By Tom Bumpous
Harley engines are "bottle-necked" by their factory exhaust pipes to comply with EPA emission requirements. Fortunately, this flow restriction can be improved by replacing the factory system with aftermarket pipes and there are hundreds to choose among for various applications.
Unmuffled "drag pipes" are commonly seen on street Harleys, and although tuned drag pipes of proper diameter and length are lightweight and work for maximum power at high rpm, they present some problems for street use. Most street riders use drag pipes for their clean appearance and wicked exhaust sound. Ah, that sound ........... music to the ears. Terrific, if that's what you want, and although a loud exhaust may be offensive to some people, loud pipes do add a measure of safety for street riders by alerting zombie cage (auto) drivers to their presence. The bad news is that, all other things being equal, drag pipes cause a significant loss of torque at low-to-mid rpm. If you replace the factory 2-into-2 pipes (and crossover pipe) with drag pipes, you may have a few more horsepower at high RPM, but you will lose measurable power (torque) at lower rpm where it is most often needed. This begs the question - How often do you ride above 4,000 rpm?
Many bikers and mechanics talk about having "proper back pressure" as though the term possessed some mystical significance that only a few engine wizards understand. As it is commonly understood, back pressure is an ambiguous, rhetorical misnomer that only confuses the complexities of exhaust theory. Assuming that an exhaust system has sufficient chamber volume (small mufflers do not have sufficient chamber volume), it is the control of reversion waves and flow velocity within the system that is important for efficient flow. Presumably, this is what so many people call back-pressure.
Exhaust theory arises from the physics of wave dynamics.
Briefly stated, the main objective of a performance exhaust design is, or should be, to reduce engine pumping losses. Strictly speaking, an exhaust system, in and of itself, can not create more engine power (more fuel must be introduced and burned for more power). An exhaust system of proper design increases efficiency by freeing power that is otherwise lost to flow restrictions (pumping losses). It accomplishes this by scavenging spent combustion gases and (ideally) by contributing to the laminar (ordered) movement of all flow events starting at the air intake, through the head, and ending at the exhaust discharge. When the exhaust valve opens, two things enter the head pipe - pressure waves expanding at supersonic speeds and the burned combustion gases which move somewhat less rapidly. These pressure waves can be likened to the concussive shock waves seen spreading outward, from the center of an explosion, or like the concentric waves caused by a pebble thrown into calm water, only much faster.
The escaping pressure waves move into the exhaust chamber (diffuser or megaphone) and are reflected off of it's inside walls, baffles and/or ambient atmospheric pressure as reversion (reversed) waves. Some of these waves will reflect all the way back to the still-open exhaust valve, and depending upon the timing of their arrival at the valve, they either help flow or impede flow.....this is where the exhaust designer's skill is tested.
The savvy designer can tune the timing of reversion wave arrival at the open exhaust valve by manipulating the effects of wave cancellation, wave resonance, head pipe diameter and length, diffuser shape and other system variables, to result in an engine that is effectively "suck-charged" (no jokes please) at a specific rpm range! This technique, along with similar intake track tuning, makes it is possible to "ram" up to 25%(+/-) more air/fuel mixture into the cylinder than the chamber volume would normally hold. When an engine enters this tuned rpm range, the increased flow efficiency can be felt as a definitive surge of power and is often referred to as being "on the pipe". However, even the best exhaust design is a compromise that yields optimal power only across a narrow rpm range, usually at high rpm's, but not at all rpm's. For the widest powerband, some of the 2 into 1 collector pipes that are available represent an acceptable compromise of pipe design by retaining reasonable low rpm power without restricting high rpm power. Also, for the record, exhaust loudness does not necessarily equate with engine power.
So, how do you choose the best pipes for your purposes? Unless you are prepared to endure the time and expense of empirical dyno-testing, the best answer you will find here is.......research and/or trial and error. Even published exhaust tests on dynamometers can be misleading unless performed on an engine identical with yours.....same heads and/or head mods, same pistons, same cam(s), same air filter and carb, etc. Very tiny mechanical differences can have very large effects upon flow efficiency. This is not meant to discourage anyone who is pondering pipe selection but exhaust theory is rather convoluted and, because of that, a challenging issue.
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Topic: Good article on exhaust (Read 2776 times)