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Piston performance

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Piston performance

Unread postAuthor: Panzerfaust » Sun Jun 10, 2007 11:55 am

Just finished my second piston gun, (first was a small diameter in-line). The gun has a five foot 2" chamber and a 1 1/2" five foot long barrel. The piston is in a tee and has a 1.25" endcap as a piston. It is ported 2 inch in, 1 inch out, 1 inch pilot/fill. I am using a modded 1" sprinkler to fire. The gun is EXTREMELY loud forcing me to wear hearing protection. What i cant understand is that i only gained about 40-60 yards compared to a mich shorter gun with modded sprinkler. Is my chamber to small to utilize my valve? the valve seems to work great, very loud opens fast dumbs chamber instantly. Unfortunately i was shooting water out of the gun, and decided to fire it without planting in the ground. Recoil was 5-6 times that of a 30-06 rifle, blew the gun out of my hands, and when it landed the fill/fire assembly snapped of. Thankfully it was removable anyways so i can replace it. Whats up with this 200-yard performance? seems low for the length of the gun and its valve.
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Unread postAuthor: hi » Sun Jun 10, 2007 12:02 pm

i could have told you that you would be on the ground, i know from experiance. :lol:

my guess is ( this is only a guess) that to much air is getting around the piston and going out the exhaust, but thats just my guess.

ps- what do you have as a bumper? i need one for a valve of simular size and cant find anything.
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Unread postAuthor: jackssmirkingrevenge » Sun Jun 10, 2007 12:04 pm

what sort of ammunition are you using? what calibre is the shorter gun?
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Unread postAuthor: Panzerfaust » Sun Jun 10, 2007 12:09 pm

Shorter gun is diameter barrel/chamber, 2" and 1.5" I am using a piece of rubber hose as a bumper, seems to work well. my first bumper was pipe insulation foam tube, but that got sucked into the pilot. Originally, i had the piston wrapped in duct tape to an extremely tight fit. Probably too-tight as it bound inside my chamber, and i was only able to get it out with extreme force and lots of dw-40. Now i have the piston wrapped in 2 layers of slippery tape. It fits fairly well, if i put it in some pipe i can blow it back and forth by mouth pressure. however, there is a very large amount of air that blows out of the firing valve when it fires. I could try wrapping it with some more tape. It could be however that i have about 4" of dead space behind the piston (4" of 2" pipe"). this was necesary however, to make the piston servicable. I dont think that dead space should be to much of a factor with my pilot being a sprinkler. i am using potatoes, they fit very tightly.

Hey, can anyone tell me how to find flow coefficient so i can have the ggdt crunch this? i know or can guestimate the other variables.
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Unread postAuthor: jackssmirkingrevenge » Sun Jun 10, 2007 12:22 pm

It could be however that i have about 4" of dead space behind the piston (4" of 2" pipe"). this was necesary however, to make the piston servicable. I dont think that dead space should be to much of a factor with my pilot being a sprinkler


That's a lot of waste, 4" is far too much - you only need the piston to move back 0.375" to get maximum flow for a 1.5" barrel.
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Unread postAuthor: Panzerfaust » Sun Jun 10, 2007 12:28 pm

The barrel is 1.5", but the porting out of the valve is 1". The dead space was neccesary to fit in male/femal adapters so that i could service the piston.
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Unread postAuthor: jrrdw » Sun Jun 10, 2007 1:41 pm

Is your barrel all gooed up with potato juice? I've read wear that robs preformance. Sounds like a small chamber also.
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Unread postAuthor: Panzerfaust » Sun Jun 10, 2007 4:05 pm

Barrel is very very new (maybe fired 20-30 times). i am really thinking i should just go for a tighter fitting piston and a bigger chamber. I have a fair amount of nsf-pw 4" pipe (You have no idea what i went through to get it), but my current chamber almost entirely empties my 5.5 gal compressor. with a bigger chamber i would need multible compressor runs to fill it, and i really dont want to have to wait through multible compressor cycles to get one shot. I could try switching to a hot glue Piston, i know they work well but i just hate the idea of my most important part being a wad of glue. Does anyone know how to calculate flow coefficient so i could put my gun into the ggdt? its air out port is 1".
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