This article concludes here.
A narrow cam lobe separation angle will help increase cylinder fill, which increases cranking compression, which will deliver more torque. Correct exhaust design for a specific engine will assist in all of this by ensuring the exhaust gases leave the engine in a timely fashion and preventing spent gases from polluting the fresh intake charge. The exhaust must be a suitable one or it will work against the engine and any potential power increases. The stock exhaust certainly is not a suitable performance choice because its primary concern is to reduce emissions, not to allow an engine to breathe freely, promoting efficiency and synonymous power. I will radically demonstrate this with drag pipes.
Drag pipes will decrease power in the low- and mid-rpm ranges on any street Harley-Davidson. However, the same pipes will assist greatly in ridding a top-fuel H-D dragster of unwanted exhaust gases, thus creating power. Different engine sizes need different diameters and lengths of exhausts. The muffler will also need to increase in volume capacity as a street engine gains cubic inches (swept volume). Mufflers need to be larger rather than smaller. Complicating the exhaust valve closing timing by a narrow cam lobe separation angle are kinetic wave pulses of exhaust gases returning up the pipes.
Many think that the exiting exhaust gases simply flow out the exhaust header into the muffler and out into the atmosphere. Well, they do, after they turn around and return up the pipe many times. One reason this occurs is that heat or temperature differentials create negative pressures (vacuum) that are greatest near the exhaust manifold with positive pressure near the muffler. This is just a fancy way of saying that it is hotter at the exhaust manifold and cooler near the end of the muffler. Heat makes things expand and cold makes them contract. The exiting exhaust gases cool as they go out the header and muffler. This creates a negative pressure back near the exhaust port, causing the cooler, positive exhaust to turn around in the muffler and travel back up the exhaust, reversing the flow of that particular exhaust pulse. The returning exhaust pulses must bounce off a closed exhaust valve to avoid polluting the fresh intake charge in the combustion chamber and cylinder.
Exhaust systems are perhaps the most complicated design part of a motorcycle. I hope that the pulses will bounce off a closed exhaust valve, but perhaps the novice fooling with his exhaust will create tight cam lobe centerlines (LCs) and narrow lobe separation angles (LSAs) [In his book, Danny explains these terms in the chapter on cams.] Narrow LSAs increase cylinder pressure and thus torque. The negative consequences of this are a rougher idle and the danger of exhaust reversion back into the chamber, past an open exhaust valve, bastardizing the fresh intake charge. If exhaust reversion becomes an issue, then changing the degreeing of the cam lobe centerline may be necessary as a slight adjustment will balance out potential exhaust reversion. Once again, cam design becomes art: where finding the fine line between increasing torque and balancing potential problems of exhaust reversion.
Torque cones, also known as anti-reversion valves, prevent reversion of exhaust gases back into the combustion chamber as they travel back up the pipes. A torque valve looks like an inverted cone where the wide diameter end meets the exhaust port. The narrowed end faces into the exhaust pipe header, catching many of the pulse waves coming back up the pipe and rebuffing them, many times breaking the wave pattern. Therefore, a torque valve preserves engine torque by preventing exhaust gas pulse reversion back into the combustion chamber. Torque valves are sometimes cast into the heads’ exhaust ports, where they meet the exhaust header pipe.
The TC 88 head is a case in point. The exhaust port end is not round but squishes in appearance. This design rebuffs returning pulses, breaking up or disrupting the wave pattern. The TC 96 head no longer has this shape, so the stock exhaust pipe has a design feature that assists in this crucial prevention of exhaust reversion, thus preserving engine torque.
However, exhaust changes will help immensely in correcting rich conditions because the backpressure pulse waves can change the ease or difficulty of scavenging the exhaust gases. Therefore, an overly free-flowing exhaust will usually cause a midrange dip of the AFR curve, which is the same as saying the AFR enriches too much below the acceptable lower range of 12:1. This enrichment increase translates into a drop in torque in this rpm range. This situation is exacerbated by performance cams, where the torque drop increases without an exhaust change. Correcting this condition through carburetor jetting alone will certainly lean out the engine higher up, to the point the increasing leanness (higher AFR) as the engine progresses into higher rpm ranges may even damage the engine.
