Fire Wood Processor

[f350ca]

Attempted a U-tube posting. Here goes.

http://youtu.be/SSx5m6ayrGo

Now for a question.
As you can hear its loud with the stock diffuser (?) on the stack. I tried a small automotive muffler on it yesterday and the result was astounding, silence. It idles smoother too (as expected with some back pressure). Is the back pressure from the muffler going to be hard on the exhaust valves due to the retained heat. I've heard yes and no. Comments please.