Exhaust pipes have 5 key parts that make them work properly. They have a header, converging cone, expansion chamber, diverging cone, and a stinger. All of these sections work together to not only tune the engine, but also to scavage the engine properly.
The header is what creates the initial vacuum when the exhaust port opens. It quickly pulls the expended gases away from the exhaust port. When the gases reach the end of the header, they enter the converging cone.
 The converging cone begins the tuning portion of the pipe. As the pulse moves down the cone, it decelerates until it reaches the expansion chamber. The pulse then maintains velocity until it reaches the diverging cone, also referred to as the “Baffle”.
The expansion chamber length and diameter assists with determining the general characteristics of the engine. The general rule is, the longer the chamber, the more torque the engine will create and the shorter, the more RPM’s the engine will make. After the pulse reaches the end of the expansion chamber, it enters the diverging cone.
The diverging cone takes that pulse and redirects it back up the pipe. The expended gases pass through the hole or holes in the baffle and exits through the stinger.
The stinger diameter and length determines how much gas and at what velocity it is able to exit the pipe. The stinger also assists the diverging cone with back pressure until the returning pulse reaches the header.
 The header then maintains that particular pulse velocity and strength until it reaches the exhaust port. Selecting the proper header length is also very important because you have to be aware of the engine stroke and piston speed. If a header is too short, the pulse reaches the exhaust port too soon and back fills the intake ports. If it reaches the port too late, the port is already closed and the unburned fuel remains in the pipe and then burns in the header causing the engine to lean out and usually leads to pre-ignition and detonation.
Usual signs of an exhaust not scavaging properly are:
- Erratic Idle
- Flat spots in power band/transition time
- Continued heat buildup and heat saturation of engine ports and overall running temperature
- Poor fuel economy
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