Spent Gases Increase Performance – The 411
writer: D. William Denish
photographer: D. William Denish, Manufacturers
website: Click Here
Staggered duals, or 2-into-2s, are popular and cost-effective power makers. This Cycle Shack 2-into-2 features tunable baffles, O2 sensor bungs, and are available with 1-3/4- and 2-inch head pipes. The pipes are available with various styled ends and fit factory heat shields.
An optimized exhaust system is critical for maximized engine performance. A few decades ago, there was little development work done on V-Twin exhaust systems. When development work was done, most was performed by the cut-and-try methods. However, with the advent of the rear wheel dynamometer and the increased popularity of the Harley V-Twin after the introduction of the Evolution motor, exhaust system development flourished.
Today, there are a plethora of exhaust systems available. When selecting a system, riders typically want a rich sounding exhaust note and bitchin’ looks that grab their attention. Riders also want real-world performance over a wide rpm range, from slow rpm trolling through the park to crisp acceleration when they whack open the throttle from a standing stop or for passing an 18-wheeler.
Nevertheless, there is no free lunch when it comes to exhaust systems. Tradeoffs exist between looks and performance because exhaust system design has a significant impact on engine performance. Header pipe diameter, pipe length, bend radius, muffler volume, and even baffle design affect performance. Make the pipe too long or too short, too big or too small in diameter, and performance suffers. Performance also suffers when airflow is restricted by the mufflers. Large displacement engines generate more exhaust and require higher-flowing mufflers than small engines, but aesthetics often suffer as muffler size increases. Designing a pipe for street riding is much more difficult than for racing because aesthetics are more important and the powerband is much wider.
Buying an exhaust system can be a daunting experience. For example, there is a overwhelming selection of exhaust system designs, from staggered 2-into-2s, 2-into-1s, and true duals for baggers to straight pipes, big and small diameter headers, short and long header lengths, and stepped headers, just to mention a few. Not surprisingly, it is hard to determine what pipe works best, let alone which one looks best. Then there are a multitude of engine displacements from 74-cubic inchers to 130-inchers and even larger that have varying performance requirements. And the larger the engine displacement, the more difficult it is to find a pipe with sufficient flow for good top-end power. Lucky for us, today we have reliable dyno charts to reference before plunking down our hard-earned cash on a fancy-looking pipe
The stock factory exhaust includes a crossover- pipe, which increases airflow by allowing each cylinder to share both small mufflers. Crossover pipes typically improve midrange performance at the expense of aesthetics.
In order to make stout power, an engine needs adequate airflow. Yet large engines generate a greater volume of exhaust gases, which requires larger volume mufflers and header pipes. Additionally, no exhaust system can deliver optimized power across the entire rpm range. Add a multitude of design options into the selection process, and pipe choice becomes confusing and time-consuming. Since optimum pipe design is determined by an engine parts combination and its most important rpm range, having a basic understanding of the exhaust process will provide you with valuable information for selecting your next exhaust system.
Exhaust System Elements
The exhaust system is an integral part of the components that regulate airflow through the engine. Other key components include the induction system, cylinder heads, and camshaft. To achieve maximum performance, these components must be tuned together as a system for maximum performance within a given rpm range. If one component is changed or modified, the entire group of components must be retuned for maximum performance.
An optimized exhaust system is designed to achieve optimized pressure balance between the engine’s intake and exhaust tracts within a given rpm range. The 2,000 to 4,000 rpm range is usually most important for street engines, while 4,500 to 5,000 rpm and higher is typically most important for race engines. Since no exhaust system is efficient throughout the entire rpm range, priorities must be determined and compromises made to achieve the desired performance characteristics. An exhaust system is fabricated from a standard variety of parts.
These include a mounting flange, header pipe, muffler and occasionally a merge collector. The diameter, length, and overall arrangement of these componentsb will have a major impact on engine performance.
Exhaust System Buying Considerations
* Design (2-into-2, 2-into-1, etc.)
* Appearance (chrome, polished,anodized)
* Pipe diameter
* Stepped vs. non-stepped
* True “dresser” dual design
* Quality of finish
* Heat shields
* Baffle design
* Removable baffles
* Mounting hardware
The rear cylinder on a stock factory dresser routes exhaust gases to both mufflers. A problem occurs because most of the exhaust flows into the right-side pipe, overloading the right-side muffler while under utilizing the left-side.
