control works off volume tuning. The larger the internal surface area, the greater the sound control.”
Muffler Tips:
Know the difference between absorption, chambered, and hybrid designs
Case volume is directly related to sound reduction potential
Material used is important – Choose stainless, aluminized steel, or powdercoated for rust resistance
Beyond size, the materials used and the internal design also affect the volume, tone, and flow of a muffler. Factory mufflers are designed with low cost, big decibel reduction, and long service life as primary goals, and the result is that the standard “multi-pass” OEM muffler leaves a lot of room for improvement. “A typical factory style muffler uses a combination of reduced-diameter perforated tubing and dead end chambers, which force exhaust through small tubing and/or small holes into other common chambers, which cancels sound,” explains Dotson. “The inherent restriction of these designs is effective at attaining the vehicle sound requirements, but retains heat in both the exhaust system and engine, and results in a loss of power and mileage.”
There are two main strategies used in aftermarket mufflers to control sound – absorption and cancellation – and most use one or the other, or a combination of the two. A good example of a pure absorption design is the iconic glasspack, where a straight-through louvered or perforated core and temperature resistant fiberglass packing combine to moderate sound. “Chambered” mufflers like the classic Flowmaster 40-series use a series of baffles and compartments to redirect and reflect sound energy, with no packing materials involved.
“Flowmaster’s patented chambered technology uses the sound energy to help both scavenge exhaust and cancel sound frequencies through its free flowing design,” Dotson claims. “This design incorporates a “V” deflector that splits exhaust gases and sound frequencies as they enter the muffler, then redirects them through a collector which produces a low pressure area behind the deflector. This low pressure area aids in not only scavenging the remaining pulses from the combustion chamber down through the muffler, but at the same time cancels the same like frequencies as they are rejoined behind the deflector. Flowmaster also has models available with a built in “tuning chambers” to reduce and control specific frequencies, controlling interior resonance inside the cab area of vehicle without affecting vehicle performance.”
Hybrid designs use both methods to tame noise. “Muffler design would take a lot more time to explain, but if you’re going for all-out horsepower you want a straight-through design,” Graves explains. “If you are looking for quiet and power, we offer the Vortex muffler which as the best of both worlds. It can support up to 800 hp with nice deep sound.”
Once again, a special case is turbo engines. “If your application is turbocharged then that helps,” says Graves. “Turbos are great natural mufflers, so you won’t need much on those – keep this in mind.” Because a turbocharger is powered by exhaust energy and that energy is part of what makes the exhaust loud, having a “whirly muffler” on your engine will help mute it as well. In this case, a free-flowing muffler is appropriate, mainly to reduce high frequency sounds rather than lower the overall volume.
“Obviously if you’re making insane power, there is not much a muffler can do past a certain point,” Graves admits. Still, there are full-race mufflers available that do more than just meet the ‘letter of the law’ standard some rulebooks have in terms of requiring a muffler of some sort. With many tracks under pressure from nearby development, everyone has to do their part to control noise whenever possible, and having an effective set of mufflers on your Mustang is a good way to help ensure there will still be places to race in the future. “A properly designed exhaust system, along with a properly designed engine combination, will generally net you the same performance both off and on the track,” Dotson concludes.
Tailpipe or Turndown?
Once the exhaust makes it past the mufflers, the question becomes, “To dump or not to dump?” The lightest and least expensive option is to simply end the exhaust at the muffler outlet, or put on turndowns, but again, class rules may require outlets behind the rear axle, and the sound may not be to your personal taste. Per Graves, “Tailpipes and turndowns mostly only affect the sound. The longer the exhaust system the quieter it will be. This does not mean that tail pipes will make less power. If you have a good mandrel bent tailpipe system you’ll make just as much power as the turndowns with less headaches. Remember, the turn downs are doubling your exhaust sound since they reflect it back up from the road. They are cool for drag racing but I hate them for street.”
Dotson has an even stronger opinion, stating, “Flowmaster doesn’t recommend running turndowns for a number of reasons, with the main one being a safety issue. With turndowns exiting under the car, the exhaust gases can get inside the interior of the car and poison the occupants while breathing the toxic fumes. Also, there is a dramatic increase in interior resonance inside with turn downs under the vehicle. It turns the rear-end cavity of the car into a giant resonant sound chamber, transmitting a loud drone into the passenger compartment along with higher sound levels outside the vehicle.”
