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cb ratio

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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cb ratio

Unread postAuthor: 91sport_tn » Mon Oct 12, 2009 2:58 pm

I have 2 feet of 3inch pvc for my chamber, and 2 feet of 2inch pvc for my barrel, about how much chamber should I cut off to make it fire efficiently?
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Unread postAuthor: theBOOM » Mon Oct 12, 2009 3:06 pm

Make your barrel longer.. Before I start explaining you your correct size... is your spudgun using metered fuel or is it a spray n spray?
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Unread postAuthor: 91sport_tn » Mon Oct 12, 2009 3:09 pm

It is a spray and pray, and this is just an experimental gun. I already purchased all of the pvc, my options are cutting the chamber or leaving it as is.
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Unread postAuthor: theBOOM » Mon Oct 12, 2009 3:15 pm

Hm I would not suggest cutting the chamber maybe cutting it 4 inches or so, what I would suggest is getting a 6 or so feet barrel to get some good results.
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Unread postAuthor: 91sport_tn » Mon Oct 12, 2009 3:19 pm

I may still get a longer barrel, but I also wanted a 4 inch chamber, but I could not find any 4 inch that was pressure rated.
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Unread postAuthor: theBOOM » Mon Oct 12, 2009 3:23 pm

Yeah a 4 inch chamber 2 feet long would require a 9.5 or so feet barrel .... a 3 inch chamber is still good just a little more compact, yet compact dosent mean less powerful.... ex. the gauntlet
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Unread postAuthor: 91sport_tn » Mon Oct 12, 2009 3:30 pm

alright, thanks man, hopefully I will find what I need.
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Unread postAuthor: SpudBlaster15 » Mon Oct 12, 2009 3:58 pm

If you want to it fire efficiently with that barrel, you're going to need a very short chamber. If you want it to be easy to fuel with aerosol sprays, loud, and capable of firing spuds a great distance, which I believe is the goal of most people, you'll want a moderate sized chamber. I used a 24" long piece of 2" pipe for the barrel of my first cannon, and decided on 12" of 3" pipe for the chamber. It was easy to fuel, loud, and had respectable power. At the time, I couldn't have been happier with it.
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Unread postAuthor: D_Hall » Mon Oct 12, 2009 7:01 pm

I won't ignore the pink elephant standing in the middle of the room....

...If you want it to fire *efficiently*, the first thing you'll want to do is meter your fuel and forget spray and pray.
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Unread postAuthor: 91sport_tn » Tue Oct 13, 2009 12:55 am

I am not going to dump an a** load of money into a cheap first pvc gun, I just want it to be a little quiter, and work a little better. So far it has only cost me $30, which makes me happy, but I am just experiment with it. I am already planning on going out to buy materials for my pneumatic. :)
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 13, 2009 12:59 am

D_Hall wrote:I won't ignore the pink elephant standing in the middle of the room...


Was that a deliberate pun? The proverbial elephant in the room has no specific colour, unless one is inebraited :)
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Unread postAuthor: psycix » Tue Oct 13, 2009 5:19 am

Don't cut it.
Spray and pray is hard to meter in smaller volumes.
On top of that, cutting the chamber would cut the power of the gun.
Even though efficiency increases, power drops even faster.

Unless a silent launcher is the goal, keep it this way.
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Unread postAuthor: D_Hall » Tue Oct 13, 2009 8:41 am

jackssmirkingrevenge wrote:
D_Hall wrote:I won't ignore the pink elephant standing in the middle of the room...


Was that a deliberate pun? The proverbial elephant in the room has no specific colour, unless one is inebraited :)


LOL... No, just an inadvertant usage. The "pink elephant" became a bit of an inside joke amongst some friends of mine many years ago. From there, it just rolled off my tongue...errr...finger tips without my realizing "where I was" when I said it.
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Unread postAuthor: jimmy101 » Tue Oct 13, 2009 3:47 pm

D_Hall wrote:I won't ignore the pink elephant standing in the middle of the room....

...If you want it to fire *efficiently*, the first thing you'll want to do is meter your fuel and forget spray and pray.

Well, actually, the first thing is you have to define what you mean by "efficient". Energy efficient? Operator efficient? Material efficient? Time efficient?

Any way...
A ~1 CB ratio would be a chamber half as long as the barrel. That's a bit small for the chamber so add 40% to get a CB near 1.5 and a chamber length of ~17". That sized chamber will get most of what is possible with 2 feet of barrel. A meter and a fan will make this an OK cannon. Muzzle velocity with half a spud of perhaps 200 feet/second.
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Unread postAuthor: Boom_erang » Fri Oct 30, 2009 6:15 pm

I would say about 12" should be right. That will give you good power but not too big of a bang... much bigger and I think you "waste" the power because so much of the combustion happens after the potato has left the barrel.
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