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Ratios? 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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Unread postAuthor: jimmy101 » Fri Sep 21, 2007 11:12 am

jipi2084@hotmail.com wrote:Hi I'm new and I already got 2 combustion spud gun !

Now i want to make one bigger and stronger ...

i have a 4'' chamber 18'' long = 226 ci
with a 2'' barrel 25'' long = 157 ci

226/157 = .69
!


That math doesn't look correct;
4*4*18/(2*2*25) = 2.88 !!!

Lets see;
volume chamber = PiR<sup>2</sup>L = (Pi)*2<sup>2</sup>*18 = 226 ci
volume barrel = PiR<sup>2</sup>L = (Pi)*1<sup>2</sup>*25 = 78.5 ci

C:B = 226/78.5 = 2.88

The gun's performance will suck. The barrel needs to be roughly three times longer for the chamber size.
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Unread postAuthor: Guest » Fri Sep 21, 2007 10:06 pm

Thanks so can you help me ?


Chamber : 4'' X 18'' = 226 ci.
Barrel : 2'' X ?'' = ??? ci.


my two first one were really not well made lol
:?


I read that your barrel should not be 1.5 time smaller than the chamber size like 4'' should not be plugged with smaller than 1.5'' barrel Is that true ?
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Unread postAuthor: Novacastrian » Fri Sep 21, 2007 11:35 pm

3.1416 x the radius of your barrel diameter squared x the length of said barrel.

So, 3.1416 x 50.8(mm) x length (in mm)
Sue me i'm metric :lol:
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Unread postAuthor: jimmy101 » Sat Sep 22, 2007 12:34 pm

jipi2084@hotmail.com wrote:Thanks so can you help me ?

Chamber : 4'' X 18'' = 226 ci.

my two first one were really not well made lol
:?

volume chamber = PiR<sup>2</sup>L = (Pi)*22*18 = 226 ci
volume barrel = PiR<sup>2</sup>L = (Pi)*12*25 = 78.5 ci

CB = (chamber volume)/(barrel volume)

Rearrange the equation to;
(barrel volume) = (chamber volume)/(CB)

A reasonable CB is anything from about 0.6 to 1.0, we'll choose the oft quoted 0.8. You've already figured out that the chamber volume is 226 ci. Plug these number into the equation;
(226 ci)(0.80) = 180.8 ci

Use the formula for the volume of a cylinder;
(volume barrel) = Pi*r<sup>2</sup>L

Since we want find L (i.e., solve for L), rearrange the equation to;
L = (volume barrel)/(Pi*r<sup>2</sup>)

Plug in the values for the volume of the barrel (180.8 ci), and the barrel radius (1");
L = (180.80)/(Pi*1<sup>2</sup>) = 57.6 inches

So the barrel should be ~58 inches long. Going up or down ~15" inches in the barrel length won't make much difference in the gun's performance.

Always a good idea to make sure the units check out. Looking at the equation above in terms of just the units;
L = (in<sup>3</sup>)/(in<sup>2</sup>) = inches

So the units are correct.
jipi2084@hotmail.com wrote:I read that your barrel should not be 1.5 time smaller than the chamber size like 4'' should not be plugged with smaller than 1.5'' barrel Is that true ?

Not really. There is really no safety issues with say a 1" barrel on a 4" chamber. There also really isn't any choke issues, the amount of choke is really just a function of the barrel diameter. A 1" barrel will have basically the the same degree of choke whtether it is attached to a 2"D or a 4"D chamber.

In general, combustions don't preform very well with barrel IDs of less than an inch or so. To get decent performance the barrel would have to be very long.
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