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Chamber and barrel 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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Chamber and barrel ratio

Unread postAuthor: topelitemarine » Wed Dec 02, 2015 10:57 am

What is the best ratio of combustion cannon? I did some research on forum and some said 80% of chamber and 20% barrel? is there any differences between pneumatic and combustion ? :?: Thanks
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Re: Chamber and barrel ratio

Unread postAuthor: jackssmirkingrevenge » Wed Dec 02, 2015 2:49 pm

In terms of volume, you want your combustion chamber to be about 70-80% of the volume of your barrel for the best performance.
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Re: Chamber and barrel ratio

Unread postAuthor: topelitemarine » Wed Dec 02, 2015 10:59 pm

Thanks how about pneumatic?? Do the same principle applies?
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Re: Chamber and barrel ratio

Unread postAuthor: jackssmirkingrevenge » Thu Dec 03, 2015 2:13 pm

Not really, it depends on what pressure you're using too for a pneumatic - it all depends what you want, do you have size restrictions, do you want to make something as powerful as possible, do you make something more quiet and efficient? GGDT is definitely your friend
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Re: Chamber and barrel ratio

Unread postAuthor: jimmy101 » Fri Dec 04, 2015 6:19 pm

jackssmirkingrevenge wrote:In terms of volume, you want your combustion chamber to be about 70-80% of the volume of your barrel for the best performance.
Actually, as jack knows, that gives the best efficiency, not necessarily the best performance. Bigger gives better performance (as long as the C:B isn't too far out of wack). Seeing as how fuel is basically free and "efficiency" isn't in most shooter's domain of concern the optimal CB ratio isn't really that much of a concern. Get HGDT and play around with it.
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Re: Chamber and barrel ratio

Unread postAuthor: jackssmirkingrevenge » Fri Dec 04, 2015 8:31 pm

Actually, as jack knows, that gives the best efficiency, not necessarily the best performance.


Hadn't it been established that beyond a certain barrel length for a given chamber performance actually begins to go down?

The graph shows a constant chamber and a changing barrel length:

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I see what you mean though, for a given barrel increasing the chamber size will give better performance even if it will be less efficient.
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Re: Chamber and barrel ratio

Unread postAuthor: jimmy101 » Tue Dec 08, 2015 3:06 am

jackssmirkingrevenge wrote:Hadn't it been established that beyond a certain barrel length for a given chamber performance actually begins to go down?
No, BurntLatke showed the optimal CB for efficiency, not for performance. And, notice that CBs in the range of 0.5 to 1 give the same performance, the small differences in the graph over that CB range are not significant, especially for the spud ammo. The CB range of 0.5 to 1.0 is a constant volume chamber attached to a barrels that differed in length by a factor of two, which is a pretty big change in the overall dimensions of the gun. The difference between the optimal CB 0.8 and 0.5 is a decrease in barrel length of 38% with a concomitant loss in muzzle velocity of just 3% (6.5% in KE). Since the shot to shot variability, especially with spuds, is probably a heck of a lot more than that the "optimal" CB ratio really doesn't help all that much.

A bigger chamber with the same barrel will have better performance (if you define performance as muzzle velocity, which is what most spud gunners are interested in). If you increase both chamber and barrel volumes then performance also increases. It is only the one case of constant chamber size that you see a very small effect of CB on muzzle velocity. So to boost performance bigger is pretty much always better and the CB affect is fairly small affect compared to chamber volume and other factors.

Overall, other things have a much larger affect on performance. For example, proper fuel ratio, proper fuel mixing, well formed and tight fitting ammo, consistent ammo mass, etc.
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