Unlike the stock factory dresser exhaust, the Bub Rinehart “True Duals” system uses a separate header pipe and muffler for each cylinder, providing a balanced flow for both cylinders. Note that the rear header does not have a crossover pipe connecting to the front cylinder header. Rinehart’s include internal baffles and signature end caps.
Header Pipe Diameter
Header pipe diameter has a major effect on exhaust gas velocity. Pipe diameter is determined by engine displacement (bore and stroke), compression ratio, valve diameter, camshaft specifications (lift, duration and timing), and the cuticle rpm band. If pipe diameter is too small, backpressure increases. Backpressure is defined as flow resistance created in the exhaust system.
The higher the backpressure, the higher the engine’s pumping loses will be, since the piston is required to physically force the residual gases out of the cylinder during the exhaust cycle.
Elevated backpressure also reduces low-lift exhaust flow during the period called “blowdown.” An effective blowdown period will efficiently use expanding exhaust gases to expel combustion residue from the cylinder. The blowdown period begins at exhaust valve opening and ends when cylinder pressure and exhaust system pressure are equalized. Camshaft timing has a major effect on blowdown.
By using blowdown to remove exhaust gases, pumping losses are reduced because fewer gases remain for the piston to physically dispel from the cylinder.
If header diameter is too large, exhaust gas velocity will be low, thereby weakening the scavenging wave and reducing its effect during valve overlap. As such, it is important to note that as blowdown pressure declines, there is an increased dependency on the exhaust system to scavenge cylinders of spent exhaust gases. Ideally, you want a balance between backpressure and velocity. Headers made of 1-3/4-inch pipe work well with stock and mildly modified V-Twin engines.
To maintain the proper backpressure/velocity balance with a street engine, it is suggested not to use 2-inch diameter or larger header pipes unless your engine is at least 100 cubic inches and preferably larger. But be aware that even in the case of a large engine there are tradeoffs, because a 2-inch pipe will bleed off some bottom-end torque for top-end horsepower.
For comparison’s sake, to optimize high-rpm power with a 120ci to 130ci race-only engine, you should start with a 3-step straight-pipe design having 2-inch, 2-1/8-inch and 2-1/4-inch pipe diameters, and then tune from that baseline.
Header Pipe Length
Pipe length is determined by the engine’s application and the most important rpm range. Pipe length is important for optimizing inertia and wave tuning, which determine the affect scavenging has on power production. Scavenging refers to the process of where a column of fast moving exhaust gases (inertia scavenging) supersonic energy pulse (wave scavenging) aids the removal of combustion residue from the cylinder while assisting the intake charge into the cylinder.
The ThunderHeader is a popular 2-into-1 pipe that merges both headers into a tapered collector. Twointo- one designs tend to increase torque below the engine’s peak torque rpm for improved low and midrange power.
The Vance & Hines Pro Pipe is a popular 2-into-1 design that is available with replaceable baffles so exhaust flow can be tuned to the engine combination and application.
The following is a brief overview of how it works: As the exhaust valve opens, gases rushing past the valve create a positive pressure wave that travels toward the open end of the header pipe. As the positive wave exits into the atmosphere, it is converted into a negative wave that moves back up the pipe toward the valve. During engine operation, these positive and negative pressure waves pulsate between the open end of the pipe and the exhaust valve.
When pipe length is optimized, the negative wave will be timed to arrive at the exhaust valve during the valve overlap period. Since the negative wave reduces pressure at the valve, it helps scavenge combustion gases from the chamber.
Because pressure waves can only be timed to help exhaust scavenging over a narrow rpm band, the engine’s most critical rpm range must be first determined so pipe length can be matched to the rpm band. A longer pipe length optimizes power at low rpm. Conversely, a shorter pipe length improves upper-end rpm performance, because as the pipe becomes shorter, the tuning effect has less time to enhance slow-speed engine’s operation.
It is important to note that an exhaust system can only be optimized over a small rpm band, typically 1,500 to 2,000 rpm. Therefore, be sure to tune the pipe diameter and length to the engine’s most critical rpm band. In the case of a drag racing engine, this band should fall between peak torque rpm and peak horsepower rpm. With due diligence, you can tune a header pipe to the middle of the rpm band to achieve the highest average power.
