 Techline-Sept 09-PERFORMANCE OBSTRUCTIONS - Part III: Exhaust systems (cont.)
This article continues here.
Exhaust Heat
There are those who espouse that a bigger exhaust is better. Well, this is not true if the muffler holds more volume, but it is true for the header pipes. As we all know, exhaust gas is hot. A goal is to keep the gases hot throughout the exhaust system because as air cools down it becomes denser, and, therefore, heavier. The engine needs to work harder to push a cooler and heavier mass of exhaust gas out of the muffler. Header pipes with a larger diameter than the stock 1-3/4" will allow the slowing of exhaust gasses. This, of course, will give the gasses more time to cool while en route. Furthermore, oversize diameter headers will allow exhaust pulse entropy, defined as the degree of disorder within a system. Before this gets overcomplicated, the entropy will disrupt header tuning, as exhaust pulses will lack the tendency to line up as they would in a smaller pipe.
Coating the entire exhaust system with an insulating material such as header wrap reduces exhaust cooling and assists in the retention of heat with too large a header pipe. The obvious fix is to reduce the pipe header diameter to match the size of the engine it serves and then wrap the headers. Harley-Davidson header pipe wrapping, which is a current 2008 customizing fad borrowed from hot rods and performance-racing vehicles, has definite functionality. The insulation keeps the exhaust pulses hot, lessening the pushing work of the engine as it expels them. It also encourages the exhaust pulses to line up and scavenge each other.
A ceramic thermal barrier coating also encourages exhaust gas pulses to retain heat for an efficient exit with less engine work and to line up for the scavenging pulling effect. Exhaust wrap currently looks cool. It comes in different colors and is inexpensive for the look it provides. Ceramic coating is more expensive, but may become a customizing trait in the future as customizing goes full cycle. The point is that both pipe wrapping and ceramic coating do work, but whether the effects are worth the trouble purely from efficiency and performance viewpoints on a street Harley is debatable.
Exhaust Sizing
Exhaust header pipe sizing on Harley-Davidsons should stay at 1-3/4" unless bigger cubic inches come into play. However, exhaust pipe length, inner diameter, and, of course, the muffling device (if any) assists greatly in controlling and directing exhaust pulses. People like Jerry Branch at Branch Flowmetrics Cylinder Head Performance Services have done much early and continuing research on this topic, including ideal exhaust pipe length, as have the pipe manufacturers. Ideal exhaust pipe is dependent on many things like the size of engine, as well as internal engine design and engine modifications. Length is also dictated by the inner diameter of the pipe and the type of muffler. Conventional wisdom 30 years ago dictated a Shovelhead’s optimum length to be 40".
I went along with this in my youth, but no longer do, as the pipe should be shorter for the volume of the engine. This drag pipe length will cause the powerband curve to rise in the rpm range. There will be poor performance at low rpm, while the midrange will be so-so. High rpm riding will grow into the nature of the pipes and produce tolerable performance on a stock Shovelhead, but will excel on one with an increase in displacement. Low-rpm riding with drag pipes is compromised by returning exhaust pulses, which bastardize the virgin air/fuel charge in the combustion chamber. At high rpm, the exhaust pulses are finding their way out of the pipe and into the atmosphere without reentering the combustion chamber. The tuning length and diameter of the drag pipe finally finds its sweet spot in the higher rpm scavenging range.
Please do not go cutting pieces off your Shovel’s 40" pipes. The correct way is to cut small pieces off and each time measure the performance on a dynamometer. Keep cutting until performance starts to dip. Of course, this is laborious and expensive. Remember there is no single best exhaust system, as different engines require different pipes, although 2-into-1 collectors reign as a category pipe. These, too, vary a great deal as header pipe length, diameter, and muffler volume play crucial roles. Let’s use an 883 Evo Sportster with stock displacement as an example. Using the standard 1-3/4" header pipe that’s been used forever on all traditional Harleys, 29" is the best length for producing optimal horsepower. Anything close to this length will work almost as well. If going to 2" inner diameter pipes, these would be better suited to a 1200 XL engine or bigger, rather than an 883. I use this only as an example since 1-3/4" is still the ideal diameter for the 1200 XL. However, the optimal length for maximum horsepower will now be around 23". Evo Big Twin engines require longer pipes while the Twin Cams a little longer again. However, all the stock engines run best with 1-3/4" header pipes.