Hopefully, we’ve made the decision on which exhaust is right for your car a little easier to make, but here’s a little parting advice from Flowmaster’s Dotson. “It’s really important to put some thought into it and to be honest with yourself about what you really are looking for. Too many times people think they want a loud “race” system, only to realize soon after that the noise can become very annoying on a day-in day-out basis,” he explains. “An exhaust system is just another important engine component that needs to be considered at the engine building stage, not an afterthought when everything else is done. It’s best to call the exhaust manufacturers’ tech lines and ask them what muffler and exhaust system is right for your combination and wants/needs. Nobody will know their product better than them.”
Size Matters
The first question you need to ask is, “What diameter tubing should I look for?” If bigger was always better, then the answer would be simple – just cram in the biggest tube that will fit between the floorboard and the pavement. But it’s not that easy, and there are several things to consider. While a larger pipe has a greater ultimate flow potential, size isn’t always an advantage.
“When it comes to exhaust, you want velocity,” explains Cherry Bomb’s Matt Graves. “If you get too big, the air gets lazy and turbulent – this will create restriction.” Dale Dotson, Motorsports Manager for Flowmaster, adds, “Running too large a diameter of exhaust tubing allows the exhaust gas pulses to expand, losing the valuable velocity that assists in carrying the spent gases out through the system. The remaining exhaust gases in the system can then contaminate the fresh, incoming fuel air charge that the engine needs for peak performance. This especially impacts torque levels at low to mid range RPM, which can be very noticeable because after all, torque is what moves the vehicle. Other negative effects will be a loss of fuel mileage due to torque losses and increased and excessive noise levels, both inside and outside the vehicle.”
It helps to picture the way a river flows – fast where it’s narrow, slow and meandering with stagnant pools where it widens out. Exhaust gas has mass and inertia, and a properly-sized exhaust system takes advantage of that fact to keep the energy of the stream high. “A common misconception is when people say you need some backpressure,” says Graves. “This is absolutely not true. It’s all about scavenging and velocity.”
It’s a horrible, overused cliché to say that an engine is an air pump, but there’s a grain of truth in that when it comes to sizing an exhaust system. Running boost or nitrous oxide will require more exhaust flow, just like more naturally-aspirated horsepower will, but since you’ve got positive pressure on the intake side, it’s not a linear relationship. “A belt driven supercharger pressurizes the cylinders, and helps force the exhaust out of the cylinders during overlap of the valves (the time both valves are open at the same time),” Dotson explains. “A turbocharged engine is a little more susceptible to exhaust backpressure at lower RPM than a supercharged engine. Larger tubing sizes will allow the turbo to “spool up” faster; however, at higher RPM the turbo still pressurizes the cylinders to help force exhaust out during overlap, more so than a naturally aspirated engine.”
Tubing Tips:
- Choose mandrel-bent tubing over crush-bent to minimize restriction
- Use an appropriate diameter – too big can be as bad as too small
- Keep bends to a minimum
- Transitions in diameter create turbulence and backpressure
“I recommend larger tubing for anything that has forced induction,” says Cherry Bomb’s Graves. “You are exponentially adding air to the engine, and you need the plumbing to get it out, especially on turbos.” Exhaust sizing is particularly important for turbo applications, because the efficiency of the turbine section is directly related to the pressure differential between the inlet and outlet. It’s one situation where it’s hard to go “too big” – the turbocharger has already extracted a lot of energy from the exhaust flow, so the goal should be to provide an easy exit route for large volumes of somewhat lazy spent gasses
With that special case in mind, it is possible to make some general recommendations, though. “We base it on engine size and power,” Graves explains. “A good rule of thumb is a 2.25-inch dual for 150 hp up to 250 hp (200-250 cid), 2.5-inch dual for 250 hp to 400 hp (250 cid to 400 cid), 3-inch dual for 400 hp to 500 hp (450 cid to 500 cid), and 3.5-inch dual for 550 hp to 700 hp (500 cid-plus). Start small and work your way up in size. You may have to experiment to find the right balance.” It’s also worth pointing out that tubing diameter has an effect on the sound of the system, regardless of the mufflers used. Dotson explains, “The larger the internal volume, i.e. tubing size, the more sound frequencies can pass through the exhaust system, making it louder. Generally, larger tubing diameter also emits a lower, deeper tone. Think of it as the difference in yelling in a tunnel versus though a hose.”
