Picked up more than 10 hp by changing covers. No distortion, the vacuum just sucked in the gasket. We saw the pan vacuum drop so we started checking for a leak. Couldn't fix the valve cover in time so swapped on the billet valve covers and gained power.

Holding pan vacuum was one of the primary reason why I designed the billet valve covers in the first place. None of the other valve covers that I tried would seal up tight so I made my own.