Originally posted by jcn30127 on another forum...
Not all exhaust exits out the exhaust valves into the exhaust manifold, and out the tail pipe.
A small amount leaks past the piston rings and into the crankcase or oil pan.
In the late 1960's/early 1970's, engineers discovered that they could run a vacuum line from the intake manifold to the crankcase and suck these "blow-by" gases back into the engine and re-burn them in the combustion process. They also learned that by placing a negative pressure on the crankcase, that the engine ran more efficiently and cleaner.
On FI engines the blow-by gases (unfortunately) tend to be higher. The OIL CATCH CAN is placed mid-stream in the vacuum line to catch any large or heavy blow-by ingredients before they reach the intake manifold. If the blow-by gases have a unusually large amount of oil in them, it CAN reduce combustion efficiency. The catch can reduces this possibility.
Ideally, piston rings would seal perfectly and their would not be any blow-by. Conversely, too much blow-by is an indication that the piston rings are not sealing properly, resulting in lower compression and power levels.