Dave68

Personally, I would not install one unless I knew more. Since I already have plenty of filter power downstream and the amount of oil collected is not incredibly significant, there really isn't a compelling reason for me to try one. One thing I would question about it is: How does the small orifice valve prevent this occurence?

* Wikipedia
While a smaller opening will certainly restrict air velocity towards the crankcase (in a reverse-flow condition), whether it is reduced sufficiently is another matter. Does this type of flow reversal happen offen? No, so I would venture to say that it is relatively safe. Still, I recommend that a coalescing filter or combination of particulate and coalescing filters be used downstream.

Dave

Dirty Howie

Well I am very suspicious too..............

How can that tiny hole allow enough flow.

It seems to me that the PCV diameter must be the same as the fresh air inlets or there will be backflow thru the fresh air inlet ...... unless there is NO ring blow by. And I am told this is not possible!!!!!!!!

I will note that SDPC motor builders does require the use of this part.....supposedly to help seat the rings. I don't understand how this would help ??????????


DH

ZeeOSix

I think I know what you're saying. The term "increased vacuum pressure" is confusing, as the term "vacuum" just means absolute pressure that it below ATM pressure. What I think you really mean is that the vacuum level inside the crankcase will be less (ie, more absolute pressure) as you add restriction (ie, a smaller fixed orifice) between the vacuum source (manifold) and crankcase.

To elaborate/rehash on the PCV system some more. The level of vacuum inside the crankcase is dependant on a many things. 1) how much vacuum is at the source (ie, inside the intake manifold, 2) how restrictive is the line between the intake manifold and the crankcase (ie, size of orifice), 3) how restrictive is the line between the valve cover and throttle body, and 4) the rate of piston ring blow-by and dirty vapor production inside the crankcase.

In an open, breathable PCV system (ie, one having both a dirty line and a fresh air make-up line), the crankcase's vacuum level, and the flow rate of vapors through the system will be dependant on the most restrictive section of the system (including both the dirty and fresh air lines), and the level of manifold vacuum. The flow rate of vapors would be the same if the same sized orifice was placed in the dirty line or the fresh air line, as it is still in series with the flow path regardless of location. The location of the orifice however will determine what the crankcase vacuum level will be. If the small orifice is in the dirty line, then the vacuum level on the crankcase will be less than if the orifice was in the fresh air line (ie, on the "other side" of the crankcase). This is because the crankcase would be experiencing the full sucking force of the manifold due to the orifice being upstream of the crankcase volume.

If the PCV system was completely "sealed" (ie, the fresh air line was blocked - meaning the orifice was closed off), then the crankcase would suck down to whatever vacuum level the intake manifold vacuum was. A smaller orifice would just mean it would take longer to suck it down. An analogy would be sucking the gas out of a sealed bottle with a straw using your mouth (vacuum source). If you had a large diameter straw, you could suck it down faster than with a very small diameter straw ... but ultimately with enough time you couldn't suck it down any farther than the vacuum level you could produce with your mouth.

If the 2.5 mm orifice PCV unit is much more restrictive than the fresh air line, then it will decrease the vacuum level (ie, raise the absolute pressure) inside the crankcase. At idle there is always the highest manifold vacuum, and the least amount of piston ring blow-by gasses ... so at idle the crankcase will be at the greatest vacuum level possible and have no problem flowing the small volume of dirty vapors into the manifold. At WOT, the manifold vacuum is very weak, and the amount of blow-by gasses is the highest ... this results in a positive crankcase pressure, which is causing dirty vapors to find any available path to escape.

As DH has mentioned, if the fixed orifice is too small, causing too much restriction in the dirty vapor line, it could cause backflow down the fresh air tube if there is large amounts of blow-by and the crankcase pressure got high enough. This is also true if other things restrict the dirty vapor line.

It seems that guys who have switched to the fixed orifice PCV unit have not reported excessive backflow in the fresh air line ... but again, maybe they don't run WOT long enough to get into a big backflow mode.

