jsefcik wrote:Does the shape of the chamber make any difference?
Yes, but it's trivial in most cases, and not simple to calculate (GGDT doesn't even try to model it, and HGDT only concerns itself with the difference it makes to igniting the mix).
Explain what you need more of,im using 3 inch pvc for the chamber defintly 5 feet , barrel i was told double my chamber which woulf be 10 im not using that long of a barrel thats insane in my eyes
That's a matter of C:B ratio, where the point at which the diminishing returns become less than worthwhile is a matter of personal preference.
You can model said diminishing returns, but it depends on valve flow, valve speed, barrel diameter, intended projectile, chamber pressure and a couple of other things. (Like whether you're fixing your chamber volume or barrel volume, although you have already said that)
I dropped this old diagram of mine into another topic recently:
This assumes a fixed barrel volume, and simulates the results of relative chamber sizes (x-axis) against relative muzzle energy (y-axis) with three different combinations of valve and projectile (blue, red and green lines).
As you can see, the difference means that these systems diminish at very different rates when the C:B ratio is changed. The blue line is still significantly improving at C:B ratios of above 3:1, but the green line is pretty much maxed out by 1:1.
My best advice on the matter is to decide how long you're prepared for the barrel to be,
then decide how big the chamber should be. For my sort of launcher, that would generally be in the same region as the barrel volume, give or take a factor of about two.
However, the only way to figure out for certain what C:B ratio is worthwhile to you is, as matti says, learning GGDT and experimenting yourself.