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C:B ratios fixed barrel length with enlarged chamber

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C:B ratios fixed barrel length with enlarged chamber

Unread postAuthor: durckelg » Sun Jul 02, 2006 10:14 pm

I have a question about C:B ratios for metered propane combustions canons. I know from reading the test results at http://www.burntlatke.com that the optimal C:B ratio for a fixed size combustion chamber is ~.7 or .8 :1. My question is, If you added a larger chamber to a gun that was already at that .7 or .8:1 ratio, will it slow the velocity of the spud? My guess is it will do little for the velocity of the spud, but make a louder bang. I realize lengthening the barrel at this point would increase velocity.

If this question has been answered, I apologize. I have tried looking all over the net for a test with a fixed size barrel and varying sizes of combustion chambers.
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Unread postAuthor: huse_spud » Sun Jul 02, 2006 10:17 pm

umm... if you look at the graphs at burntlatke you can answer your own question.

but i think he calls his preffered ratios the most effiecient and not necessarily the most powerful.

different barrel and chamber diameters and launcher configuration can also make a difference too.
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Unread postAuthor: durckelg » Sun Jul 02, 2006 10:54 pm

huse_spud wrote:umm... if you look at the graphs at burntlatke you can answer your own question.

but i think he calls his preffered ratios the most effiecient and not necessarily the most powerful.

different barrel and chamber diameters and launcher configuration can also make a difference too.


Thank you, unfortunately I was afraid this was going to be the kind of response I would get. Maybe I am not being clear enough. The graphs on burntlatke represent how to get the most out of a fixed size chamber, or transferring as much energy to the projectile as possible before friction from excess barrel slows it down. However if I were to add a large chamber (more energy) to the barrel, I do believe that the projectile would not be slowed. However the extra energy would not be transferred to the projectile because of the lack of extra barrel length. The extra energy would dissipate as sound. I realize my theory may be incorrect due to the chemistry and physics of combustion, i.e. optimal combustion pressure may not be reached before the projectile leaves the barrel.

Any help will be appreciated.
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Unread postAuthor: frogy » Sun Jul 02, 2006 11:04 pm

If you have a gun that shoots a 50 gram bullet with 25 grams of gun powder 100fps, and you add 25 more grams of powder will it shoot slower?

Your questions are pretty self explaining...

You get the most velocity out of your propane at .8:1, but if it's 2:1 it's still going to be more powerful, it'll just use more propane...
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Unread postAuthor: boilingleadbath » Mon Jul 03, 2006 12:57 pm

Yes, a larger chamber (to a degree) will give you more power.
The 'degree' being best illusrated by the salt-mine scenario: the gasses might take 15 seconds to burn, by which time the potato has long ago left the barrel. The pressure in the barrel during it's time there was pretty low, so the projectile never got much velocity.

For reasonable sized chambers, however, this isn't a concern.

Using EVBEC, here's a chart:
(2"x50" barrel, 100g projectile)
.8:1 -> 325 fps
1:1 -> 350 fps
1.5:1 -> estimated 380 fps

That's as far as latke provided us with data, though...
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