It's one that installs into the oil passage instead of a sending unit, and reads oil pressure through a flexible tube instead of sending an electrical signal. On a smallblock Chevy, the pressure port is right beside the distributor. I've discovered a bunch of inaccurate electric sending units, but they generally read too low, not higher than normal, or they are slow to indicate pressure changes.
One of my competitors on the dirt track put so many washers behind his oil pump relief spring that the load of turning the oil pump would stall the engine. When he got it started and floored the gas, the pressure split the side of the oil filter. He had the relief valve jammed down tight so there was no way to relieve excess pressure.
Jerry
