Assorted questions from a self confessed noob

Just starting to spud and wonder how long your barrel should be? Want to know how to build a better cannon? This is just the place! If you think your question is one of a newcomer, please post it in here. If your question is posted elsewhere, and it's a newcomer's type question, it will be moved in here.

Postby FLONE » Mon Dec 11, 2006 11:42 am

1) Is there a practical limit to the number of cubic inches of chamber volume that I can reasonably expect a BBQ ignitor to fire?
Not talking about total number of sparks or total spark gap.

2) Does the internal shape of the bushing connecting the chamber to the barrel have a significant affect on the gasses as they exit the chamber? The schedule 80 4" to 2" bushing I just bought is not tapered like the flat bushings I have been using.

3) Over/under combustion style. Where would I count the cubic inches of the elbows: chamber or barrel?

Thanks for all the help.
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Postby An Apple Pie » Mon Dec 11, 2006 11:56 am

1)Pretty sure it will always ignite the gas. The problem is if there is too much volume then the projectile may have already been shot by the time all of the gas ignites. Not sure though. So once you reach a certain point it either won't affect the profomance,or affect it minimally. It will however make it much louder.

2)My guess is that it does affect the profomance although you could not really notice it. So I wouldn't worry about it.

3)If you can just build the gun first and do a water test. Just before you put the barrel on fill it up with water and then measure the volume of the water.

Oops I misread your last question. Ohwell Freefall already answered it.
An Apple Pie
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Postby Freefall » Mon Dec 11, 2006 12:00 pm

1: No practical limit. As long as the mixture is good, a single spark will ignite it.

2: I thought we went over this with your first post. While there is probably some measurable effect, it's not something that will be noticeable without a chrony.

3: If you're using a fan, then that volume is most definitely part of the chamber. Without a fan, that volume may not mix well, and would simply be "dead", not really counted as part of the chamber or part of the barrel. If you're worried about getting the right mix with a meter, just start somewhere slightly rich of stoichiometric and play with it a PSI or two at a time in each direction until you find what works best for you.
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Postby FLONE » Mon Dec 11, 2006 12:11 pm

Yes, I did ask about the bushing before, it was relevant to the amount of constriction.

Thanks for the responses.
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Joined: Wed Nov 22, 2006 5:49 pm



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