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ideal chamber/barrel ratio. please help

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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ideal chamber/barrel ratio. please help

Unread postAuthor: Nertzor » Sat Dec 17, 2011 1:39 pm

ok i just typed up a long post and i accidentally closed the wrong tab, fml.

I need to know the best measurements for a 4' chamber 2' barrel spud gun, I know the optimal ratio is 1.5:1 but i never really bothered with it,


My current cannon is a 1:1 gun I think, has a 1" 4'combustion chamber and a 2" 2' barrel. Launches like clockwork about 100 yards.


would that mean for the 1.5:1 ratio i would need an 18' chamber? or is my understanding all jacked up. and would this yield me better distance.


attached is the picture of my current cannon (and its switchable barrels)
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Unread postAuthor: Gun Freak » Sat Dec 17, 2011 2:18 pm

1" 4'combustion chamber and a 2" 2' barrel

You're getting your units mixed up. Feet is ' and inches is ".

Nice cannon, the attached barrel looks way too big for the current chamber though. It doesn't look like it's even 1:1.
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Unread postAuthor: Nertzor » Sat Dec 17, 2011 5:14 pm

With that barrel. No lol. That was just for if I wanted to shoot anything bigger. The cannon was built around The smaller 2' barrel at it's side
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Unread postAuthor: evanmcorleytv » Sat Dec 17, 2011 5:41 pm

There are 2 ideal ratios, 1.5 to 1, and 2 to 1. a TON of people go inbetween with a 1.75 to 1, or a 1.8 to 1.

Sometimes this is meaningless.. I have an old combustion with a 10 inch by 3 inch chamber, and a 3 foot by 2 inch barrel, and it's a better ratio that ANY other combustion that I've made, it shoots probably 250 yards, and was ghetto chronographed at about 300 FPS.
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Last edited by evanmcorleytv on Sun Dec 18, 2011 9:59 am, edited 1 time in total.

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Unread postAuthor: pneumaticcannons » Sat Dec 17, 2011 10:07 pm

it shoots probably 25 yards, and was ghetto chronographed at about 300 FPS.

......? :?:
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Unread postAuthor: velocity3x » Sat Dec 17, 2011 10:21 pm

evanmcorleytv wrote:it shoots probably 25 yards, and was ghetto chronographed at about 300 FPS.


How can a projectile possible fly at 300 fps, yet travel only 25 yards?
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Unread postAuthor: MrCrowley » Sat Dec 17, 2011 10:25 pm

I assume he meant 250 yards.
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Unread postAuthor: Nertzor » Sat Dec 17, 2011 10:35 pm

Uh back to the subject at hand. With a 2ft 2"pipe barrel I would need a 1.5 foot, or 18 inch 4" chamber for a 1.5:1 ratio? Or is my formula wrong
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Unread postAuthor: Technician1002 » Sun Dec 18, 2011 12:08 am

To answer the question, but not spoon feed, the ratio is based on volume. Use math to calculate the volume of the chamber and the volume of the barrel.

I'm mentioning this because of the 1:1 cannon measurements don't work out.

My current cannon is a 1:1 gun I think, has a 1" 4'combustion chamber and a 2" 2' barrel.


Volume of a cylinder is area times length. Area is Pi X Radius squared.

The 4 inch combustion chamber is;
3.14159 X 2 squared. 2 squared is 4. 4 X pi is 12.56636 square inches of area. Area times 12 inches length is about 150 cubic inches.

The barrel is 24 inches of 2 inch pipe. Radius of 2 inch pipe is 1. 1 squared is 1. 1 X pi is 3.14159 square inches. Area times length = volume. Pi X 24 = about 75 Cubic inches. Your 1:1 is really a 2:1 ratio. I'll let you do the math following the example.

Learn the math.
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Unread postAuthor: evanmcorleytv » Sun Dec 18, 2011 9:59 am

MrCrowley wrote:I assume he meant 250 yards.


Indeed.. Sorry about that..
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Unread postAuthor: jimmy101 » Sun Dec 18, 2011 11:58 am

Download a copy of HGDT and play with the dimensions.

The question really isn't "efficiency", it is how much extra chamber volume are you willing to lug around for a 10% increase in muzzle velocity? (Given that most combustion guns have shot to shot variations of 10% or more.)

There are other things that'll do more to boost performance than diddling with CB ratios > 1. A fuel meter (or syringe) will have a much larger affect on muzzle velocity (and significantly reduce shot to shot variability). A chamber fan will do a lot also.
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Unread postAuthor: Essohbe » Wed Feb 22, 2012 9:27 pm

What size chamber do I need for about a 4foot long, 3inch pipe barrel?
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Unread postAuthor: battlelava » Wed Feb 22, 2012 9:32 pm

Essohbe wrote:What size chamber do I need for about a 4foot long, 3inch pipe barrel?

Like what was just said get hdgt its free and play with the numbers there is a guide on the site were you download it from
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