... when i was in school, all the diesels there except for their 2 newest cummins engines [isb [hpcr] & isx - cummins wanted filters on them as they were providing warrenty on the free engines] the older engines [n14, S60, 3126e, 6v92, dt466... ] had a metal wire screen over the turbo inlet to prevent fingers from going in, but no filter on them [the 6v71ddc had a screen over the blower]
i've run emd and ge diesel engines at work under load with a filter element removed so i could see the turbocharger compressor wheel to help in diagnosing heavy black smoke complaints [no filter there = no filter restriction]
then again, i have run emd engines with crankcase covers removed to find a cracked piston [smoke in crankcase]. . works ok on governor engines, as you can control the idle manually [down to about 100rpm] but the electronic ones can make a mess as they will idle at 200 low idle, but harder to control [but easier to kill injectors with laptop]
i someday want to get a video clip of an emd engine running with crankcase covers off . .