Overall Design The gun body consists of a 25" long piece of 4" Pressure Rated PVC. For you beginners this is extremely important; never, ever use the cheaper non pressure rated "cellcore" PVC. My local Lowes only carries the non pressure rated PVC 3" and larger. I had to go to a serious plumbing supply store and I paid about $2.50/ft. for 220 PSI rated 4" PVC. A small price to pay for not having 3rd degree burns and shards of plastic in my face. At one end of the gun I put a standard 4" "Clean Out" and on the other end a 4" x 2" Flush Bushing with a short piece of 2" PVC and a 2" Male fitting to screw on various barrels. I have a 1 1/2", 2" and a 2 1/2" tennis ball barrel and I'll be adding a golf ball barrel later. A golf ball barrel is constructed of a section of SDR-21 water pipe (try your local Well and Pump supply store). The pipe is rated at 200 PSI but MUST be sleeved with 2" schedule 80 PVC to make it safe to use as it's quite thin walled. It fits perfectly in the 2" schedule 80 pipe and that's rated at 400 PSI. Add a 2" female connector and you're in business. One note: get a laser sight if you're going to shoot golf balls. I hit a tree by mistake and the golf ball nearly took my head off when it bounced back.

The Black Box (Detail)

Construction The Ignition System I've used a Gas Heater Igniter in all of my guns so far. They're easy to adapt to a spud gun application and fairly cheap. I may graduate to modifying a stun gun someday but for now these seem to work fine. You can see in the image above that all I did was attach 40KV wire to the leads of the Igniter and embed it in Silicone with the wires exiting one end of the black box. As noted on the pages for my other guns you really must get 40KV or better wire to carry the spark to the bolts. I got mine at Surplus Sales Of Nebraska. I put heavy connectors and shrink tubing on the ends of the leads and attached them to 2" #6 brass bolts which were screwed into the combustion chamber at roughly a 90 degree angle. The gap is just about a quarter inch. Many people seem to like using multiple sparks but I have been unable to detect any difference in performance either way so I just use one large gap for simplicity. I secured the bolts with washers on both sides of the PVC and a nut on the inside just to be sure they never leak. It's probably overkill but that's how I do things.

Ventilation and Fuel Mixing Also in the black box is a pair of 9 volt batteries wired in series to produce 18 volts. They are run through a switch and then to two brass bolts in the bottom of the black box. The bolts serve two purposes; to hold the box to the gun and to carry voltage to the fan inside the combustion chamber. This minimizes the wiring inside the chamber and further simplifies the design. As you can see in the image to the right I used a standard 80mm computer case fan for the ventilation system. It gets power from the brass bolts as described above. I ground the corners of the fan down until it fit snugly inside the 4" PVC. Unfortunately, as you can also see in the image, after a few firings the frame of the fan deformed from the pressure and heat. I'll have to find a better way to secure the fan and reinstall it.

The Direct Injection Fuel System The fuel system for this gun is referred to as a Direct Injection system. For you beginners out there it's just a way to measure the proper amount of fuel for the volume of air in the combustion chamber. The concept is simple; fill a tube with gas under pressure, close one end, open the other and out squirts a specific amount of gas. This system is identical to the one used on the TA-3100. It was in fact "borrowed" from that gun and modified for this much larger gun. I had to size up from 3/8" ball valves to 3/4" pipe to keep the length reasonable. The fuel system now has a volume of 100cc and at 42 PSI it injects 290cc of fuel which is 4.6% of the 6300cc combustion chamber. For MAPP gas the proper fuel/air ratio is roughly 4.6% fuel and 95.4% air. I'm using the numbers from the Burnt Latke site. They've done some very impressive work nailing down the optimum fuel mixtures and barrel lengths among other things. A few side notes: I found a great little volume conversion tool to go from cubic inches to cubic centimeters here. I use it in conjunction with the Spud Tool on the Advanced Spuds site. The Spud Tool is extremely useful in figuring the proper length and type of pipe to use for your fuel system. I also found a good quick reference for the Inside Diameter (ID) of common pipes here. Oddly enough a 3/4" pipe isn't 3/4" inside or outside diameter. It has to do with how pipe was made with much thicker walls in the past. Rather than change the outside size of the pipes they just made them larger inside. More spud gun trivia. One thing about my gun that I haven't see elsewhere (although I certain I'm not the first to do it) is a relief valve at the front end of the gun. (See image below.) It's nothing more than a 1/4" nipple with a cap that can be removed when fuel is injected into the gun. This prevents any back pressure that can force gas to leak out at the Clean Out. Also, the fuel meter won't inject the proper amount of fuel if there is any back pressure. It may be a small error but I try to design out all the error I can. I simply remove the cap before injecting the fuel and replace it immediately afterward. It seems to give me a better fuel mix.

Leaks are Bad (Notes for Newbies) Any time you build a fuel system it's extremely important to test the system for leaks. To build the fuel system for this gun I used four size converters, four nipples and a 12" piece of pipe to make the fuel reservoir. Not the best design but I was in a hurry. First I use "pipe dope" (lithium grease with Teflon bits mixed in) and it leaked like a sieve. I then tightened it down some more and it was fine. After I strapped it to the gun it began leaking again. To test for leaks I use the same technique you might use to find a leak in a tire; soapy water. Just spray it on all the joints and look for bubbles. If you've ever smelled MAPP gas the stink alone is enough reason to be sure your system doesn't leak. Besides that a leak throws off your measurements so your mix will be too lean. There's also the possibility of igniting the gas at the leak. Entertaining as this might be at a distance it could cause problems for the poor slob firing the gun. I finally solved my leak problem by resealing the joints using Teflon tape. There are at least two types; the thinner type is used for water pipes and the thicker (usually yellow) variety is preferred for gas systems.

Putting It all Together Once you have fabricated all the parts of the gun you need to assemble them in such a way that they'll take some abuse. A good gun will create a pretty good recoil when firing big spuds from a 2" barrel. I make the brackets for securing the tank and fuel system to the gun from 3/4" plywood and self sticking felt strips. I usually use PVC scraps to draw the circles for the 4" combustion chamber on some poster board or some other kind of heavy paper. The inside of a piece of 3" PVC is almost exactly the size of a tank of propane. The steel pipe size will vary with the size of the gun. After drawing the circles roughly where they need to be in relation to one another I cut out a template and draw several tracings on a piece of 3/4" plywood. I have found that a router with a cutting bit makes short work of cutting out the brackets. A little black paint and some self stick felt strips and you've got a super deluxe bracket.

Insulated Straps I strap it all down with hose clamps. Finding large enough hose clamps can be difficult. Sometimes a boat shop will carry some large ones but I usually end up ordering what I need from Plumbing World. Please note, the clamp sizes listed are outside pipe diameter, not circumference. I cover the hose clamps in 3/8" black rubber hose to insulate them, make them less destructive to the paint job and it makes for a more "professional" look. The best technique I have found to get the rubber hose on the straps is to unwind a clothes hanger, flatten one end with a hammer and bend a hook in the end. I then slip that through the hose and hook it to the end of the hose clamp. I also squirt liquid soap into the hose to make it slippery. It's pretty easy to work the clamp through the hose using this method. Then just wash out the soap and you're in business.

Useful Links Burnt Latke is a great source for ideas and test data to help you optimize your spudzooka. The Spudgun Technology Center has some useful information and VERY cool parts like Clear Rifled Barrels! AdvancedSpuds.com has some good ideas for several aspects of spudgunning.