Whats the best C:B ratio for a combustion Cannon?

[amentic]

Im building my first combustion cannon and i got a 2" PVC pipe for my barrel and a 4" pipe for my chamber. I want my barrel to be 48" (4 feet) long and i was wondering what i should have my chamber cut to. I got it cut at 24" right now but i dont know if it should be any shorter. any suggestions?

I cut it at 24" cuz i heard that a 1:1 volume ratio was the best, but i also heard that 1.5:1 was best and also that .7:1 was best. its a big difference in length so i would like to know what anyone would think would work. Thanks guys.

[iamthewalrus]

i think u want about a 10" chamber

[Rambo]

Teoreticly the most efficient ratio is C:B 0.5:1 .This way the gun is very quiet.

[MisterSteve124]

Why do people want the gun quiet? Thats one of the best parts the Big Boom.

[FiveseveN]

Some use their cannons in combat simulation (where you wouldn't like to give away your position) or in urban areas (some neighbours may not find the Big Boom quite as entertaining). To make it louder, make the ratio smaller than 0.8/1 and/or add a burst disk. A burst disk adds a lot of boom. Par example, in my gun it goes from "fart" to "firecracker"
Also, C:B ratios depend on fuel, altitude, temperature, ignition position etc. What works for some may not work for others.

[Shrimphead]

Umm, ya Rambo, the most effecient is from .6:1-.8:1. A .5 c:b ratio will make it quieter but also less powerful.

@ Amentic- Trying to figure out the right ratio is only useful in two circumstances:
1. To find the best barrel length for a certain chamber size. If you already know how long you want your barrel to be, then just make sure that your chamber volume is at least 80% of your barrel volume.
2. To minimize the sound that the gun creates for the reasons that FiveseveN stated. If one were to make a tp gun, then they would want it to be quiet.