These comments are generalizations and there are exceptions, since other parts or designs will offset the pipe diameter. Shorter pipes for a particular cubic-inch displacement work better at higher rpm as exhaust pulses need to exit faster. Longer pipes work better at lower rpm since the extra restriction increases low-end torque. Everything must coordinate so larger diameter header pipes will usually work better in conjunction with larger exhaust valves. They would also receive some consideration if the engine design was running consistently at higher rpm. Choose your mechanic and parts adviser well. Currently, I might choose a 2" inner diameter header for a SE CVO 110" Twin Cam and certainly would for a 116-incher. The S&S 124" Twin Cam might work better with a 2-1/8" or 2-1/4" header pipe while S&S’ 145" may have an ideal pipe diameter of 2-1/2".
Leaving Harleys for a bit, generally, for an engine making 250 to 350 horsepower, the ideal pipe diameter is 3"-3-1/2". Bigger engines require up to 4", which is pretty much the maximum header pipe size. Exhaust diameter becomes complicated quickly, as header pipe length and muffler size and type will compromise the above diameter choices up or down. Moreover, there is no accurate or simple calculation for optimal exhaust pipe diameter. This is mainly due to the styling and necessities of an exhaust system. Bends in the piping, whether for styling or to get around a frame member, transmission, saddlebag, or whatever, along with temperature fluctuations, and differences in muffler design, make selecting pipe diameter little more than an educated guessing game.
One exhaust system that I will state is categorically better than others is a properly tuned 2-into-1 header pipe feeding into a collector muffler of sufficient size to have at least 10 times the volume of the swept volume of the cylinders. Internal muffler construction will have the three-layer outer shell for pulse absorption, perforated tubing, and sufficient volume and expansion chamber length to destructively cancel and subtract wave pulses, but not at the expense of high-end power, unless, of course, the design purpose is to assist in building low-end torque. This may sound simple in the beginning but it gets exponentially complicated. This is why I shudder when watching riders drill muffler holes, remove baffles, cut baffles, and whatever else they do without a clue. The seemingly innocuous muffler involves more physics and design principles than most other components of a motorcycle.
Exhaust Affects Tuning
An astute technician-operator will address exhaust backpressure before tuning a carburetor because backpressure increases or decreases will have a major effect on a problem air/fuel ratio (AFR) at a specific rpm range, but not affect another. Carburetors have overlapping jets and needles. It is best to leave fine-tuning until after other major changes are made.
One might logically think that the optimal volumetric efficiency of a cylinder would be 100 percent. Through headwork and longer duration cams, which open the valves for a longer time beyond the theoretical 180 degrees of engine rotation, in combination with high rpm, the principles of momentum or scavenging will come into play. Exiting exhaust gases will act as if there is a vacuum behind them and help suck in fresh intake gases at a greater rate than normal. This is sort of like a reverse supercharger.
The truth is, an air/fuel charge can be packed into the cylinder to achieve beyond 100-percent fill. In fact, it’s not out of the question to obtain up to 130-percent fill. Scavenging does not occur at lower rpm because the exhaust gases are not moving fast enough to act like a vacuum.
Therefore, the maximum torque moves up the rpm range because the maximum cylinder fill has moved from a lower rpm to higher rpm. Since all things are not equal, the modifiers to this general discussion, other than the intake components listed earlier, will be combustion chamber configuration, squish band, spark plug location, number of plugs per cylinder, compression ratio, engine head temperature, quality of the octane gas, and the exhaust system.
Horsepower and torque have the mathematical relationship: if torque increases, then so does the other at a specific, constant rpm. Let us look at some basic engine principles that create torque. Compression equals torque. Compression equals cylinder fill. Compression depends on cylinder fill.
Cylinder fill of 100 percent is optimal at lower rpm, but can easily increase to 130 percent by stacking in the air and fuel via the scavenging forces of momentum as exiting exhaust gases suck in the intake charge while both valves are open during the overlap period. With increased cylinder fill comes increased cranking compression since the piston must compress a bigger volume of air and fuel molecules. Of course, the burning of this increase in air and fuel, and thus cranking compression, creates more heat, which creates more pressure to drive the pistons down on their power strokes, developing more torque. Scavenging and exhaust backpressure assist mightily in creating more horsepower and torque.
This article concludes in the next thread. Check back issue for pix and extra information. |
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