Crossover pipe question
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Seasoned HDF Member
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From: Kalifornia
Crossover pipe question
I put my new (to me) SE slip ons on the bike today, and noticed the crossover pipe is anything but round. Is this normal?
If yes, are the aftermarket headers like this too, or is the crossover a separate pipe from the headers? I guess the correct question is, do aftermarket headers COME with a ROUND crossover pipe. I cannot see this being good for performance at all.
If yes, are the aftermarket headers like this too, or is the crossover a separate pipe from the headers? I guess the correct question is, do aftermarket headers COME with a ROUND crossover pipe. I cannot see this being good for performance at all.
Stellar HDF Member
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From: NW Kentucky
Most aftermarket exhaust systems have a round crossover. Some have a full size and some have a reduced size that is similar to HD original size and use HD hardware. There are all kinds out there. Look at the specs before you buy. I have a Magnaflow Tru-X head pipe which is round full size all through the system. Some head pipes use the stock HD crossover pipe. I don't know the flow numbers or advantages but I believe the round full size systems should be better, especially if you start adding cubic inches.
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Seasoned HDF Member
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From: Kalifornia
Copied this from a air conditioning site, should explain the difference somewhat.
They compared a 6 round duct to two forms of oval duct made from a 6 round: 3 x 7.71 and 3.25 x 7.57. Their measurements showed that those two sizes of oval duct are equivalent to round duct at 5.02 and 5.18, respectively. (See page 20.)
Just because a 6" oval comes from the same piece of metal as a 6" round doesn't mean they have the same cross-sectional areas. They don't, so you would expect them to move air differently. If you need to move the same air as in a 5" round, you've got to use a 6" oval. For a 6" round, you need an 8" oval.
So based on this information, if the HD uses a 1.75" round exhaust pipe for an oval crossover to allow the same or similar air flow, it would need to be a 2.1" pipe, in oval form. NOT a pipe 2.1" wide, but a 2.1" pipe compressed to oval. Would make it about 3.5" wide x 1.25" tall. Those measurements are estimates, but you get the picture. A considerably larger crossover is needed to equalize flow to the mufflers, since HDs crossover is most likely allowing between 50-75% the airflow of their round pipes.
So obviously a true diameter, round, crossover is needed.
They compared a 6 round duct to two forms of oval duct made from a 6 round: 3 x 7.71 and 3.25 x 7.57. Their measurements showed that those two sizes of oval duct are equivalent to round duct at 5.02 and 5.18, respectively. (See page 20.)
Just because a 6" oval comes from the same piece of metal as a 6" round doesn't mean they have the same cross-sectional areas. They don't, so you would expect them to move air differently. If you need to move the same air as in a 5" round, you've got to use a 6" oval. For a 6" round, you need an 8" oval.
So based on this information, if the HD uses a 1.75" round exhaust pipe for an oval crossover to allow the same or similar air flow, it would need to be a 2.1" pipe, in oval form. NOT a pipe 2.1" wide, but a 2.1" pipe compressed to oval. Would make it about 3.5" wide x 1.25" tall. Those measurements are estimates, but you get the picture. A considerably larger crossover is needed to equalize flow to the mufflers, since HDs crossover is most likely allowing between 50-75% the airflow of their round pipes.
So obviously a true diameter, round, crossover is needed.
Stellar HDF Member
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From: NW Kentucky
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Club Member
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From: Beyond Fucital
The cross over is to just ballance pressure and used to increase exhaust flow for narrower pipes.
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I'm pretty sure H-D doesn't use the Residential Duct Systems manual (Manual D) to design their exhaust systems. 
J/K
The post above answers why the crossover pipe is oval. Clearance issues is another reason it's oval.
J/KThe post above answers why the crossover pipe is oval. Clearance issues is another reason it's oval.
