SpudFiles

Hi, this is my first post on a question about my potato cannon. Upon building my potato gun, tests showed that it was very effective, however, I was wondering if there could possibly be too much space in the combustion chamber, creating less pressure upon ignition. The design is in-line, and the main size is 4 inch schedule 40 pvc. The length of 4 inch is 18 inches, and added to the chamber is a three inch reducer, and a three inch screw type coupling. The length of that apparatus is 9 inches. With the items I launch (ie. cans, cups, anything three inch with foam wrapping), I am able to push it all the way to the 4 inch, but then there are obstacles for the objects to push through. I was wondering what kind of feedback I could get from the experts here, thank you.

As far as I am aware, the size of the chamber is essentially irrelevant because you must meter in the correct amount of fuel to achieve ignition in the first place. If you somehow managed to ignite, say, a 3" chamber's worth of fuel in a 4" chamber, then you would lose power, but that is absurd, given that the fuel should distribute itself through the whole chamber if mixed. In summary, you shouldn't have performance issues stemming from chamber size.

Anyone who actually builds combustions, correct me if I'm wrong

So basically your gun has no barrel? Try to accurately describe the gun. What I see in your post is:
Chamber: 4"D x 18"L
Chamber to Barrel junction: 4" reduced to 3" threads, overall length 9"
Barrel: ???"D x ???"L

Yea I'm very confused as well, jimmy.