For a lower-rpm street engine, it is normally best to tune pipe dimensions to peak torque rpm. Larger diameters and shorter length header pipes optimize high-rpm operation, while smaller diameters and longer pipe lengths favor low-end power.
Stepped Header Pipes
Various exhaust systems are designed with a stepped-header pipe. A stepped header includes the placement of pipediameter differentials in the pipe. The differentials are referred to as steps. Stepped headers are divided into two or more pipe sections (usually two or three) with each successive section at least 1/8-inch larger than the previous section.
Exhaust port shape and valve size are crucial for determining the optimum size differential and location ofsteps in a header pipe.
Although a stepped header generates more low-pressure waves than a non-stepped design, its waves are weaker. Steps help maintain a higher average gas velocity over the total length of header pipe. A stepped header won’t necessarily make more power than a non-stepped pipe, but it can broaden the engine’s torque curve by widening the scavenging wave’s effect, which increases the time of negative depression.
This can result in a win-win situation: High torque at low rpm while maintaining high horsepower at high rpm. Since the engine views a stepped header as a tapered pipe, the greater the number of steps in a length of pipe, the greater the taper angle and the stronger the pressure waves. But as the pipe angle increases, the rpm band it affects gets narrower. On the otherm hand, a longer pipe with fewer steps results in a narrower angle, thereby widening the working rpm range but lessening the strength of the pressure wave. Stepped headers are most beneficial when used on large-displacement and/or high-rpm engines.
A reverse cone is fitted to a collector to augment, extend and smooth out the peak horsepower band. A reverse cone can improve top-end power without compromising low-end torque. This pipe includes heat shields to retain the show-chrome look.
A cam with a long overlap can benefit from a pipe design that produces a wide exhaust-scavenging wave because a greater portion of the overlap period is effectively covered. A stepped header is an excellent design for this application as long as the pipe taper is not too steep. Conversely, if the scavenging wave is too narrow, the overlap period will not be covered effectively and performance will be reduced. Increasing the amount of time the scavenging wave covers the overlap period tends to broaden the tuned rpm band. Combiningm a relatively wide scavenging wave with a short overlap or generating a wider wave with a long overlap are two ways to broaden the rpm band over which the exhaust system is effective.
V-Twin engine builders have various philosophies for stepped headers. Some favor equal length sections for a stepped pipe while others prefer unequal lengths. Still others achieve good results by making the first section 10 to 12-inches long, and then splitting the remainder of the header into equal length sections. Some tuners base the length of the first stepped section on valve diameter. Moreover, the location of the bend in the header immediately after exiting the port and the design of the bike’s frame can dictate the length of the first step.
Most stock Harley mufflers are relatively small for aesthetic reasons, and include a healthy dose of internal baffling to satisfy EPA noise limitations. As a result, airflow is restricted and so is power. Large, touring model Harleys ship with large volume mufflers and a crossover pipe. This large interconnected design produces a few extra ponies over the Dyna/Softail models with smaller volume mufflers.
Adequate muffler volume for the engine displacement is important for keeping exhaust backpressure low at high rpm. Generally, muffler volume should be roughly 10 times the cylinder volume to make good high rpm power. However, horsepower is also a factor because the more horsepower an engine makes, the more exhaust flow it generates.
In other words, as engine airflow increases exhaust gas volumeor also increases. With increased exhaust gas volume, muffler airflow and volume must also be increased. For example, assume we have two 113 cubic inch engines, one producing 100 horsepower and another producing 125 horsepower. A muffler that barely provides adequate airflow for the 100 horsepower engine will surely be inadequate for making good top-end power with the 125 horsepower engine.
Large engines require a muffler with a large main body, free-flowing baffle and unobstructed exit. Since large mufflers do not look aesthetically pleasing on a V-Twin, it is difficult to make a pipe for a large-displacement engine that satisfies both aesthetics and performance. The easiest and least costly method for improving exhaust system performance is to modify the system.
Most 2-into-2 systems are tunable by modifying the internal baffles. Modifying baffles, especially those used in small-volume mufflers, can improve power. Increasing the number and/or size of holes in the baffles or shortening the baffles reduces backpressure and helps top-end power. However, remember that increasing flow too much can reduce bottom-end torque. Furthermore, tunable 2-into-1 systems offer a clear advantage over non-tunable collector systems, especially if engine displacement is large.
