Cox Dragonfly SPI?

BWH wrote:I just purchased a dragonfly without the muffler.  I intended on running a muffler but noticed it has SPI.  Is this right? I assumed because it came with a muffler it would have been non SPI? Thanks, Brandon

Brandon, someone replaced the cylinder. Fact you already brought it up means you are already familiar.

A muffler defeats the purpose of SPI. Without access to outside air, one gets recirculated exhaust gases and thus a reduction in oxygen. May 1967 Aeromodeller, Peter Chinn had an excellent article in May 1967 Aeromodeller on the Cox .049 QZ, which was the first of the modern non-SPI engines such as the Sure Starts. He explains in the article why SPI was eliminated, so engine could be muffled. http://sceptreflight.com/Model%20Engine%20Tests/Cox%20049%20QZ.html

I'd save the higher performance SPI cylinder and piston for use in another engine, replace with one without SPI, if it were me. Or, you could use unmuffled with an appropriate throttle ring. But I imagine that you wanted it muffled for a specific reason.

Thanks guys, I will look for a Sure Start for this engine and use this one somewhere else. It is going in a cowl and I wanted the muffler to be able to run the slime outside it.