The Catch Can setup continues...
06-07-2017, 05:54 PM
#21
You have to alleviate the pressure somehow and venting it to the atmosphere isnt only bad for the env and illegal in some states and cities but also is a bad idea.
In a normal operating system the air rushing past the tube creates a low pressure system that the higher crankcase pressure flows to. Otherwise it provides clean air into the crank case when the air pressure is higher than the crankcase pressure. it serves a dual purpos and why alot of systems make the distinction of clean air and dirty air.
The oil mist can be pretty thick (thats what she said) but on the valve covers there are baffles to help keep the mist to a min and on the ls6 valley its chambered also for the same reason. The driver valvecover sometimes has the pcv valve like early ls trucks then swapped to a metered orifice. The fbodys had some horrid plastic fitting that (ls6 style) then tied into the pass valve cover then went into an inline pcv valve. That is how they limit/meter the amount of air passing through the system and back into the engine as well.
On aftermarket covers with no baffles youll notice a large diff in pcv oil mist issues theyll also just have an open elbow often but some have pcv gromemts for valves. The Holley covers (most) have internal baffles so they are a good alternative to stockers.
The catch can further filters this already baffled oil mist.
Most guys just dump the can and not replace or fully clean the filter element, whatever it may be. Some use steel wool/chore girl style filter and some use a scotch brite style. Those are the most common. They should be cleaned as well or replaced if possible every oil change or every other at the most.
In a normal operating system the air rushing past the tube creates a low pressure system that the higher crankcase pressure flows to. Otherwise it provides clean air into the crank case when the air pressure is higher than the crankcase pressure. it serves a dual purpos and why alot of systems make the distinction of clean air and dirty air.
The oil mist can be pretty thick (thats what she said) but on the valve covers there are baffles to help keep the mist to a min and on the ls6 valley its chambered also for the same reason. The driver valvecover sometimes has the pcv valve like early ls trucks then swapped to a metered orifice. The fbodys had some horrid plastic fitting that (ls6 style) then tied into the pass valve cover then went into an inline pcv valve. That is how they limit/meter the amount of air passing through the system and back into the engine as well.
On aftermarket covers with no baffles youll notice a large diff in pcv oil mist issues theyll also just have an open elbow often but some have pcv gromemts for valves. The Holley covers (most) have internal baffles so they are a good alternative to stockers.
The catch can further filters this already baffled oil mist.
Most guys just dump the can and not replace or fully clean the filter element, whatever it may be. Some use steel wool/chore girl style filter and some use a scotch brite style. Those are the most common. They should be cleaned as well or replaced if possible every oil change or every other at the most.
06-07-2017, 07:30 PM
#22
Your analogies of a straw in a lid and bleeding are far off what goes on inside an engine or in how the pcv actually operates.
The catch can doesnt add air to the system. Do you have any idea how fast air pressure moves inside or even outside an engine? Why does gm put pcv tubes into the air stream before the tb? And again why is there one after? Its a simple balance.
Adding a catch can does not change the vacuum signal at idle. If you had an oil cap breather it would, but that is dependend upon the size of the possible unmetered air going in. There are offsets in factory computers for the airflow caused by egr and other things as well.
The catch can doesnt add air to the system. Do you have any idea how fast air pressure moves inside or even outside an engine? Why does gm put pcv tubes into the air stream before the tb? And again why is there one after? Its a simple balance.
Adding a catch can does not change the vacuum signal at idle. If you had an oil cap breather it would, but that is dependend upon the size of the possible unmetered air going in. There are offsets in factory computers for the airflow caused by egr and other things as well.
If you want a more complex series of examples I posted some on LS1 tech elsewhere, and also other forums such as here:
http://www.nsxprime.com/forum/showth...=1#post1909285
more here:
http://www.nsxprime.com/forum/showth...=1#post1909293
A more advanced example:
http://www.nsxprime.com/forum/showth...=1#post1909408
Next, to your statement "adding a catch can does not change vacuum signal at idle". Couple of issues, where are you taking a measurement? As you stated earlier several times: there is a differential that exists, and it exists both in the intake manifold and the crank case as two separate (ideally) yet intertwined spaces (volumes). Idle vacuum source is strongest where? Perhaps by the intake valve from which it originates. And where is the source weakest? At the farthest reaches from that valve(s). So distance from point of origination through a given volume deteriorates vacuum signal present by the valve(s). Any length of tube, whether it be a hose, a runner, a plenum, from 0.00001" to 1000000.00" and you will have a resulting differential based mostly on total volume and source signal strength and available molecules/time to replenish whatever our source signal removes. If you increase the length of the tube to infinity, the vacuum at the end of the tube resting in the atmoshere is atmospheric regardless of how much original vacuum signal source strength exists, i.e. the source (intake valve) has an x/infinity relationship with the end of the tube resting at infinity (effectively zero) because any number divided by infinity is essentially back to zero, the differential is at its maximum value (highest on one end, lowest on the other).
In an ideal PCV system, the differential at the end of the PCV suction (to intake manifold source of vacuum) tube would be nearly ZERO when compared with the pressure at the other end of the tube (the crank case). That means if we have 15" of vacuum on the intake manifold side, we have 14.99999" Of vacuum on the crankcase side of the tube (very little differential). Again, to re-iterate what was said above, imagine the tube between intake manifold and crankcase becomes longer and longer. 10" becomes 100000'. As the tube becomes longer what happens to the pressure at the end of the tube? We will no longer have 14.9999" of vacuum on the end of that tube, its length (volume) becomes an issue. The larger the volume (length/size of tube) the less vacuum will be presented to the crankcase. A catch can is essentially a large tube. It represents an additional volume through which the source vacuum signal from the intake manifold must persist in order to reach the crank case (if the catch can is installed between crankcase and intake manifold vacuum).
04-20-2018, 02:18 PM
#23
Just to revisit this, I have had the MM can on for awhile and have zero blow-by. Before I had the RX performance can on and was still getting a fair amount of oil in the intake. I was pretty pissed because the POS RX performance can was not cheap by any means.
05-05-2018, 08:58 AM
#24
TECH Senior Member
05-05-2018, 12:01 PM
#25
11 Second Club
iTrader: (3)
Just pulled the intake on my 03 Silverado today in order to replace the knock sensors and harness. Engine has over 300,00 miles on it and still has a good compression readings on my cylinders but the intake manifold and the runners are caked up with oil deposits from running a PCV system. It's going to be a messy job but since this is not my performance engine I'll clean everything up and replace the PCV valve and get on down the road.