2-Into-1 Collector Systems
A 2-into-1 exhaust system terminates both header pipes into a tapered merge collector, which includes baffling that acts as a muffler. Some collector systems for the V-Twin are designed with replaceable baffles, which allows the tuner to select a baffle offering the optimum amount of airflow and backpressure for the application.
Since collector systems tend to increase torque below peak torque rpm, they typically improve low and midrange power. Highrpm race engines, such as Top Fuel motors, generally do not use a collector because the engines do not drop below peak torque rpm.
Early model Screamin’ Eagle pipe by SuperTrapp Industries includes a series of diffusion discs at the pipe’s end for tuning purposes. Between 15 and 23 discs are often used with various end-cap designs. Within limits, installing more discs increases airflow, sound and performance.
Optimum collector design requires cut-and-try testing, so there is no one size fits all solution. However, in general, the longer and smaller the diameter of the merge collector, the lower the point where peak torque rpm will occur. A long slow-tapering collector effectively spreads the reflected wave over a wider rpm band. Spreading the wave essentially tricks the engine into thinking the header pipes are longer than they actually are. On the other hand, a short and wide-angle collector creates the strongest return pressure wave but works over a smaller rpm range. Additionally, the merge angles of the header pipes and the internal design of the collector can have an influence on power production.
A slow-flowing port typically allows excessive amounts of exhaust gases to backup in the port and re-enter the combustion chamber (called reversion). The exhaust gases dilute the intake charge and ruin carburetion and throttle response. An anti-reversionary (AR) flange is often installed in a header pipe where the pipe intersects with the exhaust port. This feature helps when the exhaust port is inefficient and slow flowing. Essentially, an AR flange shrouds the port, thereby catching much of the back-flowing exhaust, which improves performance, particularly at low rpm.
Some exhaust ports are designed with a miss-match at the bottom of the exhaust header pipe. The miss-match functions in the same fashion as an AR flange by reducing exhaust backflow. The mismatch creates a ledge at the bottom of the port that stops reversionary exhaust gases from backing into the combustion chamber.
Stepped headers are divided into two or more sections, with each section being a minimum of 1/8-inch larger in diameter than the previous section. Stepped headers produce more but weaker low-pressure waves, which broadens the engine’s torque curve by widening the scavenging wave’s effect.
Key exhaust system considerations include pipe diameter and length. Both pipe variables should be optimized based on engine displacement, rpm band, cam timing, and application. If a collector is used, its diameter and length also must be considered. Bike weight, gear ratios, and the number of gears along with the application also enter into the equation for optimum exhaust design.
Header diameter (inside diameter) is typically the most important factor in exhaust system design because it sets the torque curve. Increasing diameter generally improves top-end power at the expense of low-end torque. Changing pipe length will move the torque curve either up or down the rpm scale. A shorter pipe favors top-end horsepower while a longer pipe caters toward low-end torque.
Some engine builders have found great power numbers using an exhaust system that appears contradictory to known rules: long large-diameter or short small-diameter header pipes. However, these designs can offer the best of both worlds-great low end torque and top-end horsepower. Straight pipes typically improve power above roughly 4,000 rpm, which is great for an engine that never drops below this rpm. However, at low rpm, straight pipes generally create big dips in the torque curve, reduce throttle response, and make jetting difficult.
2-into-2 straight pipes can improve top-end performance at the expense of low and midrange torque. Performance and throttle response below 3,800 rpm is usually poor and the pipe is excessively loud for street use. Straight pipes are best used on the racetrack.
Exhaust systems for ’06 and all ’07-up models have bungs welded into the pipes for EFI O2 sensors, which are required for maintaining EPA compliance. For these models, be sure to buy a pipe with bungs. Since a performance exhaust system increases airflow through the engine, the carburetor jetting or EFI fuel map must be altered to ensure a proper air/fuel mixture.
Since some years and models of bikes may have warranty implications if the exhaust system is changed, be sure to check your warranty status before buying an exhaust upgrade
Finally, consider the exhaust system an integral part of components that regulates airflow through the engine. For optimum performance, it is important the exhaust and induction systems, camshaft and ignition timing be tuned together as a complete system. If a component is replaced or modified, the engine must be retuned for best performance.
writer: D. William Denish
photographer: D. William Denish, Manufacturers
website: Click Here