ZeeOSix

I always thought that increasing the vacuum level inside the crankcase helped seal the rings better. When I think about it, I can see how the fixed orifice would probably flow better at idle. The original spring loaded PCV valve is sucked to it's closed position at high manifold vacuum during idle, so it may not flow much at all at idle, and would also prevent much crankcase vacuum to occur at idle due to high restriction to flow. This means that with the 2.5 mm orifice PCV unit, the crankcase vacuum level at idle is probably higher than with the original PCV valve ... this could help the rings seal and seat in better during break-in.

Dirty Howie

Well no valve at all or no restrictive oriface is going to flow the most and seal the rings by highest vacuum isn't it??


DH

ArKay99

This is just an empirical observation observation but something I thought I would add.
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When I was running a stock cam and heads I monitored my manifold pressure at 28-32kPa at idle, depending on temp. After I changed my cam and intake I monitored my manifold pressure at idle to be 52-55kPa. I imagine this change in pressure would now keep the PCV valve open during idle and change it's operating parameters. Also, now the only time manifold pressures would go below 30kPa are during hard deceleration with the engine in gear. When taking this into account the PCV valve is open almost all the time so I imagine the opportunity for oil getting through the PCV system is also greatly increased.
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This may one reason a fixed orifice will perform better for cam modded engines with reduced manifold vacuum.

Dirty Howie

So do you think the new PCV fixed restrictive oriface fitting is designed to prevent oil syphoning at low RPM/High Vacuum or somehow that I don't quite understand, to seal the rings better and there for prevent oil usage thru the rings...............

And I don't see how a good catch can setup wouldn't work better in the case of syphoned oil at low RPM


DH

ArKay99

I honestly don't know that answer. I was simply stating what I observed while monitoring MAP while tuning. As a matter of conjecture I am proposing that a PCV valve doesn't work as designed with altered MAP's associated with cams that change MAP.
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It seems logical to me that oil siphoning will occur at a greater and greater degree as the vacuum goes up. However as siphons go, all that is needed is the initial vacuum to 'prime' the siphon and then gravity takes over. I can see this happening if the catch can is lower than the source. Also, the siphon will occur until the vacuum is broken, either by a bubble in the flow, or when it dumps into the can. What that means is if your doing a high rpm sprint you can be filling your valley cover baffle with oil and with not much vacuum, not much else will happen. Let your foot off the gas and you'll get a burst of vacuum that will pull oil out of the baffle. It's possible the weight amd/or inertia of the oil coming out will keep the PCV valve open, the spring is very weak, and the siphoning effect will start and continue until there is a bubble in the flow from the baffle. Then if vacuum is strong enough the flow will stop and the valve will shut. This almost describes the oil baffle as a reservoir. I have a baffle in my valley cover (LS2), and get a lot of oil in my catch can. I have removed the baffle from my valve covers (Jesel rockers), and get nothing there. I'm only suggesting this as a possible cause for this.
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A fixed orifice PCV has a much smaller opening than a variable orifice and it's possible the smaller opening creates a lower pressure drop on the engine side of the valve reducing the siphoning effect.
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Also remember that the cfm of the gaseous flow has been measured to be between 1.2 and 1.4 cfm, but what would be the volume if it was oil? I'm sure it's significantly less, but even .12 cfm of oil is a lot.
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I guess what I'm suggesting is there are other factors at work here, in addition to air flow.
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I wish my motor was together so I could verify some of this. Sorry...

Dave68

Zee, let's look at it this way: Let's say you have a vacuum pump that is pulling air from a source that is open to atm pressure. With a large enough hose ID, vacuum pressure within the hose will be next to zero. Now, you add a ball valve in line with the hose; this ball valve is effectively is a variable flow restrictor. As you close the valve, vacuum pressure will increase (higher vacuum) until the valve is completely closed, when vacuum pressure will at its highest level. During this whole scenario, we are assuming that the pump exhaust is not changed. Since your engine's fresh air source flow rate does not change (at constant engine speed), replacing the PCV with a fixed orifice of smaller diameter should result in higher vacuum pressure downstream of the restriction. Now, I don't know if at idle, the orifice hole is smaller that the opening of the PCV, but assuming it is, there should be an increase in vacuum pressure, however slight. Yes, I know that it's a bit more involved that that, but hey, it's easy to check with a vacuum gauge and the orifice valve.

