Rambler 195.6 OHV crankcase ventilation (PCV)

15 sep 2022

positive crankcase ventilation, aka PCV

Crankcase ventilation is absolutely necessary. even the best rings in the best engines don't seal perfectly; water is a major combustion byproduct and it definitely ends up below the rings and needs to be removed before it condenses and ends up in the oil.

AMC tried out various crankcase-ventilating schemes between 1956 when this engine was new through it's final year in 1965. this was originally a road draft system; a 1" or so tube dangled down to the ground where it was hoped a mild vacuum would be generated by the car moving along the road, and draw out "fumes". mainly it served to spray dirty oil and mist over the chassis and of course the roadways and the air. (older drivers well recall the black stripe down the middle of every roadway.) adding to this mess in some years the valve cover has vents at both ends that both let air in and oil mist out, at random.

the goal of crankcase ventilation

piston rings do not seal perfectly when new, and leak more with wear. the major chemical components of ring blowby are air, exhaust, water and fuel. (reference: Blowby Gas Composition in SI Engines.pdf by Karel Pav of Skoda Auto.) added to this is a large volume of lubricating oil mist generated by the whirlwind of crankshaft and connecting rod motion.

if there were no oil mist, crankcase ventilation would be a lot easier.

water is about 4% of ring blowby (see referenced paper) and ends up in the crankcase. It's not a liquid until it condenses, which requires removing heat. in an active ventilation situation (PCV) the water remains a gas and is sucked into the intake (which already has ordinary atmospheric water anyway).

PCV system experiments

In the last 10 years I did a lot of experimenting with PCV in my roadster, to clean up the engine compartment and keep oil mist out of the intake. Most of the problems I had I created myself by doing foolish things with the engine, like using it in a sports car environment, sloshing oil in every hard turn causing oil pressure loss (air under the pickup). I ran an extra quart of oil for a while and finally settled on an expensive Accusump system. That solved the problem.

The PCV systems below all worked fine, and the stock one does get some oil mist desposited up into the floor of the intake manifold, but it's not enough to be worth any effort to solve (unless you like to cause your own problems, like I do.) But the systems below all worked, and if you really need a catch-can then a carefully chosen spin-on oil filter and adapter and hose will set you back $40 in 2020 money, though it won't say MISHIMOTO on it (a Sharpie will cost you another buck). It will work better too.

PCV, velocity-chamber cover, external catch can/filter

though the velocity chamber valve cover (details below) by itself was fairly effective, certainly better than stock, it still passed about a teaspoon of oil every 1000 miles or so into the intake (determined by installing a clear fuel filter in the PCV hose). catch-cans being needlessly expensive, and a life-long adherent to Rambler Mentality, i refused to pay $200 for a pretty Mishimoto "billet" can. installed here is an inexpensive Derale remote oil filter adapter and a tall filter without an anti-drainback valve (WIX WL10100).

catch cans work because they slow the velocity of the gases so that fine droplets coalesce into larger droplets and separate, either via gravity or filter mesh. depending on where they are mounted they may remove heat too. if cool enough the catch can will also undesireably condense water vapor. the collected water will trap dissolved gases and particulate matter that would otherwise be better off fed through intake to combustion chamber, where it would go out the exhaust. i'm convinced that this is the cause for the alarming catch-can contents in all those youtube videos -- without an aftermarket catch can that junk wouldn't be produced in the first place. that said, this Engineering Explained youtube video seems mostly sensible.

valve cover velocity chamber

since the top of the valve cover is the highest point in the engine, and with relatively low oil flow up there, it seems the most sensible place to draw from. (the subsequent 40 years of engine design is a big hint too.) to that end i built this velocity chamber equipped valve cover. a large internal baffle blocks direct flow to a .75" hole that feed the "chamber" on top. the chamber is packed with coarse bronze wool. the inner baffle is very wide, the valve cover is relatively cool and provides area for condensing droplets.

the stock cover vents are welded shut and the side cover outlet blocked. with engine running my hand sealing the oil filler neck tube rapidly draws solid vacuum at idle. construction details below.

