I thought the valve cover vent topic was better understood by now. I’ve posted on this at length in the past but I’ll give my take on it again.
First off, it sounds as though some of you are routing things in such a way that the front cover vent is routed to the pressure side of the intake. That does not seem to be a wise approach since it effectively plugs the vent whenever you are on boost.
The reason oil gets pushed out the vent in the extreme cases experienced by some people is not blow-by as such, although blow-by will exacerbate the problem. What happens is that sustained high lateral G’s can slow the drainage of oil from the head down to the pan. Oil flowing to the head can be “pinned” against one end of the valve cover then tossed to the other end, etc., with less than normal making it to the drain passages down. Remember that the V design of the engine means that the passages are “ramps“, not vertical passages, which may also slow the downward flow under G’s, especially acceleration in the case of the rear bank. So at the track you are at constant high RPM and therefore oil pumping volume. Occasionally at the track you gat just the right combination of moves to trap enough oil in the rear cover that it swamps the vent holes in the baffle (underside of the valve cover. see pick in my old post.) and gets up into the baffle itself, where only air and gasses were meant to be. Once there it has no easy way out, but as you continue around the corners is finds it’s way to the end with the exit vent and “belch” out it goes.
Why do we see this only (or at least mostly) with forced induction cars? It is important to first understand the stock venting design of the crankcase. It is designed to allow air to flow in the front cover, down to the crankcase, then up to the rear cover, and out that vent to the intake, taking “normal“ amounts of blow-by gasses along with it. (later cars flow in the opposite direction) That flow is aided by the inherent light vacuum effect where the rear vent connects to the intake. The fumes are sucked out of the crankcase and back to combustion with the front cover vent providing fresh replacement air and allowing for even mild vacuum to be enough. Even without emissions concerns the crankcase must be ventilated or it will pressurize, reducing power and blowing the dipstick through the engine cover. J
OK, now we add forced induction and can’t leave the rear vent connected to the stock location because it would be pressurized. Most people just route them both to a catch can, but that means you no longer have the “positive” aspect of the crankcase ventilation. And consequently more of the normal pressure is allowed to build up. Add to that the fact that you are burning more fuel which generates higher cylinder pressures than a normally aspirated car and even at normal leak-down figures you are forcing more gasses into an inadequately vented crankcase. The result is a pulsing of pressure at the vents. Once you get some oil trapped up in the baffle and it finds it’s way around to the vent tube, it gets expelled with some force.
If you find my old post you will see where I recommended drilling additional holes in the baffle to allow trapped oil to return to the head. Subsequently someone else (Gerry J. I think) said that he had some the same thing and solved the problem. Of course, the real problems are still there, oil collecting in the top of the engine rather than draining down, and poor ventilation of the crankcase, but at least the expulsion of oil into the engine compartment or even a catch-tank can be fixed, and the need for drains from there back to the dipstick or pan largely unnecessary.
First off, it sounds as though some of you are routing things in such a way that the front cover vent is routed to the pressure side of the intake. That does not seem to be a wise approach since it effectively plugs the vent whenever you are on boost.
The reason oil gets pushed out the vent in the extreme cases experienced by some people is not blow-by as such, although blow-by will exacerbate the problem. What happens is that sustained high lateral G’s can slow the drainage of oil from the head down to the pan. Oil flowing to the head can be “pinned” against one end of the valve cover then tossed to the other end, etc., with less than normal making it to the drain passages down. Remember that the V design of the engine means that the passages are “ramps“, not vertical passages, which may also slow the downward flow under G’s, especially acceleration in the case of the rear bank. So at the track you are at constant high RPM and therefore oil pumping volume. Occasionally at the track you gat just the right combination of moves to trap enough oil in the rear cover that it swamps the vent holes in the baffle (underside of the valve cover. see pick in my old post.) and gets up into the baffle itself, where only air and gasses were meant to be. Once there it has no easy way out, but as you continue around the corners is finds it’s way to the end with the exit vent and “belch” out it goes.
Why do we see this only (or at least mostly) with forced induction cars? It is important to first understand the stock venting design of the crankcase. It is designed to allow air to flow in the front cover, down to the crankcase, then up to the rear cover, and out that vent to the intake, taking “normal“ amounts of blow-by gasses along with it. (later cars flow in the opposite direction) That flow is aided by the inherent light vacuum effect where the rear vent connects to the intake. The fumes are sucked out of the crankcase and back to combustion with the front cover vent providing fresh replacement air and allowing for even mild vacuum to be enough. Even without emissions concerns the crankcase must be ventilated or it will pressurize, reducing power and blowing the dipstick through the engine cover. J
OK, now we add forced induction and can’t leave the rear vent connected to the stock location because it would be pressurized. Most people just route them both to a catch can, but that means you no longer have the “positive” aspect of the crankcase ventilation. And consequently more of the normal pressure is allowed to build up. Add to that the fact that you are burning more fuel which generates higher cylinder pressures than a normally aspirated car and even at normal leak-down figures you are forcing more gasses into an inadequately vented crankcase. The result is a pulsing of pressure at the vents. Once you get some oil trapped up in the baffle and it finds it’s way around to the vent tube, it gets expelled with some force.
If you find my old post you will see where I recommended drilling additional holes in the baffle to allow trapped oil to return to the head. Subsequently someone else (Gerry J. I think) said that he had some the same thing and solved the problem. Of course, the real problems are still there, oil collecting in the top of the engine rather than draining down, and poor ventilation of the crankcase, but at least the expulsion of oil into the engine compartment or even a catch-tank can be fixed, and the need for drains from there back to the dipstick or pan largely unnecessary.
If you have something to say, by all means do so.
