I've finally found time to work the TD7G. It has a naturally aspirated Cummins four cylinder 3900CC, the same engine as my Case backhoe. It performs well, but is incredibly loud. My friend lives a mile away, and came to investigate. I wear earplugs, and muffs, still my ears ring after running it. I'm told they all are, and left the factory loud. The muffler is a convoluted thing under the hood, and stubs through a rectangular box that looks like an add on, but others have it too. It is cut through the hood, and looks crude with less than attractive weld. The exhaust tube from the muffler passes up through this box, and stops. At the top of the box directly above the muffler is a 3?" pipe, very substantial short tube sticking up. Over this is a stack big enough to fit over the pipe on the hood, then narrowing to 2?". At the intersections of muffler out pipe, and box, then to stack there is a lot of gap. It doesn't appear to have ever been closed. I tried a short section of tube fitting tightly over the muffler out, extending up into the stack hoping it'd quiet it some. Nothing changed. I will presume it uses convection, and venture to collect exhaust, as no dirt accumulates under the hood. How can I tame this beast?
You are spot on about the venturi effect to keep the dust and heat out of the engine compartment. As far as muffling a naturally aspirated 4 cylinder Cummins I'm not sure what would work best. I had a Skytrak telehandler that had a 4 cylinder NA Cummins with a pretty substantial vertical muffler that was fairly quiet, You might look into what they used. I can't remember the model number though.
My first impulse is to add a secondary muffler above the hood. The pressure would prevent the venturi from working. It would need a flexible connection through the hood. The backhoe muffler was about $350. It's more quiet, but too bulky for the crawler.
I know it doesn't help but I have the same motor in my Komatsu (4BT), but mine is turbo'd, quietest motor I have ever been around in any machine.