Knowledgelable members please advise.

In case you want to fly a model without attaching a muffler to the glow engine in it, What changes do you make and what effects does it have on the engine performance? While breaking in a new engine do you need to attach a muffler to it ?
Does any one have an internal diagram of a glow engine muffler?

I am more familiar with the diesel engines that is why I need to know.
thanks