This article concludes in this thread. Tunable Exhausts
Let us look at some exhausts like Hooker, which makes a wide range of stylish pipes for Harley-Davidsons. This premier company, part of the Holley dynasty, divides each performance style into two sizes or classifications: exhaust systems for 80"-95" engines and ones for 96" and larger displacements. Add to this that some Hooker exhaust systems, like the bagger mufflers and 2-into-1 collectors, have a means to control the amount of backpressure. In other words, they’re tunable. Tunable exhausts fit a wider range of engines and produce optimal power within a usable powerband. A Hooker exhaust has a rotating disc that can be adjusted for different power-output engines.
SuperTrapp, a well-known performance exhaust manufacturer that has been around a long time, also makes the Kerker line of performance exhaust. SuperTrapp is also a tunable system, but, and this is big for the performance-oriented, it can actually move the engine’s powerband up or down the rpm range. A progressive fellow, Paul Moller, invented tunable disc technology way back in 1971. Riders can easily add or subtract discs from the end of the muffler to customize it to their performance needs and/or simply achieve a pleasing sound. Adding more discs will increase the exhaust outlet and decrease backpressure. It also widens the powerband at the top end for highway riding and increases the exhaust tone. The tuner must be aware that the AFR will also lean out and fuel adjustments may be necessary when discs are added. Removing discs decreases the exhaust opening, which increases exhaust backpressure and narrows the engine’s powerband. This also creates more low-end torque and decreases the exhaust tone. The AFR will also lower (richen) the fuel and air mixture. An overly rich AFR will decrease power, so it may be necessary to lean out the air/fuel ratio.
Other Systems
In my humble opinion, the best-known true duals (a current fad on FLT baggers) are from Rinehart Racing. The company derives its name from Gerald Rinehart of NASCAR fame. BUB Enterprises, which is currently challenging Land Speed Records at the Bonneville Salt Flats, manufacturers the pipes for Rinehart Racing. Rinehart is no slouch in the performance game, and its true duals are strikingly beautiful. Both Rinehart Racing and BUB manufacture quality products and many of their exhaust systems combine beauty and performance.
The much-maligned stock crossover pipe on Harley baggers serves a performance function: it attempts to equalize pipe pressure. The crossover balance tube assists in bottom-end and midrange performance, while its elimination moves the powerband up into the mid- and top-end range. I recommend leaving the stock header pipes in place unless you’re increasing engine displacement substantially and use a tunable, big-volume muffler like Hooker or SuperTrapp.
There are many other exceptional exhaust systems in the marketplace. If a particular exhaust goes unmentioned in this article, it does not mean anything negative. However, there are many more lousy exhausts — in terms of performance — than good ones. After absorbing this information, the reader will be more than capable of conducting his own competent analysis and, therefore, making an educated choice.
Air Equals Power
Modern Harley-Davidsons, certainly later Evos and the Twin Cams, have breathing restrictions. Gasoline needs air in certain ratios to produce the highest percentage of burn, thus creating the heat that expands the gases in the cylinder. The expanding gases create the pressure needed to drive the piston down on its power stroke. This produces the twisting force called torque, which turns powertrain components and, ultimately, the rear wheel. Horsepower is only an arbitrary figure that is calculated by multiplying torque by rpm. The stock cam(s) and air cleaner will limit the performance of your engine, even with the installation of a high-performance exhaust system.
An engine is like a human lung. The lung must be able to breathe in as much air as it needs through its induction system: air cleaner, fuel mixer, intake manifold, head intake port, and intake valve. When using a performance cam, the intake valve stays open longer (duration) and opens higher (lift), which allows more air into the lung (engine). The exhaust tract and exhaust valve, in combination with a performance exhaust, allows all the spent gases to escape the lung. If any burnt gases are left in the cylinder, it will dilute the incoming air/fuel charge and reduce the power output of the engine.
The exhaust also creates returning pulses that it must discourage from reentering the lung. Furthermore, the exhaust creates the pressure of momentum to draw additional clean air into the lung. The exhaust system will have a dramatic effect on how an engine operates throughout its rpm range. The exhaust system affects torque, peak horsepower, and rpm, as well as influencing throttle response, gas mileage, vibration, and engine temperature.
Engine temperature is a big deal now that Harley-Davidson uses closed-loop electronic fuel injection, also known as a feedback system. The O2 sensors send exhaust composition analysis reports, via electrical pulses, to the electronic control module (ECM). The ECM regulates fuel delivery and ignition timing to create a stoichiometric 14.7:1 air/fuel ratio (AFR) during hot idle and cruise situations. The engine’s sensors constantly feed information to the ECM, so it can achieve this lean and hot air-to-fuel ratio. The exhaust system can assist in lowering injurious engine temperatures, or it can exacerbate the problem, further adding unwanted heat. It’s clear that the addition of a high-performance exhaust system alone will not lead to maximum torque and, thus, horsepower. The engine must have the proper team of components necessary to produce maximum horsepower output.
Conclusion
With the installation of breathing components, which are a high-performance cam(s), carburetor/EFI, air cleaner, ignition, and exhaust system, the expectation of huge percentage increases in torque and horsepower are a given. How much? Well, who can say without more specific information? But a 30-percent increase is easy to achieve over stock. Headwork, which will increase air flow substantially, in combination with supporting performance parts that assist in the task, can surely take the increase to 50 percent.Increasing cubic-inch displacement and/or utilizing forced induction will create different exhaust system demands for performance, but can result in increases of 100 percent or more over stock.
Next month, we’ll continue discussing various obstructions to power production on the Twin Cams.
Donny Petersen
Tattoo Tony’s Heavy Duty Cycles
Toronto, Canada
www.HeavyDutyCycles.com 
Techline-August09-PERFORMANCE OBSTRUCTIONS - Part II: Heads & exhaust sys. (concl.) 
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