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Size question

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Size question

Unread postAuthor: niceguyjps131 » Thu May 19, 2005 8:47 pm

Can anyone tell me if they think a spud gun wih a 5ft, 2in pvc barrel and a 4ft, 4in pvc conbustion chamber would be TOO big, wouldnt work or impracticle, and how much propane (without propane meter or injection, just a bernzematic torch) would be needed to fill the chamber without killing myself.
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Unread postAuthor: CS » Thu May 19, 2005 9:26 pm

I would say thats a little to big i would try more like 4" chamber that is 2ft long and a 1 1/2" barrel that is 5 feet long. Your plan for a gun would be a little of a overkill i think unless you had a 10 to 15 foot barrel (that would suck). I dont know what to tell you on the propane because for me at least it is really hard to fill up your gun unless you have a propane injection thing.
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Unread postAuthor: boilingleadbath » Fri May 20, 2005 1:58 pm

I'm not Mr. combustion, but I'm going to extrapolate from my experience with a 'noise maker' I made (icecream container with hole in lid & ignition at bottom). That was 2 liters, compared to the 8 liter volume of your chamber. I used about... 3/4-1 second (at most), so you want 3-4 seconds of pure propane.

Don't wory about killing yourself. The pressure generated is the same no matter what the chamber size. I would place the ignigter in the middle; a chamber that long and that narrow will result in a high pressure flame front - the pressure spike is not good for PVC, and the preformance gains are small.

On the chamber:barrel ratio, you can't make it too big. You will get more power than with a smaller chamber. I would make it smaller for weight reasons, but you don't have to.

Of course, such a chamber would be annoying to get the fule to mix in. You definataly can't rely on diffusion as I did in my noise maker - you'll need a chamber fan. And it won't vent well. I'd go with a smaller chamber - 2' maybe. That still gives you a 2:1 ratio - even an infinite ratio will only give you about 15-20% extra energy over that.
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Unread postAuthor: spudshot » Fri May 20, 2005 3:00 pm

a 4ft combustion chamber would be much to large to easily fuel and to get to fire, 1ft-18" of 4" is about the normal chamber length, and if you go with a 4ft combustion chamber a 5ft 2" barrel will be WAY undersized for it, you'd need about a 10ft barrel to get good ratios with such a large chamber, you'd also need a chamber fan to get such a beast to fire

you being new to the hobby, i'd like to tell you that you DO NOT ever want to place the ignitor directly in the chamber or cap, use 2 screws about 1/4" apart as electrodes, and wire the bbq ignitor to them. also dont use thin clean out caps, some brands are thinner than others, cleanouts are not strong to begin with, if you cant find a good one buy a male adapter and a glue on plug or a glue on cap
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Unread postAuthor: foxcrf450rider » Fri Jun 03, 2005 9:31 am

i am building a pneumatic spud gun that has a 8" x 63" chamber and a 2" x 78" barrel. i know the chamber/barrel ratio is way too high but what is a good ratio to go by?
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Unread postAuthor: boilingleadbath » Fri Jun 03, 2005 9:46 am

Dude, the power increase from the increased volume you get by using 8" instead of 4" is minisule... and 8" fittings cost like 50$
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Unread postAuthor: foxcrf450rider » Fri Jun 03, 2005 9:50 am

i have alraedy got the supplies and was just woundering about the ratio not nessesarily the power
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Unread postAuthor: spudshot » Fri Jun 03, 2005 10:42 am

anywhere from 1:1 to 4:1 is good, are those fittings NSF-PW? because i've never seen 8" PW fittings anywhere
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Unread postAuthor: foxcrf450rider » Fri Jun 03, 2005 10:51 am

I am not sure what NSF-PW is but when i get home i will check. Thanks for the help!
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Unread postAuthor: spudshot » Fri Jun 03, 2005 4:14 pm

NSF-PW is a marking that is on any pressure rated pipe or fitting, those that arent will say NSF-DWV. with such a large gun the 8" has a fairly low pressure rating, and it becomes even more dangerous to have DWV fittings or cellular core pipe (well, in any size cell core will explode) if your fittings arent pressure rated dont even build a pneumatic. DWV is fine for a combustion, so if your fittings arent PW go with a combustion launcher if you cant return your fittings
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