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chamber to barrel ratio question

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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chamber to barrel ratio question

Unread postAuthor: beans » Sun Apr 06, 2008 12:38 pm

Hey guys, new here, new to the whole spud cannon thing. I've always wanted to make my own spud cannon since I saw one in highschool (crap, that was like 7-9 years ago...) but never got around to it because I didn't want to fiddle with something that could potentially blow up on me. Then recently I saw that someone made one out of copper piping (I think it was american chopper) and used compressed air rather than combustion...and I thought to myself, "that's gotta be safer and just as fun...." and as far as I know, it is! I made a simple copper canon that's just an air chamber, ball valve, and a barrel. So far, it's working great for me, but I'm always looking for ways to improve things. So, I want to make the air chamber bigger...but before I do that, I want to know how I would benefit from it. That is, how much bigger should I make it? Right now, my barrel to air chamber ratio is practically 1:1...the barrel is a 1 inch pipe that's got to be at least 4-5 feet long...the airchamber is a u shaped assembly I made out of 1 1/4 pipe that has a little bit more volume than the barrel, but pretty much like I said, 1:1. Do I really need more volume in the air chamber, or am I pretty much good to go? If I do add more volume, what kind of results will I get at 100-120psi? sorry if I'm not being clear, but I would appreciate the help.
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Unread postAuthor: Sergeant Dotson » Sun Apr 06, 2008 12:58 pm

Making the chamber ratio larger could greatly increase velocity and accuracy. I would suggest maybe a 1 1/2 foot 2 inch galvanized steel chamber. It is a bit expensive, but no more than copper. Hope I was a help.

P.S.
100-120 would bring uber ownage... but if you do use copper or steel you could easily get from 200-300 PSI.

-Sergeant Dotson 8)
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Unread postAuthor: beans » Sun Apr 06, 2008 1:00 pm

well I do have about another 5 feet of copper 1 inch (had to buy a 10 foot pipe for the barrel, and just the barrel...) so I could just add that. Thanks for the help though, I really appreciate it.
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Unread postAuthor: beans » Sun Apr 06, 2008 1:01 pm

I'm using a tire valve to as the inlet, so I don't know if it can handle that...I'm not sure...either way, I'm just using a bike pump, so to get it that high takes a bit of effort...and I'm kinda lazy. How much can it handle, though?
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Unread postAuthor: Sergeant Dotson » Sun Apr 06, 2008 1:03 pm

Well, it's not really the length of the chamber, but the size (ex. 2 inch.)

Also, if you ever need me, just PM me... :D
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Unread postAuthor: SpudFarm » Sun Apr 06, 2008 1:03 pm

you do not get so much more increase in performance with bigger chamber..
you have to make the barrel longer also and don't go lower than you are..

if you make the chamber and barrel twice a long you get a extreme increase in performance..
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Unread postAuthor: SpudFarm » Sun Apr 06, 2008 1:05 pm

it is not either of length and size it is volume.
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Unread postAuthor: Sergeant Dotson » Sun Apr 06, 2008 1:05 pm

Actually man, not to be argumentative, but I think a 5 ft. barrel is long enough. But I agree that you definately should not decrease chamber size.

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A tire valve can probably hold in the one-hundreds. If I were you, I would go buy a shrader valve or a snap-in valve from Pep Boys.
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Unread postAuthor: SpudFarm » Sun Apr 06, 2008 1:08 pm

there i not wourth to make the chamber bigger without follow up with the barrel.
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Unread postAuthor: jrrdw » Sun Apr 06, 2008 1:12 pm

Rule of thumb is 2:1, longer barrel gives better acceleration to the ptojectile, there is a limit. I'm not good with the math, hopefuly someone who is will chime in and help you out.
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Unread postAuthor: Sergeant Dotson » Sun Apr 06, 2008 1:15 pm

I heard through the grapevine that for every foot of chamber, you should have 2 1/2-3 feet of barrel.
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Unread postAuthor: beans » Sun Apr 06, 2008 1:16 pm

I was just saying that I have an extra 5 feet of copper piping, so I was saying I could use that to add volume to the air chamber, not length.
this is pretty much the valve I got:
http://www.stockcarproducts.com/acc21.htm

the only mod I want to make is make the chamber bigger, I'm not planning on touching the barrel.
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Unread postAuthor: SpudFarm » Sun Apr 06, 2008 1:18 pm

depends of you chamber diameter..

my marble cannon has a ratio of 0.7:1 and i have chronoed 1046fps.
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Unread postAuthor: D_Hall » Sun Apr 06, 2008 1:28 pm

Talk all you want of barrel/chamber ratios... Your biggest improvement at this point would come from ditching that ball valve and getting something faster. The standard "quick and easy for the beginner that still works well" valve is the modified sprinkler valve.

Use the search feature of the board, there should be PLENTY of information available.
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Unread postAuthor: beans » Sun Apr 06, 2008 1:30 pm

I'm looking to get a QEV, but I don't know from where....mcmaster has them, but I can't find them on their site...
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