X (or H) Marks the Spot
Another hot topic in exhaust system design is the debate between H and X style crossovers. For the uninitiated, a crossover links the two sides of a dual exhaust system. The firing order of most V8’s puts sequential pressure pulses on the same side of the car, and a crossover will generally help even out the flow. This is advantageous not only for making power, but also preventing your exhaust from sounding like a UPS truck. “Both H and X crossovers have multiple benefits, and should be considered a mandatory feature of a properly designed exhaust system,” Dotson states. In theory, an H-pipe merely balances pressure, with pulses traveling back and forth across the crossover without much actual flow between sides, while an X merges the exhaust stream from each side before diverging again.
“An X pipe will generally shows gains over an H pipe on smaller cubic inch engines, or engines with restricted intake inlets,” says Dotson . “As the exhaust pulses travel past the common area of the X, it creates a vacuum on the opposite side of tubing and actually helps scavenge the exhaust through the X with both sides feeding off each the other. At the same time, when the sound frequencies meet at the X, there’s a cancellation of sound frequencies (generally the lower frequencies) and it gives it a higher pitched tone while helping reduce the overall sound level.”
“A traditional H pipe or balance tube also relieves the pressures that build up,” Dotson continues. “An H pipe will generally provide an increase of horsepower and torque on most engines regardless of cubic inches. The exhaust gases don’t flow through the H pipe, but the sound frequencies do, and the cancellation that takes place creates a deeper, smoother tone than the X design, and also eliminates back rap on deceleration.”
In practice, it’s harder to quantify whether an X is really “better” than an H. “Here is my theory,” expounds Graves. “It’s mostly installation preference. If your pipes are too far away to merge into an X pipe, run an H-pipe. As far as any power differences, I have never seen one – at least not anything really measurable. The main thing these do is balance the pulses so it sounds better. It can take away the deceleration pop in some cases.”
When it comes to locating the H or X in the system, Graves explains, “You want the crossover to be as close to the engine as possible. When the exhaust is hot, it expands really fast and that’s when you want to get those banks together. When it cools down its expansion rate has slowed.” To determine the right place, Graves has a clever technique. “A good trick when you have unbalanced pipes on a car and you want to install a crossover is to take a crayon or spray paint and start at the header, making a line back toward the muffler. Run the car to warm it up. Where the flow slows down the most, you’ll see the mark stop melting or burning – that’s the best spot for the crossover.”
Silence is Golden – Choosing the Right Muffler
An open exhaust is fun in small doses, but unless it’s a track-only car, your ride is going to need a muffler of some sort as part of the exhaust system. In some cases, even dedicated race cars need mufflers to meet class rules or track decibel limits. Whatever the case, flow, tone, size, and weight are all considerations. Though clever design and materials have made smaller, quieter mufflers possible, Graves advises, “The longer or bigger the body is on the muffler, the quieter it will be. Most muffler sound control works off volume tuning. The larger the internal surface area, the greater the sound control.”
Muffler Tips:
Know the difference between absorption, chambered, and hybrid designs
Case volume is directly related to sound reduction potential
Material used is important – Choose stainless, aluminized steel, or powdercoated for rust resistance
Beyond size, the materials used and the internal design also affect the volume, tone, and flow of a muffler. Factory mufflers are designed with low cost, big decibel reduction, and long service life as primary goals, and the result is that the standard “multi-pass” OEM muffler leaves a lot of room for improvement. “A typical factory style muffler uses a combination of reduced-diameter perforated tubing and dead end chambers, which force exhaust through small tubing and/or small holes into other common chambers, which cancels sound,” explains Dotson. “The inherent restriction of these designs is effective at attaining the vehicle sound requirements, but retains heat in both the exhaust system and engine, and results in a loss of power and mileage.”
