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Old 01-02-2010, 12:29 PM   #1 (permalink)
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Crossover pipe

Found this simple enough for me to understand,,,Figured I should pass It Along ::

Q
Stock HD exhausts have a crossover pipe, after-market exhausts do not. What is its purpose? Also, after-market exhausts are short or long, and with or without baffles. What is the use/function/purpose/advantage/disadvantage of each combination?
Billy Cole
Amissville, Virginia
A
Your first question is easy to answer; the second would take a book to be anywhere near complete. Let's look at the easy one.
Most Harley motorcycles have small mufflers relative to their engine displacement. Those small mufflers must also be quiet, and that combination results in limited airflow which, in turn, limits power output. A crossover pipe allows the exhaust gasses from one cylinder to flow through both mufflers, thus reducing the restriction and adding a bit of power.
Crossover pipes can also improve mid-rpm torque if they are properly located. Triumph first discovered this when they introduced an exhaust crossover pipe to their American Triumph Bonnevilles in 1964. The original purpose was to increase the airflow through the smaller mufflers that we Americans preferred. However, they also discovered that the crossover added performance in the mid-rpm range.
Aftermarket exhaust systems tend to not have crossovers because most buyers do not like their looks. Most Evo Harley owners change the header pipes simply because they think the crossover is ugly. Harley learned its lesson and now hides the crossover pipe, sometimes incorporating it as part of the muffler bracket.

Now, about that second question. I believe that most aftermarket Harley exhaust system sales are based on appearance and not necessarily performance. Most of us buy what we want to see and be seen with. Pipe makers build what they can sell. If someone wants 18-inch open pipes that point straight up, someone else will make them. Unfortunately, few buyers ask for proof of performance beyond a verbal assurance that the selected pipe sounds great and kicks, er, behind.
Your Harley's engine, on the other hand, responds only to the physical dynamics of the exhaust system. For example, the length and diameter of the header pipes (the portion of pipe from the cylinder-head port to the muffler) are critical. The flow volume and acoustic characteristics of the mufflers are equally important.
Evo, Twin Cam and Sportster engines mainly run in the middle part of their rpm range, from 2,000 to 5,000 rpm in the case of the Big Twins and maybe 3,000 to 6,000 for Sportsters. Many owners modify these engines to increase total airflow by way of accessory air cleaners, exhausts and sometimes cams. These engines, modified or stock, work best with header pipes that are 1.75 inches in diameter and have a length of between 27 and 32 inches. Peak power drops of dramatically outside these dimensions.
Let me give some examples: VHR (Vance & Hines) makes a series of three exhaust systems that differ mainly in their length, but all use 1.75-inch header pipes. The "Straight Shots" have header lengths of about 30 inches. The "Short Shots" are under the 27-inch minimum and the "Long Shots" are over the maximum. The Straight Shots function about the same as stock Harley headers (30-inches long) equipped with loud Screamin' Eagle or Cycle Shack short mufflers and are a very satisfactory compromise between appearance and performance. The other two, however, are difficult to deal with. They adversely affect torque output and make it difficult to tune carburetor and EFI systems. Mind you, VHR does not misrepresent their pipes and the construction quality is superb; they are merely giving the public what it insists on having.
A successful muffler does two important things: It allows sufficient unimpeded airflow, and it reflects pressure waves. If you build an engine that only runs in the top quarter of its rpm range, like a road racing engine, an open megaphone is still hard to beat. However, useful power through the middle half of your engine's rpm range requires a muffler design that delivers "back pressure."
I've never been fond of the term "back pressure" as it implies pressure in the muffler and isn't entirely accurate. What you do want is a strong positive pressure wave sent back up the exhaust pipe to reduce air/fuel mixture losses from the combustion chamber. When the engine is running slower than is optimum for the cam design, incoming air/fuel mixtures tend to continue to flow across the combustion chamber and out the exhaust port. A timely pressure wave slows this loss and the engine makes more power at that rpm.
When properly implemented, a muffler with back pressure does not limit cylinder fill at high rpm because the pressure wave arrives at the exhaust port after it closes. This is why baffles, which reflect such pressure waves, are located at the rear of the muffler.
The trick is to get a strong pressure wave without overly impeding flow volume. The success of the SuperTrapp and White Bros. "E" series muffler designs is that they send powerful pressure waves without substantially reducing maximum flow volume. The small mufflers, like the ones built into the VHR Straight Shots or the small Screamin' Eagle mufflers simply cannot send the wave without greatly restricting flow.

If a muffler has enough volume, it can be very quiet and still flow enough air for the intended application. The problem is, of course, that Harley owners do not like large mufflers. So, the muffler designer must find a compromise that combines acceptable silencing with enough airflow for the task at hand. The practical 'bottom line' is that small mufflers must be rather loud to make power and only two-into-one and touring mufflers are large enough to be both reasonably quiet and flow enough to make racing-class power. Back when Harley published peak horsepower numbers, the touring models with their large mufflers made about three horsepower more than the Big Twins equipped with shorty duals.
If you are interested in performance, be skeptical when you shop for a system. As you might expect, I prefer performance to elegance - you can have both. Look for header pipes of the diameter and within in lengths I specified. Buy the largest muffler you can stand to look at. Consider two-into-one systems.
Be sure to prefer mufflers with perforated tube baffle tubes over those with louvered baffle tubes. The louvered core material promotes turbulence, restricts flow and doesn't silence as well either. It is cheap, though.
Ask for performance data in the form of torque and horsepower dyno charts. Unless your intent is strictly racing, be sure to compare system performance in the 2,000 to 5,000 rpm range (3,000 to 6,000 for Sportsters) where your engine spends its time.

Last edited by Osco; 01-02-2010 at 12:30 PM. Reason: speeling
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