I had a year old Velocette Thruxton, which came new with an inlet stack:

 

only one word to describe it "SWEET"
although by today's standards where style trumps practicality the older machines had an appeal.
the indian line of motorcycle (original) were interesting. i had a vertical twin that i inherited but was offered too much money for the parts value and i let it go. now if it were the inline 4, would probably still have it. over the years, i had over 40 diff machines. usually got them as seized or wrecks and rebuilt them. my father was a naval machinist so parts were not an issue.
the across the pond machines all had style.

 

Living across the pond, we had tons of wartime Harleys and some Indians left behind, when the US forces returned home in the '40s. I had a 500cc Indian twin for a while, a non-runner, but the cost of replacement parts to rebuild the engine, in the late '60s early '70s, was eye-watering over here, so I let that go eventually.

 

this was an indian scout 249 which was 440cc. it resembled brittish machines and was way advanced over usa competition. this had twin ignition systems and gear shift indicator. i had the complete machine including the original battery. it was in my uncle's barn in illinois for decades, he quit riding when a truck passed too close with west coast mirrors and put a metal plate in his head, doctors forbade him to ride.

in its heyday would have been like this
http://www.themotorcyclemuseum.com/i...Scout-1080.jpg

 

I've never seen a real one of those 249s in the metal, but they do look odd, in as much as the primary is on the right, whereas so many brands have it on the left. Good looking bikes:

 

That works.
Scott

 

in times past, bells were added to air cleaners, VW and MG as examples. however, this only equated to only 1>2 hp gains that could be proven. also in the past, funnels were sold to mount on the bottom of air cleaners, i even ran one. a stack inside of an air cleaner can be counter-productive since it adds a restriction to the volume of the air cleaner. they make a thing that is called a pod, if i remember correctly which mounts to the carb throat and adds a radius leading to the venturi opening but is only around 1" in protrusion. a big issue with a stack in a filter is the way the air flows through the media. will it work, hummmm, good question??? it definitely would be a bear to tune if you are looking for that. another issues as the media clogs, so does cause and effect.
if setup correctly, a butterfly cleaner in a ram mode probably would be better if the shutters were timed right. if not, the ram air could upset carb function as the are not a positive pressure devise as a norm.
i do not know if hd tunes the cleaner to the system but other makes have a science of it. look at the rice super slab machines, everything matches up for max performance.


i was wrong, not called a pod. the jpeg below is what i was referring to.

 

Scott did prove on the dyno with my 98 Evo that a velocity stack can make a difference. IIRC it was 2-3 hp... The S$S carbs work on the same theory, increased velocity.. Instead of running a velocity stack a 1" spacer gave close to the same results.....

 

very true very true
the extra plenum volume holds the extremely well mixed fuel aided by the reversion or reverse pulse wave, same as the stack, burning twigs vs logs, more power.