What is exaust scavening? Off season boredom for me is time to learn about the things I think I should know about. Which is better for scavenging: two into one exaust or true duals?

 

Exhaust Scavenging
When an engine starts its exhaust stroke, the piston moves up the cylinder bore, decreasing the total chamber volume. With the exhaust valve open, the high pressure exhaust gas escapes into the exhaust manifold or header, creating an exhaust pulse comprising three main parts: The high-pressure head is created by the large pressure difference between the exhaust in the combustion chamber and the atmospheric pressure outside of the exhaust system. As the exhaust gases equalize between the combustion chamber and the atmosphere, the difference in pressure decreases and the exhaust velocity decreases. This forms the medium-pressure body component of the exhaust pulse. The remaining exhaust gas forms the low-pressure tail component. This tail component may initially match ambient atmospheric pressure, but the momentum of the high- and medium- pressure components reduces the pressure in the combustion chamber to a lower-than-atmospheric level. This relatively low pressure helps to extract all the combustion products from the cylinder and induct the intake charge during the overlap period when both intake and exhaust valves are partially open. The effect is known as scavenging. Length, cross-sectional area, and shaping of the exhaust ports and pipeworks influences the degree of scavenging effect, and the engine speed range over which scavenging occurs.

The magnitude of the exhaust scavenging effect is a direct function of the velocity of the high and medium pressure components of the exhaust pulse. Performance headers work to increase the exhaust velocity as much as possible. One technique is tuned-length primary tubes. This technique attempts to time the occurrence of each exhaust pulse, to occur one after the other in succession while still in the exhaust system. The lower pressure tail of an exhaust pulse then serves to create a greater pressure difference between the high pressure head of the next exhaust pulse, thus increasing the velocity of that exhaust pulse. In V6 and V8 engines where there is more than one exhaust bank, Y-pipes and X-pipes work on the same principle of using the low pressure component of an exhaust pulse to increase the velocity of the next exhaust pulse.

Great care must be used when selecting the length and diameter of the primary tubes. Tubes that are too large will cause the exhaust gas to expand and slow down, decreasing the scavenging effect. Tubes that are too small will create backpressure against which the engine must work to expel the exhaust gas from the chamber, reducing power and leaving exhaust in the chamber to dilute the incoming intake charge. Since engines produce more exhaust gas at higher speeds, the header(s) are tuned to a particular engine speed range according to the intended application. Typically, wide primary tubes offer the best gains in power and torque at higher engine speeds, while narrow tubes offer the best gains at lower speeds.

Many headers are also resonance tuned, to utilize the low-pressure reflected wave rarefaction pulse which can help scavenging the combustion chamber during valve overlap. This pulse is created in all exhaust systems each time a change in density occurs, such as when exhaust merges into the collector. For clarification, the rarefaction pulse is the technical term for the same process that was described above in the "head, body, tail" description. By tuning the length of the primary tubes, usually by means of resonance tuning, the rarefaction pulse can be timed to coincide with the exact moment valve overlap occurs. Typically, long primary tubes resonate at a lower engine speed than short primary tubes.

Some modern exhaust headers are available with a ceramic coating. This coating serves to prohibit rust and to reduce the amount of heat radiated into the engine bay. The heat reduction will help prevent intake manifold heat soak, which will decrease the temperature of the air entering the engine.

(I stole this from Wiki)

To put it differently, a proper exhaust system will create a "siphoning" effect on the motor that will help clear the exhaust out of the cylinder and help draw in more fresh air/fuel, thus creating more power.

If your looking for no compromise power, 2 into 1 ehaust systems win it hands down. Look at the sport bikes. they are almost always 2 into 1, 3 into 1, 4 into 1, etc. Some v-twin or large displacement 4 cylinder models (aka Suzuki Hyabusa or the Suzuki TL 1000) do use multiple mufflers or what appear to be "dual exhaust" or 4 into 2 exhaust but they do so for noise reduction and they still use crossover pipes.

Dual exhaust on HD's are sommon because they look fantastic sound even better. I was torn when I was deciding what to put on my SG. The reality of it is, unless your doing some tuning at the same time, your not gonna do yourself any favors in the power department by installing true dual exhaust.................

I was an auto mechanic for about ten years before I got out of the business about 4 years ago. Exaust theory can actually get quite complicated but this is the jist of it.

..................and don't even get me started on the 2 strokes.

I hope this helps buddy.

 

Very will said even if it was pulled from wiki. +1 two in one is always going to be better then true duals.

Keep in mind that your stock pipes or most after market pipes that keep the exhuast on the right side have a crossover pipe that works very much like a scavanger. It is not near as good as a 2in1 but way better then true duals.

If you go with duals you really should get a tune to regain some of the low end you loose.

 

Thanks for the info. I'll stick with my cross overs.

 

I would like to see a before and after dyno comparison with only a ceramic coated stock exhaust header change.

 

this is from the thunderheader web site. D&D has similar information on their site.

It is possible to use the energy from these forces to help extract exhaust gases from the engine. This process is called “exhaust scavenging”, as it uses the energy from the inertia force created from the out going exhaust pulse, to cause a low pressure area in the exhaust port & combustion chamber during the valve overlap period (when both exhaust & intake valves are open at the same time). If the negative pulse is timed to reach the open exhaust valve during the overlap period, it will draw in a fresh mixture of air & fuel from the intake before the piston descends on the intake stroke. Timing of this pulse is controlled by; engine displacement, RPM, primary tube diameter & length, and Reverse Flow Impedance (RFI™). The velocity can be slowed by increasing the primary tube diameter, however by slowing the velocity, you decrease the scavenging effect.

 

I always heard that One sucks and One blows to help each other prime the pistons.



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