Aos

[JFP in PA]

Originally Posted by The Radium King

i understand. what i am wondering is is whether the crankcase needs vacuum or atmospheric pressure.

the pistons are slapping around, fluids are heating, pressure increases in the crankcase above atmospheric pressure. as a result, air leaks past rings, pistons have to work to compress the air in the crankcase, etc. etc.

the 1955 solution is to vent the crankcase to atmosphere so that pressure doesn't build. this venting, however, lets bad gasses out into the environment. to address this, in modern cars the gasses are now vented back into the intake to combust and eventually get scoured by the cats instead (via an aos to remove as much liquid from the gas as possible).

so, you can vent the aos into the intake before the throttle body or after. if you vent after (as per the boxster) you will get vacuum. i see the benefit to vacuum as it will provide some scouring of the system; vent to before the throttle body and you'd get all kinds of deposition of crud.

given that, the requirement for vacuum is not of any benefit to the crank and engine longevity/efficiency. but rather to facilitate introduction of crankcase gasses into the intake. if that is the case, then redirecting the output of the aos to a catchcan at atmospheric pressure is a valid thing to do for engine health (not environmental health, however). dumping it to ground as per TPC isn't, as there is risk that liquid oil can get past the aos and make a big mess.

Actually, the slight vacuum in the crankcase is critical to the low ring tension design rings creating a seal to the cylinder walls. Eliminate the vacuum below the rings and ring seal deteriorates significantly, blow by becomes rampant, ring “flutter” starts scoring the cylinder walls, and you loose significant HP, not to mention engine longevity. Do not run one of these engines without some level of crankcase evacuation, even if it comes from a supplementary source such as a belt driven vacuum pump, common on high output race engines. On a street engine, it also aids in pollution control, but it cannot be removed without consequences………