If a crossover isn't necessary on a tuned exhaust, then why does the Screaming Eagle set still use one?

 

That would be because Screamin Beagle exhaust is built for Stage 1 applications and retaining bottom end power is important.
When you get intohigher HP applications you can lose the cross over and not notice the small amout of bottom end power lose.

 

I'm not sure what "Screaming Eagle set" you are refering to. All the SE exhausts I see listed in the current catalog are mufflers only. Step Tuned exhaust systems refered to above are complete tuned header pipe systems from the jug back, not replacement mufflers. Changing mufflers only won't eliminate the need for the crossover.

 

So I know this is a old thread so I'm bumping it back up with a question.
So if your pipes are different lengths you need a cross over pipe, does it matter where the cross over pipe goes? How far from the engine?

 

I'd like to know this too.

 

How does this apply to "true duals" exhaust systems or does it at all? Curious because on my 08 FLHR the rear cylinder has a "Y" so it has a balanced look dual exhaust.

 

Exhaust Scavenging

When an engine starts its exhaust stroke, the piston moves up the cylinder bore, decreasing the total chamber volume. When the exhaust valve opens, 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.
The whole purpose of the cross over is to keep the "low pressure tail" pressure low and not let the "low pressure tail" pressure build up. As the exhaust pressure hits the mufflers on a stock Harley exhaust, there is a resistance that without the crossovers would allow the exhaust pressure to build up and reduce the scavenging effect. The crossover keeps the pressure low by allowing the exhaust to be divided between the two mufflers, thus dissipating the pressure faster. There is no reverse pulse "per say" that will go back up into the cylinder but there would be a pressure increase all the way up to the cylinder which would decrease the reduced pressure in the cylinder and thus reduce the amount of extra air/fuel mixture that is sucked into the cylinder while both valve are open and just before the intake stroke of the cylinder. If you have a tuned exhaust or straight pipes this usually eliminates the build up of pressure in the "low pressure tail" segment of the exhaust.