An exhaust system change will do much to correct this AFR dip. Carburetor tuning is then used to finesse the AFR. Carburetor jetting, EFI fuel modification, ignition timing, sufficient air intake, and sufficient air outtake, also known as exhaust backpressure, all interweave to increase or decrease power substantially. Tuning each in isolation can afford a few extra ponies, but the next tuning operation may take away from the first. Tuning must encompass all these factors in combination with each other, not in isolation. Different problems require differing techniques or order of tuning. I favor installing a correct exhaust after a free-flowing air breather, since the engine, much like the human lung, must breathe without impediment. With hemi-head engines, exhaust tuning moves up the priority scale for Harley-Davidson motors like the Shovel, Pan, Knuckle, and Ironhead XL, as this is where the major problems first occur.
A camshaft’s LCA affects valve overlap around the piston’s top dead center point. In addition, the LCA establishes the amount of intake or exhaust valve closure delay beyond the theoretical 180-degree end of the relevant stroke. As the piston descends to the bottom of the cylinder, but before the piston reaches the bottom, the exhaust valve begins to open. Most of the fuel has burned by this time and the remaining cylinder pressure will begin to push the burnt exhaust gases into the exhaust port, out through the exhaust headers and into the mufflers.
After the piston passes bottom dead center, it begins ascending the cylinder on the exhaust stroke. This upward action forces the remainder of the spent exhaust gases out of the cylinder and combustion chamber to make room for the fresh charge of air and gas. As the piston moves toward the top of the cylinder, the already opening exhaust valve goes through maximum lift and begins to close. Just before the piston reaches the top of its exhaust stroke, the intake valve begins to open, but the exhaust valve is not yet fully closed. Theoretically, this should not work unless other forces of physics assist, like the Principle of Momentum.
The piston’s upward exhaust stroke removes just about all of the spent gases by the time the piston approaches top dead center. However, just before this, the intake valve begins to open slowly. Scavenging (Principle of Momentum) begins. The remaining exhaust gases rushing out into the exhaust will draw in the start of the intake charge. Conversely, the developing momentum of the intake charge also assists in pushing out the remaining exhaust gases. It becomes a symbiotic affair, first with exhaust pulling and then a little intake pushing. Valve opening and closing timing is crucial.
Finding the sweet spot where pollutants fully exit, but the beginning of the fresh intake charge is not lost, is a science and many times the art of valve timing. Therefore, the exhaust valve must close at precisely the right point after top dead center to prevent any burnt gases from reentering. Overlap is this area around top dead center where both the intake and exhaust valves are open.
This is one of the most critical moments in the engine’s 720-degree rotational cycle. To complicate matters, the functional design of the motorcycle changes this positioning for different types of riding and other factors. Different types of riding demand different overlap periods. As the engine passes through the overlap phase, the intake valve is opening while the exhaust valve closes just after the piston starts descending.
Conclusion
Many modern manufactured exhaust pipes are all about styling with no regard for performance. They are too skinny, too fat, too long, or too whatever to produce good gains in horsepower. There is no single production exhaust that I or anyone else can say is the best overall. In general, I can probably narrow the search quite effectively to a 2-into-1 collector system with a bigger muffler and a restrictor pipe with the same diameter as the header pipe. However, the consumer still needs much more information to decide on which collector system is best for his application, needs, and wants.
Donny Petersen
Tattoo Tony’s Heavy Duty Cycles
Toronto, Canada
www.HeavyDutyCycles.com 
Techline-Sept 09-PERFORMANCE OBSTRUCTIONS - Part III: Exhaust systems (concludes) 
Linear Mode
