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If I have a pressure chamber 15 Inches long with 1.5" diameter PVC pipe how long will a 3/4" barrel have to be to have maximum performance? Couldn't find anything on search specific to this.

if i remember correctly for pneumatics, it is .2:1 ratio is the minimun for the best performance in pnuematics. I always forget for combustins but i think it is .8:1 or 1:1 but dont make my word for the combustion. Could someone clarify for me if i am (which i probably am) wrong.

But barrel length should be nothing to worry about. You could have double the barrel length to the chamber since your are using exactly 1/2 of the chamber volume for the barrel volume. I have a gun similar to what you are building and i get pretty good performance out of mine but i would say that i could make mine longer and get a little better performance. You can see the gun here.

Oh i get it so make the barrel 2x the length of chamber. I think thats what you meant.

Yea pretty much because you could have 1:1 ratio and still be fine in performance.

You are wrong:
A double diameter pipe has MORE then double volume inside
pi*radius*radius*length=volume

so
pi*0.75*0.75*15= volume of chamber =26.5 cubic inches
pi*0.375*0.375*30= volume of your barrel = 13.25 cubic inches
this would give a 26.5/13.25= 2:1 ratio and NOT a 1:1

Edit: you should get the longest barrel you still think its mobile enough.
If you want to run around much with ur gun, you want a short barrel, if u want raw power but not being mobile, just get one helluva long barrel. 120" if you want.

ahh another factor is the valve, what valve are you using?

the valve will affect the length of barrel needed becasue a valve with a slow opening time (a ball valve) need a long barrel to get good performance but a fast opening valve (a sprinkler, qev or piston valve) will get better performance than a slow opening valve from a short barrel.

We can only answer this question properly if you give us the pressure you intend on using.

Assuming you are using 120 psi, you will need a 3/4" barrel approximately 110" long for maximum performance. At 240 psi, you could use a barrel about twice as long.

If you would simply download GGDT, you wouldn't need to ask these questions.

Psyicx wrote: A double diameter pipe has MORE then double volume
so
pi*0.75*0.75*15= volume of chamber =26.5 cubic inches
pi*0.375*0.375*30= volume of your barrel = 13.25 cubic inches
this would give a 26.5/13.25= 2:1 ratio

you are contradicting yourself. I am pretty sure that 2:1 is double. I SAID that 1:1 is something your aiming for in a C:B ratio. But, it isnt 2:1 because your calculations are incorrect. Example:

Chamber = 3.14 * (.75 * .75) * 10 = 17.6625
Barrel = 3.14 * (.375 * .375) * 10 = 4.415625

17.6625:4.415625 = 4:1 C:B ratio

That means that you can multiply the barrel length * 4 so that you get a 1:1 C:B ratio which is what i told him to aim for. So therefore, i am wrong in my first post, but you psicyx didnt correct me, you just added more false info. So you can make the barrel 4 times the length of your chamber to get a good C:B ratio so that it would equal 1:1 which is what i told him to aim for.

The formula for the volume of a cylinder (which pvc sections are cylinders) is ¶r2h. So based off this formula, you get a 4:1 CB ratio.

your error up in your post is that you doubled the length of the 3/4 inch barrel. And you want to use the same length of pvc so that your calculations are accurate.

In most cases, 1:1 or higher is a horrible waste of gas. The only reasons to make a cannon with a C:B that high are
1. Using pressures below 50 psi
2. You can't make the barrel any longer (my problem)

so then what would be a reasonalbe ratio that would be gas effieicent?

At 120 psi, for a given chamber volume, 0.2:1 will be pretty close.

You can find the answers to all these questions by using GGDT. Don't tell me you're a 1st Lieut. and still don't have it!

I am trying to download it now but its not working. I got both of the files in a directory but it still has the run time error. Can you help me?

Ask someone like Pete for help, he really knows what he's doing when it comes to comps.

I will be using, hopefully, 100 PSI with a standard sprinkler valve found on the tutorial. Please make this clearer because several people are saying each other are wrong. just tell me how long i should make this 3/4" barrel to 15 inches of 1.5" chamber. It needs to be mobile.

LikimysCrotchus5 wrote:

Psyicx wrote: A double diameter pipe has MORE then double volume
so
pi*0.75*0.75*15= volume of chamber =26.5 cubic inches
pi*0.375*0.375*30= volume of your barrel = 13.25 cubic inches
this would give a 26.5/13.25= 2:1 ratio

you are contradicting yourself. I am pretty sure that 2:1 is double. I SAID that 1:1 is something your aiming for in a C:B ratio. But, it isnt 2:1 because your calculations are incorrect. Example:

Chamber = 3.14 * (.75 * .75) * 10 = 17.6625
Barrel = 3.14 * (.375 * .375) * 10 = 4.415625

17.6625:4.415625 = 4:1 C:B ratio

That means that you can multiply the barrel length * 4 so that you get a 1:1 C:B ratio which is what i told him to aim for. So therefore, i am wrong in my first post, but you psicyx didnt correct me, you just added more false info. So you can make the barrel 4 times the length of your chamber to get a good C:B ratio so that it would equal 1:1 which is what i told him to aim for.

The formula for the volume of a cylinder (which pvc sections are cylinders) is ¶r2h. So based off this formula, you get a 4:1 CB ratio.

your error up in your post is that you doubled the length of the 3/4 inch barrel. And you want to use the same length of pvc so that your calculations are accurate.

Look over my calculations: i didnt use the same length for the chamber as for the barrel but I used 15" chamber (the one he has) and the 30" barrel (wich you supposed to do the chamber length x2 as barrrel lenght and then said something about a 1:1 ratio.)

My calculations arent incorrect, i just used other numbers.


Edit:

Oh i get it so make the barrel 2x the length of chamber. I think thats what you meant.

Yea pretty much because you could have 1:1 ratio and still be fine in performance.

And i proved it was NOT a 1:1 ratio.

Edit 2:
Ive read over some of your posts again and i conclude we had a miscommunication.
You are thinking I wanted to calculate a 1:1 ratio and made a mistake, but I just wanted to prove that 2x chamber lentgh barrel is NOT a 1:1 ratio.