PCV valve

PCV valve flow rate is an esoteric and mystical subject, i assume, because there seems to be zero information available. i stood in the back of an Autozone, gently lifting the business end of each valve out it's container and sucking on it to guesstimate relative flow rates ("a lot" ... "not much"). looking like a crazy person For Science. i'd made a list of candidate part numbers via images on web search and via my rigorous scientificalish characterization process above determined that the valve for a 1980 Honda Civic had the highest restriction and the right physical form factor. If you have numbers on these things, by all means contact me.

the valve i'm using has the pleasant part number of PCV1234. common as dirt and very cheap.

(as an aside, using engine vacuum as the source of air movement in the crankcase seems like a primitive holdover from the 1950s. a better system would be a dedicated pump and separators. then actual gases, minus even water, would be a no-brainer to return to engine intake.)

oil filler cap and dipstick

for a real PCV system the closed 1965 filler cap is best. it doesn't have the large area vents and steel wool that was necessary for low restriction with the old road draft system (the cap was retained in most 195.6 PCV systems until 1965).

with positive ventilation, the (reduced) flow through the unvented cap will be fine. it also means when shut off, oil mists won't waft out the cap all over the engine compartment. with adequate ventilation, that won't matter.

early factory experiments in crankcase ventilation

in hindsight it all seems quite haphazard.

road draft: this lovely but poorly photographed 1961 color diagram shows fairly well the mostly-useless road draft tube but also the oil-mist/crankcase vapor volume that needs scavenging. the filler neck is not shown, but is on the right side lower crankcase and extends up to nearly valve cover height.

early PCV: this excerpt from the 1962 TSM shows the side cover with 1/2" hose nipple, the most commonly seen system on most 195.6's. the TSM page shows the PCV screwed into the edge of the L-head but on the OHV it's on a brass street elbow.

looking at an actual engine, you might assume that the baffled side-cover location would be fairly free of mist and flying oil, but you would be wrong. every system i used had much oil mist out that cover. in addition the covers leak very nicely.

this system works poorly; i doubt it can clear the crankcase; there are too many openings to the outside (oil filler, two locations in the valve cover).

late PCV: final model year 1965 had more or less all of the PCV variants available at once. 1V carb OHV, 2V OHV, and L-head each had their own arrangements. the drawing below are detailed and acccurate, but very hard to read even in print; suffice to say in real life it looks like most 1960's..1970's PCV system. no big deal here.


the above two components are desireable for an improved PCV system. of course everyone knows that the chromed valve cover makes the car much faster.

PCV improvements

valve cover PCV with velocity chamber construction

here are details on the 2019 scheme.

this is a '61 or '63 valve cover, which had open ports fore and aft, which i tapped flat and welded up.

the draw hole was placed in the frontmost full ribbed section. the ribs provide the gap for the baffle. the draw hole and two drain-back weep holes live under the fitted velocity chamber which was welded onto the cover.

there's a bit of 80%-open steel screen over the draft hole. coarse bronze wool is very loosely packed in, then a rectangle of screen slips over it.

JB Weld was smeared over the welds to seal any pinholes and the cover got painted, wool stuffed and screen and grommet inserted.

demist-can PCV

this was the prototype for the velocity-can cover. it worked OK, it's just ugly.

after the first pass at valve cover PCV, i went back to a side-cover draw but with a special can between side cover and PCV valve. the hose from the side cover feeds the bottom of the can, the valve in a grommet on top. inside are two perforated separators and coarse bronze wool.

the operation of the can isn't immediately obvious -- it is a widening of the hose to drop velocity, it is not a catch can. whatever the air flow (fractional CFM) is inside that half-inch hose, it is fast enough to keep oil mist in suspension. the same volume of gas in a three-inch hose is far, far slower; this allows mist to fall out, and condense/collect, onto the bronze wool. this system works great. oil that condenses in the can drains/dribbles back into the side cover, and out the top is nearly dry.

this system has worked well for half a year, but it's ugly and inconvenient under the hood, the velocity chamber solved the basic problem.

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