Decisions on my Last Combustion Gun

Pete Zaria · Posted: 11/20/2006 4:36 AM Post subject:

I decided to build one "perfect" combustion gun, and throw away all my old and jury-rigged ones (Hey, it's a learning process). I'd like some opinions from the experts here on a few things.

I'm going to base this gun on
This design , which is over/under, propane metered/injected, stungun/sparkstrip ignition, ball valve venting, cam-lock pass-through barrel, fan, etc... Gort has pointed out that the main problem with this design is the lack of a way to access the chamber. I play to attach the back 4"-2" reducer (that connects the venting valve to the barrel) with 8 or so screws, instead of glue, to allow access to the chamber if repairs are needed. Any objections?

I'm going to use ABS. I've pressure tested it and feel safe with it. It won't shrapnel if it fails (as PVC will) and is much cheaper than NSF-PW PVC parts. Plus I already have a lot of ABS stock laying around.

I'm going to mount it on a stand with wheels anyway, so I'm going for a 10', 1.5" (or 1.5" SDR21) barrel. So, obviously, I have to pick my C:B ratio. I want to use 4" for the chamber. I have 5' of 4" ABS, plenty to play with.

10' of 1.5" ABS sch40 is approx. 206 cubic inches of volume.

I was thinking about a 1.5:1 ratio, which would be roughly 25" of 4" pipe. Any suggestions on this? Is 1.5:1 a good number for a large combustion gun?

Where in the chamber would be the best place for the spark strip, and the fan? Is it worth it to try to suspend the spark strip in the center of the chamber, or just mount it to the side of the chamber, near the back/middle?

The last thing I'm unsure on is what kind of stand to build. I'm kind of clueless here. I was thinking of using the bases from two office chairs and some 2x4"s to build an adjustable stand, or maybe two big wheels in the back and an adjustable height bipod in the front. Suggestions?

Thanks a ton for the input, guys. I'll post pics of the build process and finished product.

Peace,
Pete Zaria.