Copper or Brass exhaust manifold.

[MachX]

Hello All,

I am drawing up a design for a water-cooled exhaust manifold for a small engine. It would be a simple design of an exhaust exit pipe mounted to a flange, with a tubular copper heat exchanger liquid filled coil wrapped around the exit pipe right near the cylinder.

The copper would have to be soldered or brazed. Normal plumbing solder melts at around 400 degrees F. I know exhaust gases can exceed this, therefore I would worry about the joint melting and the exhaust assembly falling apart.

My question is, if I were to braze this copper together with a silver compound with a higher melting temp, would it hold? What exhaust temperatures would I expect straight out of the cylinder of a 35cc 4-stroke engine? I would like to use copper or brass for the heat conductive qualities and corrosion resistance, and to be able to braze/solder the coiled heat exchanger to the exhaust pipe. Any thoughts? should I use aluminum?

 

[pantaz]

Exhaust gas from automotive and motorcycle engines can exceed 1800°F near the cylinder head. Since the manifolds and tubing typically dissipate heat rapidly, the external surface temperature drops pretty quickly along the length of the system.

I don't have data for smaller engines, and without measurements from your actual system, I can't even guess what you should expect to encounter.

A local welding supply store should be able to provide the proper brazing flux and filler rod for handling temps well over 1000°F.

 

[SteamKing]

I would not use copper since it is a very soft metal and subject to failure by fatigue.

 

[MachX]

Thanks for the input. Someone mentioned somewhere that exhaust gasses through copper will create copper disulphide. Not sure about this, but I don't want to create any extra carcinogens. The main reason I wanted to use copper was to pull heat away from the engine, but as mentioned I feel like if I make a manifold, the heating and cooling effect would eventually fatigue and stress the joint enough to fail. I think I am going to use stainless for the pipe and manifold itself, and then use silver solder to braze the copper coil heat-exchanger to the outside of the stainless tube. This way at least it won't fail and fall apart.