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barrel size

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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barrel size

Unread postAuthor: jackh » Tue Jan 22, 2008 4:39 pm

i have one gun that has 2 feet of 3 in pipe for combustion chamber and 4 ft of 1.5 in barrel. it shoots farther/better than my other gun that has 4 ft of 4in combustion chamber and 4 ft of 2 in barrel. is this just because i dont spray enough right guard? or will the second gun shoot better with a 1.5 in barrel? it seems it would because all the energy would be more compressed in the smaller barrel.

i do about a 3.5 second spray of right guard in the smaller gun and about an 8 second spray in the bigger gun. i havent tried more in the bigger gun yet havent gotten enough time.
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Unread postAuthor: daberno123 » Tue Jan 22, 2008 4:46 pm

I'm guessin that your ratio is better on the bigger one, plus the bigger chamber.But don't go by that because i'm too lazy to take the time to figure them out. The perfect C:B ratio is .8:1

BTW there is no way to spray in to much right guard because if you do it simply won't ignite
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Unread postAuthor: BigGrib » Tue Jan 22, 2008 4:59 pm

Yeah dude, check out this deal right here cause this will help you out in your quest for a c:b ratio. This deal will calculate everything for you, just punch in the numbers and bingo.

http://www.advancedspuds.com/SpudToolonline.htm
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Unread postAuthor: jackh » Tue Jan 22, 2008 11:05 pm

dam so i should really have a barrel thats 4 times as long as my chamber for it to shoot well? thats gay i thought that if i just had a smaller barrel the energy would be compacted for a farther shot. i guess the pressure builds up throughout the barrel
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Unread postAuthor: jimmy101 » Wed Jan 23, 2008 4:12 pm

jackh wrote:dam so i should really have a barrel thats 4 times as long as my chamber for it to shoot well?

No, the barrel length to chamber length ratio depends on the diameters of the two pipes.

With a 4"ID chamber and a 2"ID barrel the relative area is;
4*4/2*2=4
For a 1:1 chamber to barrel volume ratio, the barrel should be 4X longer than the chamber.

With a 3"ID chamber and a 1.5"ID barrel the relative area is;
3*3/1.5*1.5=4
For a 1:1 chamber to barrel volume ratio, the barrel should be 4X longer than the chamber.

With a 4"ID chamber and a 1.5"ID barrel the relative area is;
4*4/1.5*1.5=7.11
For a 1:1 chamber to barrel volume ratio, the barrel should be 7.11X longer than the chamber.
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Unread postAuthor: OuchProgramme » Wed Jan 23, 2008 4:27 pm

I thought right guard didn't work anymore....?
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Unread postAuthor: jackh » Wed Jan 23, 2008 6:10 pm

OuchProgramme wrote:I thought right guard didn't work anymore....?


ya thats what that huge thread says but i bought a can on saturday and it works fine...
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Unread postAuthor: jackh » Wed Jan 23, 2008 6:12 pm

jimmy101 wrote:
jackh wrote:dam so i should really have a barrel thats 4 times as long as my chamber for it to shoot well?

No, the barrel length to chamber length ratio depends on the diameters of the two pipes.

With a 4"ID chamber and a 2"ID barrel the relative area is;
4*4/2*2=4
For a 1:1 chamber to barrel volume ratio, the barrel should be 4X longer than the chamber.

With a 3"ID chamber and a 1.5"ID barrel the relative area is;
3*3/1.5*1.5=4
For a 1:1 chamber to barrel volume ratio, the barrel should be 4X longer than the chamber.

With a 4"ID chamber and a 1.5"ID barrel the relative area is;
4*4/1.5*1.5=7.11
For a 1:1 chamber to barrel volume ratio, the barrel should be 7.11X longer than the chamber.


thats a lot of math. so if i follow these guidelines, my gun should shoot well? so basically i just need to make my barrel longer?
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Unread postAuthor: jimmy101 » Wed Jan 23, 2008 7:11 pm

jackh wrote:thats a lot of math. so if i follow these guidelines, my gun should shoot well? so basically i just need to make my barrel longer?

Yes, your barrels are too short.

But, more important than the barrel size, you need a better way to mesaure your fuel. "Squirt-and-screw" ("spray-and-pray") fueling sucks. It is too inconsistent if you are trying to get the best performance from your hunks of PVC pipe.

Get a disposable butane lighter ($0.99) or a Bernzomatic torch ($15) and a syringe ($2). (like this or this)

A chamber fan will also help a lot.
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Unread postAuthor: jackh » Thu Jan 24, 2008 3:54 pm

thats too complicated for me at the moment. i just like the extremely simple design of the spray and prays. i guess ill just trash the big gun i dont want a 12 ft gun lying around i dont have any place for it. is there any way to get the peices apart? i glued them with pvc primer and pvc cement
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Unread postAuthor: daberno123 » Thu Jan 24, 2008 9:11 pm

No there is no way to get them apart. Once glued with primer and cemnt they are solvent welded together until the end of time. Build a new gun with C:B in mind
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Unread postAuthor: jackh » Thu Jan 24, 2008 10:43 pm

in theory, with the right C:B ratio in mind, the larger the chamber the more distance you get correct?
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Unread postAuthor: jimmy101 » Fri Jan 25, 2008 1:07 pm

jackh wrote:in theory, with the right C:B ratio in mind, the larger the chamber the more distance you get correct?

In general yes. If you increase the size of both the chamber and barrel you'll increase the muzzle velocity.

But, the larger the chamber the more difficult it is to get it to fuel properly. "Squirt-n-screw" is going to really suck as a way to fuel a large chamber.

A smaller chamber with a meter and fan will probably out perform a much larger chamber fueled with "Squirt-n-screw".
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Unread postAuthor: jackh » Fri Jan 25, 2008 4:17 pm

what does the fan do? air it out after each shot? id rather just save my money and blow it it after each shot if thats the case
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Unread postAuthor: DYI » Fri Jan 25, 2008 4:57 pm

The fan's primary purpose is to induce turbulence in the gases, increasing the burn rate significantly. This has been shown to increase performance by over 30% in some launchers.

In terms of performance gains, fans are more effective than metered propane and multiple ignition points combined.
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