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Decisions on my Last Combustion Gun

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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Decisions on my Last Combustion Gun

Unread postAuthor: Pete Zaria » Mon Nov 20, 2006 4:20 am

I decided to build one "perfect" combustion gun, and throw away all my old and jury-rigged ones (Hey, it's a learning process). I'd like some opinions from the experts here on a few things.

I'm going to base this gun on
This design, which is over/under, propane metered/injected, stungun/sparkstrip ignition, ball valve venting, cam-lock pass-through barrel, fan, etc... Gort has pointed out that the main problem with this design is the lack of a way to access the chamber. I play to attach the back 4"-2" reducer (that connects the venting valve to the barrel) with 8 or so screws, instead of glue, to allow access to the chamber if repairs are needed. Any objections?

I'm going to use ABS. I've pressure tested it and feel safe with it. It won't shrapnel if it fails (as PVC will) and is much cheaper than NSF-PW PVC parts. Plus I already have a lot of ABS stock laying around.

I'm going to mount it on a stand with wheels anyway, so I'm going for a 10', 1.5" (or 1.5" SDR21) barrel. So, obviously, I have to pick my C:B ratio. I want to use 4" for the chamber. I have 5' of 4" ABS, plenty to play with.

10' of 1.5" ABS sch40 is approx. 206 cubic inches of volume.

I was thinking about a 1.5:1 ratio, which would be roughly 25" of 4" pipe. Any suggestions on this? Is 1.5:1 a good number for a large combustion gun?

Where in the chamber would be the best place for the spark strip, and the fan? Is it worth it to try to suspend the spark strip in the center of the chamber, or just mount it to the side of the chamber, near the back/middle?

The last thing I'm unsure on is what kind of stand to build. I'm kind of clueless here. I was thinking of using the bases from two office chairs and some 2x4"s to build an adjustable stand, or maybe two big wheels in the back and an adjustable height bipod in the front. Suggestions?

Thanks a ton for the input, guys. I'll post pics of the build process and finished product.

Peace,
Pete Zaria.
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Unread postAuthor: killagorrila99 » Mon Nov 20, 2006 4:36 am

Sounds alright, Should be BLOODY POWERFULL with a 10' barrel.
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Unread postAuthor: rna_duelers » Mon Nov 20, 2006 5:04 am

I believe the 'best' ratio for a metred propane injection combustion is a .8:1 C:B.The 1.5:1 is said to work better for the more conventional spray arosol(sp) type cannons.
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Unread postAuthor: BC Pneumatics » Mon Nov 20, 2006 2:54 pm

rna_duelers wrote:I believe the 'best' ratio for a metred propane injection combustion is a .8:1 C:B.The 1.5:1 is said to work better for the more conventional spray arosol(sp) type cannons.


***With a fixed chamber size
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Unread postAuthor: mark.f » Mon Nov 20, 2006 10:38 pm

Exactalmente.

Just use the whole 10' stick of 1-1/2", and adjust chamber size accordingly. When I do this, I usually just set it at a simple 1:1 ratio.
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Unread postAuthor: Pete Zaria » Mon Nov 20, 2006 11:50 pm

Markfh, that's exactly what I said I'm doing; a 10ft 1.5" barrel, and 4" x ?? combustion chamber. I'm trying to decide how big a chamber to use. Since it's an over/under, size isn't really a problem. I could do as big as 500ci, but it seems just unnecessary and ridiculous. I think I'm gonna go for 1.5:1 . My primary ammo will be golf balls. What do you think?

Peace,
Pete Zaria.
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Unread postAuthor: SpudBlaster15 » Tue Nov 21, 2006 1:18 am

The larger the chamber, the more power, but at a certain point the gas costs start to outweigh the minimal performance gain. Stick with 1:1 or 1.5:1.
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Unread postAuthor: abcdefg » Fri Nov 24, 2006 6:09 pm

if u have a ten foot barrel how do u move the gun...

is the pipe presure rated, cause it wouldn't be fun blowing yourself up with all that power :shock:
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