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Moderator
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Discussion Starter #1
Can someone please explain what the velocity stacks do for our bikes? Why are they different lengths for non equal length exhaust and equal lengths for equal length exhaust? How did HD choose the length that they chose. The reason I ask is I am going with the airbox cover with the gages mounted in it and with that airbox cover the front velocity stack needs to be cut down. How will this affect performance. I am going with SuperTrapp exhaust until someone come out with the PERFECT exhaust (performance and style)

Thank You in Advance
Frank
 

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Geezer
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13,343 Posts
Frank - there is a wave front phenomenon established by the intake valve pulses, and probably the exhaust as well with a high performance engine. The velocity stack established a uniform pulse. The length of the velocity stack probably affects the "resonant frequency" of the entire intake - exhaust system. No stack is bad. Chganging the length will have some effect, but probably only at certain power settings.
 

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Ghost Rider
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533 Posts
We have a similar phenomena on old carbureted race engines, longer stacks are better for higher rpm ranges and vice versa. It depends on where you expect to have your normal operating range or your shift points.

As a practical example, on my racecar we use long stacks as the peak torque and HP goes from 6700 to 8500 RPM and we then geared the transmissions to take advantage of this accordingly. The settings being that when you upshift at approx 8250 RPM the next gear comes in at 6700 RPM thus in the thick of the power band.

On my application all stacks are equal length as are the exhaust headers.

In some modern cars, the intake runners are variable in length to take advantage of boundary air layers applicable at different engine speeds. The idea being maximizing intake velocity and thus the volume of air being drawned in.

Hope this helps. Juan
 

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I just added a 7 mm velocity stack to my 47 Indian. The stack looks like a thin aluminium donut.
I havent riden it yet, but have been advised that when they dynoed the test bike in Melbourne, it produced an extra 6 HP (no idea about torque reading).
6Hp is a lot on a 50 HP bike!

Best wishes.

Ken.
 
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