Dave

ZeeOSix

It can also depend on the restrictiveness of the fresh air line. As I discussed above, if there is more restriction in the fresh air line vs the dirty line, then the guts of the engine will go to a lower pressure (more vacuum) than if the dirty line was restricted and the fresh air line was not.

I have to believe that GM came up with the size of the fixed orifice (2.5 mm = 0.10 inch) based on some flow balancing of the PCV system. It’s still strange that they didn’t list the LSx Vette engines in the application, as we all know the 2004+ LSx series engines have the 2.5 mm orifice built into the valley cover hose nipple. The fixed orifice PCV unit should therefore act the same as the 2004+ valley covers.

ZeeOSix

But the flow of fresh make-up air will change based on the restriction in the dirty vapor line. The size of the orifice will directly dictate what flow rate goes through the PCV system. Think of this -- if you totally blocked the dirty air line the fresh air flow would go to zero. The air flow is like electrical current ... what goes out the dirty line must come in the fresh line is everything is working properly.

Yes, if you increase the restriction more and more between the valley cover and the intake manifold, then the vacuum level on the engine side of the restriction will become less (ie, higher absolute pressure). If you totally blocked off the dirty vapor line, then the crankcase would go to ATM pressure (no vacuum at all).

If the fixed orifice is smaller in diameter at idle than the variable PCV at idle, then the vacuum level inside the crankcase will be less ... meaning the absolute pressure will be higher. Again, if you completely blocked off the dirty air line (orifice dia = 0), then there would be no vacuum on the crankcase. The vacuum level inside the intake manifold is constant at idle ... consider that the constant vacuum source.

Dave68

Yes, it is a constant vacuum source with a controlled leak - plug up the leak and vacuum pressure should increase slightly, at least from my experience with vacuum pumps (I have one in my garage). The thing that is tricky here is that one of the vacuum paths (PCV line) is connected to the pump itself (the engine). I think this is what you are including in your theory and it may make things harder to conclude. Still, as I mentioned previously, a test is very easy to do. Maybe after my PCV line temperature test is complete, I'll T in a vacuum gauge to see how it responds to a gradual shutoff of flow in the PCV line.

vjjack04

This discussion is some sort of confused dialogue between vacuum and manifold absolut pressure; folks, the more modern LS engines have very little need for PCV - as did the earlier V-8s - emission control Under full throttle very little vacuum maters anyway. I suggest you guys look at the logic for PCV and then the efficiency of an LS engine; ...a catch can is the solution. Also a good reference to understand the engine and controls is: Corvette fuel injection by Charles Probst.

Y2Kvert4me

Since you're well-versed on the topic, why did GM make the change from the PCV valve to the CCV metered orifice in 2004?


That's the point of, and still the only unanswered question in this thread.

ZeeOSix

IMO, I don't think a PCV valve (fixed orifice or not) is going to change the vacuum level inside the intake manifold, as the "leak" is just too small to make any noticeable difference. Now if the "leak" was pretty big, then yes ... you'd probably see a slight change in manifold vacuum. Even if you just pulled the PCV line off the intake manifold and let it suck directly from the ATM the vacuum level inside the manifold probably wouldn't change very much if any to measure accurately. But if you put a 1/2+ inch hole in the side of the manifold it would have a much bigger effect. This is getting off on a tangent in the discussion somewhat, as our real goal is to understand why GM went from a spring loaded, variable orifice PCV to a fixed orifice unit.

I bet you wont see any measurable difference, as the "leak" via the PCV system is just too small to effect the vacuum level in the manifold.

ZeeOSix

I agree to that ... the terminology is getting confused sometimes. A vacuum is absolute pressure that is less than ATM pressure. Positive pressure is anything above ATM pressure. In the "gauge pressure" scale, the gauge reads 0 psi at ATM, reads negative (ie, vacuum) if less than ATM and reads positive if higher than ATM.

If using the absolute pressure scale, then "0 psi absolute" = a perfect vacuum. 10 psi absolute would be the same as a 4.7 psi vacuum. 14.7 psi would be considered nominal ATM pressure.