There are two main strategies used in aftermarket mufflers to control sound – absorption and cancellation – and most use one or the other, or a combination of the two. A good example of a pure absorption design is the iconic glasspack, where a straight-through louvered or perforated core and temperature resistant fiberglass packing combine to moderate sound. “Chambered” mufflers like the classic Flowmaster 40-series use a series of baffles and compartments to redirect and reflect sound energy, with no packing materials involved.
“Flowmaster’s patented chambered technology uses the sound energy to help both scavenge exhaust and cancel sound frequencies through its free flowing design,” Dotson claims. “This design incorporates a “V” deflector that splits exhaust gases and sound frequencies as they enter the muffler, then redirects them through a collector which produces a low pressure area behind the deflector. This low pressure area aids in not only scavenging the remaining pulses from the combustion chamber down through the muffler, but at the same time cancels the same like frequencies as they are rejoined behind the deflector. Flowmaster also has models available with a built in “tuning chambers” to reduce and control specific frequencies, controlling interior resonance inside the cab area of vehicle without affecting vehicle performance.”
Hybrid designs use both methods to tame noise. “Muffler design would take a lot more time to explain, but if you’re going for all-out horsepower you want a straight-through design,” Graves explains. “If you are looking for quiet and power, we offer the Vortex muffler which as the best of both worlds. It can support up to 800 hp with nice deep sound.”
Once again, a special case is turbo engines. “If your application is turbocharged then that helps,” says Graves. “Turbos are great natural mufflers, so you won’t need much on those – keep this in mind.” Because a turbocharger is powered by exhaust energy and that energy is part of what makes the exhaust loud, having a “whirly muffler” on your engine will help mute it as well. In this case, a free-flowing muffler is appropriate, mainly to reduce high frequency sounds rather than lower the overall volume.
“Obviously if you’re making insane power, there is not much a muffler can do past a certain point,” Graves admits. Still, there are full-race mufflers available that do more than just meet the ‘letter of the law’ standard some rulebooks have in terms of requiring a muffler of some sort. With many tracks under pressure from nearby development, everyone has to do their part to control noise whenever possible, and having an effective set of mufflers on your Mustang is a good way to help ensure there will still be places to race in the future. “A properly designed exhaust system, along with a properly designed engine combination, will generally net you the same performance both off and on the track,” Dotson concludes.
Tailpipe or Turndown?
Once the exhaust makes it past the mufflers, the question becomes, “To dump or not to dump?” The lightest and least expensive option is to simply end the exhaust at the muffler outlet, or put on turndowns, but again, class rules may require outlets behind the rear axle, and the sound may not be to your personal taste. Per Graves, “Tailpipes and turndowns mostly only affect the sound. The longer the exhaust system the quieter it will be. This does not mean that tail pipes will make less power. If you have a good mandrel bent tailpipe system you’ll make just as much power as the turndowns with less headaches. Remember, the turn downs are doubling your exhaust sound since they reflect it back up from the road. They are cool for drag racing but I hate them for street.”
Dotson has an even stronger opinion, stating, “Flowmaster doesn’t recommend running turndowns for a number of reasons, with the main one being a safety issue. With turndowns exiting under the car, the exhaust gases can get inside the interior of the car and poison the occupants while breathing the toxic fumes. Also, there is a dramatic increase in interior resonance inside with turn downs under the vehicle. It turns the rear-end cavity of the car into a giant resonant sound chamber, transmitting a loud drone into the passenger compartment along with higher sound levels outside the vehicle.”
Hopefully, we’ve made the decision on which exhaust is right for your car a little easier to make, but here’s a little parting advice from Flowmaster’s Dotson. “It’s really important to put some thought into it and to be honest with yourself about what you really are looking for. Too many times people think they want a loud “race” system, only to realize soon after that the noise can become very annoying on a day-in day-out basis,” he explains. “An exhaust system is just another important engine component that needs to be considered at the engine building stage, not an afterthought when everything else is done. It’s best to call the exhaust manufacturers’ tech lines and ask them what muffler and exhaust system is right for your combination and wants/needs. Nobody will know their product better than them.”
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