[V8] V8 Crankcase Ventilation (PT/ABH)

Tue Oct 2 18:52:01 PDT 2012

We all know the CV (crankcase ventilation) system on these engines "blows"! Really, the poor design of this system literally blows oil into the intake stream as it does not have good separation/condensation characteristics. 

Not a rant, just a fact.

So, in replacing the engine in my 91 Automatic car (engine going in this week) I decided to do some experimentation with trying to improve it. Ultimately, I fear the only way I will see marked improvement is with the addition of a "catch-can",and possibly a PCV valve, but I don't want to go there unless my first attempt (revision 01) doesn't yield the results I'm looking for.

The valley cover (the "labyrinth" as its called in the manual) does not have much of a labyrinth to it at all, hence the propensity to allow excessive amounts of oil to be drawn into the intake.

If you study the flow of the system, there are two main paths for vapors to take on their way to the intake manifold: 

1: when the throttle plates are closed (idle), vapors flow through the very small hose at the back of the intake. It's the small hose that loops over the intake where the air box connects, from the labyrinth on the left, to a restrictor nipple on the right rear of the intake.

Just behind this port on the left side of the labyrinth is the large port that connects to the left side cam cover. This connects the crankcase (blow-by below the piston rings), to the cam covers (blow-by past the valve guides). The other large and very pricey hose that rounds out the picture is the one that connects the two cam covers together, to balance out crankcase blow-by pressure.

2: this path is through the large hose that goes from the front of the labyrinth under the intake, to the throttle body. This path is used for everything except closed throttle, and is of course a higher flow volume path.

Now, let me digress a bit: My stick car behaved as follows: 
A) Let it idle for a bit and then immediately upon tipping into the throttle the blue clouds begin!
B) After a long deceleration, upon giving it throttle, again blue! 
In fact the mountain I live on is a .9 mile stretch from my house to the main road, and its all downhill, so the throttle is closed 99% of the way down. When pulling away from the stop at the bottom, you got it,  I'm the "bug man", emitting clouds too thick to see through.

Okay, I concede this is symptomatic of a wheezing engine in need of repair, but to a great degree, the CV system is a large contributor to this mess as well!

So lets analyze the situation for a moment. Under idle/decel, engine vacuum is high. The small right rear port is the only one open to intake vacuum. So, crankcase vapors, and lots of lubricating oil that could not properly drain back to the pan, get sucked through this high vacuum port. This oil puddles on the floor of the intake as even though vacuum is high, velocity through the runners is low.

Upon opening the throttle, the increase in velocity then is able to pick up this oil and pull it through the combustion chambers. The latent oil that was picked up at idle and was coating the exhaust now also has the oxygen (with an open throttle) to burn off at a much  higher rate: a perfect storm if you will to spray the neighborhood for bugs! LOL!

The objective is to prevent, as much as possible, oil from being drawn up into the intake. It's funny, because the CV system is designed purposefully to draw IN crankcase vapors so they CAN be burned, versus venting them to the atmosphere ala the days of the old "road draft tubes", prior to the early 60s and the start of PCV (positive crankcase ventilation) systems, the very first emission control.

Now, without pics it's hard to explain, but under the labyrinth sits a separator plate. This plate (a piece of formed - stamped sheet metal), has the job of being a baffle of sorts to separate the connection of the crankcase and cam covers, to the labyrinth and the ports which feed vapors to the intake. Both the labyrinth and the plate have to work together to separate these sections, feed vapors, and try to condense oil from the vapors before they are fed to the intake, and allow drain back to the oil pan.

One of the primary ways that this is done is through a complex of passages that force the gasses to twist and turn about, so that condensation, AND drainage can occur, and therefore theoretically only the finest of vapors gets drawn back through the combustion chambers.

Funny, upon closer inspection I expected to see this maze of passageways in the so called "labyrinth", but NOT! I think that must have been the point when the Fräulein  distracted the engineer and things went awry! 

This in my humble opinion is the biggest shortcoming with this system.

What I have done is used a material that should help separate these distinct areas and allow better filtration and condensation of the oil in the gasses. What remains to be seen is if it will allow adequate drainage of the condensed/filtered oil back to the pan.

If not, this is where the catch-can and PCV valve would come into play. But, one step at a time.

Ultimately, the addition of a complete PCV system may be what's needed, as the PCV valve itself, by design is a flow regulator controlled by engine vacuum. This would allow proportionally correct amounts of vapors to be drawn into the intake based exactly on only what's needed.

For now, I added the stainless steel "filtering" material under the separator plate in the valley, through the large hole in the center of the plate, as well as up into the rear of the labyrinth where the small idle flow restrictor is located. This is important as vapors off-idle to the large hose at the front of the labyrinth are drawn through this point also. So this should cover 99% of the vapors.

We'll see how well this first revision works before we go any further, if necessary. The spark plugs in this donor engine tell me that the engine is in pretty good shape, so I didn't bother taking any compression or leak down readings before yanking it out. I do plan on doing that for baseline purposes once it is in and running.

More to come.

Thanks,

ProfessorGT
(Sent from my iPhone4)