Seems to me this test should be as valid as a dyno. If a diesel is truly with an excess of air, there will be no difference in any filter.
But if a vacuum is shown with a gauge, there is not an excess of air, yes? I may have to buy a spare elbow and drill it for a hose and just see if nobody beats me to it.

I personaly *hope* the stock setup is not restrictive, but..

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One more Radar Lover gone...
1982 VW Caddy diesel 406K 1.9L AAZ
1994 E320 195K