Quote:
Originally Posted by The Radium King
moment of clarity for me. you don't want to equalise crankcase pressure to atmospheric, you want to equalise to the vacuum on the other side of the piston, otherwise the pressure will try to equalise from crankcase to combustion chamber via the rings.
so what to do in a forced induction situation? checkvalve ont he aos? get your vacuum for a vacuum pump instead?
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You always want to have lower pressure (read a level of vacuum) below the rings in low tension motors than you have above them during the engine's compression cycle on each cylinder; this will aid the low tension rings in obtaining optimum seal to the cylinder walls at the moment they need it most.
How you obtain this condition can vary all over the map. Production cars tend to use manifold vacuum, as the M96 does via the AOS system, and other makes do using a positive crankcase ventilation valve. On a race engine, some use a vacuum pick up in the header collector (slightly behind the point where the pipes come together in the collector, a properly shaped tube mounted on an angle and sealed to the collector housing with create a venturi effect and generate a slight vacuum signal, which can be routed to the crank case with hoses), or using a belt driven vacuum pump (more reliable and tunable) from suppliers like Moroso and others, which also utilize a catch can to trap oil. Either way, you get to the same place: lower parasitic drag engines with higher power output.
Non-atmospheric engines (turbo or blower) tend to go the pump route as manifold vacuum becomes non existent the instant the engine goes into boost. Just routing the AOS line down at the ground or putting in a check valve is obviously not going to provide any crankcase evacuation, and the lack of same is going to cause the rings not to seat properly...............