Not too sure I agree that the LSx doesn't need a PCV system. All engines have blow-by and combustion contaminants in the oil. If you don't have some kind of effective PCV system those contaminants will eventually build up (sludge) and do internal harm.

Maybe you mean the LSx engine can still pass emissions without a PCV system ... maybe so, but even if it did I wouldn't run it without a PCV system, as the crud really needs to be evacuated from the internals of the engine. I guess if you raced and tore the engine down every other weekend then a PVC system wouldn't be needed ... just a pain old breather to ATM so the crankcase didn't build up any pressure.

ZeeOSix

I'm sure some of you guys have seen these tech articles on the fixed PCV unit.

http://www.mightyautoparts.com/pdf/articles/tt122.pdf
http://www.mightyautoparts.com/pdf/articles/tt118.pdf

After reading them both again, I focused on a couple of things:
"It has been determined that under certain operating conditions, the original equipment variable type PCV valve flow rate may promote excessive oil consumption. The original equipment valve may promote a siphoning of the oil back into the intake manifold where it would be consumed through the combustion process.

The excessive oil consumption is promoted by an excessive PCV valve flow rate under given operating conditions. The PCV valve hose is routed to the intake manifold and it should be examined for evidence of excessive oil. The filtered fresh air for the PCV system is routed up-stream of the throttle plate to the right valve cover, via a rubber hose. A back-flow of the PCV system can result in oil depositing directly onto the throttle plate, promoting a deposit formation that can result in a sticking throttle valve."

They mention that the fixed orifice PCV unit should be used to also cure the sticky throttle plate ... this means that the fixed orifice must cut down on the dirty vapor backflow down the fresh air line, as that is the cause of a sticky throttle plate.

Not sure what they exactly mean by "under certain operating conditions" ... they could have elaborated, but didn't.

I believe that the spring loaded variable PCV valve almost closes at idle due to high intake manifold vacuum, and as such doesn't flow very much - probably less than the fixed 2.5 mm orifice does. This could also be true at light throttle loads, which could mean that some oil vapors might be back flowing down the fresh air line at a light throttle load - causing the stick throttle plate problems on some vehicles. With the fixed orifice, I believe there is better idle and light throttle flow down the PCV line to the manifold, which would prevent backflow down the fresh air line to the throttle plate in the throttle body.

It's possible the spring loaded variable PCV valve flows too much at some larger throttle opening (not WOT, but say 1/8 to 1/2 throttle), and if so then the fixed orifice unit would cut down on the "oil siphoning" at higher engine loads.

It would be hard to determine exactly what the spring loaded PCV valve variable opening does at various throttle positions without measuring it or seeing a graph ... but I'm sure it's not linear. Maybe GM determined a fixed orifice PCV does what they wanted, and therefore didn't need to redesign the spring loaded PCV valve to try and "calibrate" it to the system.

I think I'm going to try the fixed orifice PCV unit in my Zee and see what happens.

Dave68

According to the articles, the fixed orifice unit is intended for vehicles exhibiting excessive oil consumption (1 quart every 2000 miles or less). I wonder whether we can assume that oil consumption will be ruduced, regardless of how little oil the engine is "consuming" currently. In other words, if you are adding a quart every 5000 miles, you may not notice any difference by swapping in a small orifice flow restrictor.

Of course, it would be nice to know why GM is NOT recommending this part for 97-03 C5s. Until I know more, I'll stick to my coalescing filter combo.

Zee,

What is your current oil consumption rate?

Dirty Howie

I went and bought one of these today. Don't know when I will install it.

I called SDPC engine builder who insists these be placed on their new engines. I asked if it was to help create vacuum for seating rings. The answer was no .... it is reccommended only because they have seen engines without it suck so much oil that a low engine oil condition occurred!!!!!!


DH

ZeeOSix

My Zee burns uses about 1 quart every 3500 miles. I've had an Elite Engineering catch can on it for about a year, and it might have caught about 1/4 quart in 3500 miles. So apparently, the other 3/4 qt must have been burned up by going past the rings, etc. It's also possible that the catch can isn't catching 100% of the oil heading to the intake manifold either.

I'm not too bothered about the oil use rate on my Zee, but it would be nice to cut it down even more, especially any oil going into the intake manifold if I can.