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C/B Ratio and PSI questions

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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C/B Ratio and PSI questions

Unread postAuthor: BrianMigs » Sat Jan 26, 2008 2:43 pm

I tried to search for these topics but couldn't find anything, or I just am searching the wrong keywords.

I finished my first potato gun last weekend, and I had a bunch of friends over last night to shoot batteries, apples, potatoes, heads of stuffed animals, and other fun stuff.

It seemed that whenever I had launched something I also had some air left over. My chamber is a piece of 2'-3" diameter pipe along with a reducer, and two 90' degree elbows to attach to my 1" sprinkler valve and then to my 1-1/4" barrel that is about 26 inches. Would increasing the diameter and length of my barrel cause my projectiles to reach a higher muzzle velocity and longer distances since I would theoretically be using more of the air in my tank?

Also, because of this current problem would using 70 PSI be more effective than using 50PSI if there is still air left over in the chamber regardless? On mythbusters, when they shot their "chicken gun", they said there was no % increase using 150 PSI over 125 PSI, which makes me curious for the answer to my question.

I am new to this forum and it is one of my favorite places to click around.

Any help is greatly appreciated. Please dont flame me if the answer is here somewhere, please trust that I did use the search button.

Thank you! :D
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Unread postAuthor: Hailfire753 » Sat Jan 26, 2008 3:12 pm

You said you were using a sprinkler valve? Is it modified for higher power? If not, do so.

The leftover air probably will not affect performance, as the projectile will have likely already left the barrel before the sprinkler would be able to seal again. If you want to test this, try holding the valve open longer.
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Unread postAuthor: bigbob12345 » Sat Jan 26, 2008 3:22 pm

You will get better performance if you triple the barrel length but if you dont want a barrel that long double it.I would also think about using a 1.5in barrel which should be atlest 4ft long.And yes there will be an increase in performance if you increase the pressure it wont be that dramatic but it will be noticable.It will also make a louder noise which makes it seem more powerful and as hailfire said mod the valve if you havent already.
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Unread postAuthor: daberno123 » Sat Jan 26, 2008 3:24 pm

Check to make sure that your reducer is pressure rated, it will say NSF-pw if it is. Usually they aren't. If you're only using 70 psi it should be ok but go any higher and you should replace it with a pressure rated bushing.

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Unread postAuthor: BrianMigs » Sat Jan 26, 2008 3:29 pm

Well here is the valve I am using; Rainbird CPF-100

Image

If you have ever seen one of these valves, i cant really drill straight down through the center because of the flow control.

I mean all I would need to do it is 4 hose clamps, some hose, 2 barbed male connectors, and a blowgun tip right?
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Unread postAuthor: daberno123 » Sat Jan 26, 2008 3:33 pm

Not sure about this because i don't know the exact model you have but you might want to try unscrewing the flow control handle (if you can) and then drilling through there.

Looks like you have the basic idea of modding, but you'll only need 2 hose clamps, one for each barbed connector
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Unread postAuthor: DYI » Sat Jan 26, 2008 3:33 pm

To get the most performance at 120 psi out of that chamber with a 1-1/4" barrel, the barrel would need to be ~ 47 feet long. Your chamber is a complete waste of air. You current C:B is about 5.3:1. Increase the porting on the valve, and use a 2" or larger barrel as long as you can carry - with a chamber that size, you don't need to worry about making the barrel too long, unless it's bigger than 4" ID.
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Unread postAuthor: bigbob12345 » Sat Jan 26, 2008 3:34 pm

Just drill a hole in the side and I think you know the rest and you only need two hose clamps not four.You should try to get the orbit watermasters from now on, they are cheaper and easier to modify.
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Unread postAuthor: BrianMigs » Sat Jan 26, 2008 3:44 pm

So drilling anywhere through the top half of the valve should do the trick?

I dont have any drill taps, so if I just drill through with a bit, thread in the barb connector as much as I can, and then use a bunch of epoxy over that, and any other holes will work correct?

I am thinking of going to pick up 3-4 feet of 1.5 inch PVC, that would be better overall for what I am doing.
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Unread postAuthor: daberno123 » Sat Jan 26, 2008 4:27 pm

yes you'll need a half inch drill bit for a 1/4" barbed hose connector.

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Unread postAuthor: Gippeto » Sun Jan 27, 2008 9:21 am

"you'll need a half inch drill bit for a 1/4" barbed hose connector. "

The correct drill size for 1/4" NPT is 7/16" not 1/2"
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