Exhaust it's a gas, gas, gas

It is no secret to seasoned riders and mechanics that an internal combustion engine is just a large air pump, air goes in and air goes out – in between there is the nifty combustion that does the “work”.  It is also no secret that the more efficient you can move the air through this “pump” the more efficient the engine is at doing the “work”.  In this article I will break down some of the theory, physics and even some misconceptions about the exhaust.

First a very brief introduction.  The exhaust system is designed to evacuate gases from the combustion chamber quickly and efficently. Exhaust gases are not produced in a smooth stream.  Exhaust gases start as pulses from the opening and closing of the exhaust valve(s). For example, a 4 cylinder motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pules and so on. The more pulses that are produced, the more continuous the exhaust flow.  Some terms that are associated with exhaust(s) are: Velocity, Backpressure, and Scavenging.

Backpressure and Velocity and their relationship:

Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It is not hard to see how this misconception is appealing – wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn’t “wider is better” a good rule of thumb for exhaust upgrading? In a word VELOCITY.
The astute exhaust designer knows that a of balance flow capacity with velocity is the best recipe for a well performing exhaust pipe. You want the exhaust gases to exit the combustion chamber and speed along at the highest velocity possible. If you have two exhaust pulses of equal volume, one in a 2″ pipe and one in a 3″ pipe, the pulse in the 2″ pipe will be traveling considerably FASTER than the pulse in the 3″ pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity. Backpressure in it’s most extreme form can lead to reversion of the exhaust stream – that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you’d want a narrower pipe than if your powerband is located at 8-9000RPM.

Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly.  The best example that I can think of this is some of Eric Buell’s motorcycles.  Buell has developed a variable volume exhaust that adjusts “on the fly” the volume and size of the muffler via vaccum and cable operated baffles in atempts to adjust the exhaust’s ability to maintain the exhausts maximum velocity.  It suprisingly works well-downside is it is very complicated, more expensive to manufacture and package or translate to more traditional sport bikes designs (the muffler resides under the motorcycle on Buell motorcycles)  It also adds a bit more maintence and moving parts to wear out over time.

So why is exhaust velocity so important?

The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit too complicated to get into in this article, but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle – dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle.

The conclusion of all this is: any exhaust upgrade WILL change the engines performance (good and bad) as long as the exhaust was designed with the specific engine and most importantly the RPM range that the engine will be using most often.

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