An optimized exhaust system is designed to achieve optimized pressure balance between the engine's intake and exhaust tracts within a given rpm range. The 2,000 to 4,000 rpm range is usually most important for street engines, while 4,500 to 5,000 rpm and higher is typically most important for race engines. Since no exhaust system is efficient throughout the entire rpm range, priorities must be determined and compromises made to achieve the desired performance characteristics. An exhaust system is fabricated from a standard variety of parts. These include a mounting flange, header pipe, muffler and occasionally a merge collector. The diameter, length, and overall arrangement of these componentsb will have a major impact on engine performance.
Exhaust System Buying Considerations
* Sound
* Design (2-into-2, 2-into-1, etc.)
* Appearance (chrome, polished,anodized)
* Performance
* Pipe diameter
* Stepped vs. non-stepped
* True "dresser" dual design
* Quality of finish
* Heat shields
* Ceramic-coated
* Baffle design
* Removable baffles
* Anti-reversion
* Mounting hardware
* Warranty
* Price
 4. The rear cylinder on a...  4. The rear cylinder on a stock factory dresser routes exhaust gases to both mufflers. A problem occurs because most of the exhaust flows into the right-side pipe, overloading the right-side muffler while under utilizing the left-side. |  5. Unlike the stock factory...  5. Unlike the stock factory dresser exhaust, the Bub Rinehart "True Duals" system uses a separate header pipe and muffler for each cylinder, providing a balanced flow for both cylinders. Note that the rear header does not have a crossover pipe connecting to the front cylinder header. Rinehart's include internal baffles and signature end caps. | |
Header Pipe Diameter
Header pipe diameter has a major effect on exhaust gas velocity. Pipe diameter is determined by engine displacement (bore and stroke), compression ratio, valve diameter, camshaft specifications (lift, duration and timing), and the cuticle rpm band. If pipe diameter is too small, backpressure increases. Backpressure is defined as flow resistance created in the exhaust system. The higher the backpressure, the higher the engine's pumping loses will be, since the piston is required to physically force the residual gases out of the cylinder during the exhaust cycle.
Elevated backpressure also reduces low-lift exhaust flow during the period called "blowdown." An effective blowdown period will efficiently use expanding exhaust gases to expel combustion residue from the cylinder. The blowdown period begins at exhaust valve opening and ends when cylinder pressure and exhaust system pressure are equalized. Camshaft timing has a major effect on blowdown. By using blowdown to remove exhaust gases, pumping losses are reduced because fewer gases remain for the piston to physically dispel from the cylinder.
If header diameter is too large, exhaust gas velocity will be low, thereby weakening the scavenging wave and reducing its effect during valve overlap. As such, it is important to note that as blowdown pressure declines, there is an increased dependency on the exhaust system to scavenge cylinders of spent exhaust gases. Ideally, you want a balance between backpressure and velocity. Headers made of 1-3/4-inch pipe work well with stock and mildly modified V-Twin engines. To maintain the proper backpressure/velocity balance with a street engine, it is suggested not to use 2-inch diameter or larger header pipes unless your engine is at least 100 cubic inches and preferably larger. But be aware that even in the case of a large engine there are tradeoffs, because a 2-inch pipe will bleed off some bottom-end torque for top-end horsepower. For comparison's sake, to optimize high-rpm power with a 120ci to 130ci race-only engine, you should start with a 3-step straight-pipe design having 2-inch, 2-1/8-inch and 2-1/4-inch pipe diameters, and then tune from that baseline.
Header Pipe Length
Pipe length is determined by the engine's application and the most important rpm range. Pipe length is important for optimizing inertia and wave tuning, which determine the affect scavenging has on power production. Scavenging refers to the process of where a column of fast moving exhaust gases (inertia scavenging) supersonic energy pulse (wave scavenging) aids the removal of combustion residue from the cylinder while assisting the intake charge into the cylinder.