DIY Smoke Muffler: Turn your plane into airshow performer!

Nothing adds excitement to your RC flights more than a smoke system. Do a loop or a roll and add that dense, long-hanging smoke trail and you have an airshow! Instead of shelling out the coin for a commercial smoke muffler, save a few bucks and modify your stock muffler. Then apply your savings to the cost of a good smoke pump and the required hardware. Here’s how I made my smoke muffler for my Zenoah G20–powered Hangar 9 1/4-scale J-3 Piper Cub.

Image above: A smoke system is a great way to add a lot of fun to your RC flying.
right: The stock muffler that comes with the Zenoah G20 engine makes a great smoke muffler.

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Getting Started

Remove the stock muffler from the engine, and clean it up with some acetone solvent to degrease it. I used a new single-edge razor blade to carefully remove the old exhaust gasket. Here, you can see the new one that will be used after the muffler has been modified.

For plumbing, you’ll need a mix of Tygon gasoline fuel line and heat-resistant silicone tubing that holds up well to the heat of the muffler. To modify the muffler, all you need is a large brass tube that fits into the muffler’s exhaust pipe (used as a baffle tube) and a length of thin 3/32-inch copper tube for the oil preheat and injection tube. I used Du-Bro and K&S tools to bend and cleanly cut the metal tubes. For the smoke oil tank, I used a Sullivan 10-ounce tank with a standard two-tube setup. Be sure to use tie wraps to secure your tubing. A piece of plywood along with double-sided foam tape or Velcro makes your tank and smoke system easy to remove for servicing.

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Injector Tube

Start by bending and preparing the end of the injector tube. To make a fine atomized spray mist of oil inside the muffler, I used a wire cutter to snip off the end of the copper tube; this squeezes and seals the end and gives it a fishtail shape. Use a thin cutoff disc with your rotary tool to make a small nick in the middle of the fishtail; this produces an opening about half the size of the stock tube. Bend the tube to shape so that it’s long enough to enter the side of the muffler, pass through to the other side, and bend 180 degrees to re-enter the muffler.

Drill three holes in the sides of the muffler. Depending on your muffler and engine installation, these holes have to be custom laid out. On mine, the single entry hole is on the muffler’s aft side, and the two others are on the front side. Drill the holes slightly larger than your injection tube’s diameter. I used a drill bit slightly smaller than 1/8 inch (about 0.1 in.). Using a grinding bit, remove the black finish on the muffler exposing the bare metal. This provides a clean surface for the metal epoxy to stick to.

Here, the injection tube has been installed and is ready to be sealed with J-B Weld metal epoxy. Clean the injection tube as well as the surface of the muffler with more acetone, and when dry, mix up some epoxy and build up neat fillets around the tube/hole areas. Smooth the fillets with a wet finger and then set the muffler aside to fully cure (about 24 hours).

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Baffle Tube

Increasing the metal area to help transfer the heat to the smoke oil within the muffler produces denser smoke. The easiest way to do this is to add a baffle tube. Slip a snug-fitting brass tube into the muffler’s exhaust pipe; the tube should be longer than the exhaust tube already within the muffler. Slip the baffle tube into place until it bottoms out against the inside of the muffler, and mark the length. Use a K&S tubing cutter, and cut it to length.

To determine the length of the internal tube, bend a piece of wire as shown and slide it into the exhaust pipe. Feel for the end of the internal tube, and with your thumbnail, use the wire as a depth gauge. Transfer the length to the brass tube.

This small area at the end of the brass tube is the baffle section, and several holes need to be drilled through the tube. I used a 1/8-inch drill bit to drill eight holes around the end of the tube. Remove any burrs from the holes and then sand smooth.

Slip the baffle tube into the muffler until it bottoms out and is flush with the end of the exhaust pipe. Drill a hole in the side of the pipe and through the tube, and secure in place with a panhead sheet-metal screw. Loosen the screw slightly, add a drop of red thread-lock, and retighten.

Reattach your DIY smoke muffler to your engine, and if necessary, trim the engine cowl so that it does not touch or interfere with the smoke plumbing.

Internal Setup

A removable smoke system is a good way to go as you can remove the overall system for servicing. It is a lot easier to work on everything outside of your model than with it crammed into the fuselage. For my Piper Cub, I simply removed the cabin details and seats, and used the pilot area for the smoke-system installation.

You need to regulate your smoke-fluid flow. Injecting too much smoke oil into your muffler is worse than using too little. The excess oil actually cools your muffler, and this reduces the amount of smoke it generates. This also shortens your “smoke-on” time by burning up the fluid faster than needed. I used an auxiliary needle-valve assembly from an old glow engine to fine-tune the amount of oil entering the muffler.

Be sure to use fuel filters between your smoke tank and your transfer pump. Try not to switch on your smoke pump when your engine is at low throttle or at idle; this can cause the engine to drown on the unburned oil, which increases back pressure. A good idea is to mix the smoke switch with your throttle channel and set the “on” position to 1/4 to 1/3 throttle.

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Final Installation

Drill a hole in the firewall, and route your smoke lines into the cabin area. A stiff length of wire fed through the hole can be used to guide tubing into the cabin. Attach the smoke line to the wire, and pull it into place. At the muffler end of the smoke line, insert a short length of aluminum tube and slip on a 1-inch-long piece of glow-fuel silicone. Attach the silicone tubing to the muffler’s injection tube. Silicone stands up well to the muffler’s heat. Connect the smoke line to the needle-valve assembly, and connect the needle valve to the output fitting of your smoke-oil pump.

Attach the smoke-oil tank to the pump’s input fitting, and plug the pump’s power lead into your receiver. I use a T-fitting between the tank and the pump for filling and emptying the tank, and use a fuel dot to seal the fill tube. Be sure to also attach a vent tube to your smoke tank, and route it out of the fuselage.

Check out the smoke density! This is what it looks like when the oil flow is reduced (only one turn open). This gives a great “smoke-on” time, and my 10-ounce smoke tank lasts for more than 10 minutes.

Give this technique a try! This DIY smoke muffler works just as well as any commercially available muffler. Smoke on, and have fun.

TEXT & PHOTOS BY GERRY YARRISH