Crankcase ventilation

[MBF]

I know there are threads about this in the other forum. On one of my parts trucks I found where the previous owner (a very skilled mechanic) had cut the tube off the road tube and welded up a fitting to clamp a piece of hose on. He had a fitting under the carb on the intake(where the wiper gets its vacuum source) that he ran the hose to, with a hooded/filtered vent that he fabricated for the oil filler tube. I hooked this up w and w/o a pcv valve in the hose, but used a filtered cap from a slant six on the filler tube. After a slight adjustment of the idle circuit the truck ran fine. Goes down the road well, doesn't ping (well not in February). When idling, I can feel the vacuum on the intake of the oil fill. Is this too simple to work or what? Mike

[grey beard]

Mike,

What you just described is a home-made scavenging system that will work quite well to keep smelly blow-by from stinking up your cab on hot, hard pulls. One of my B series trucks had this problem - so much blowby that it smelled bad on hard pulls up hills. The cab even began to fill with smoke from the road draft tube blowkby..

On antique tractors, I've often seen these setups, where someone directed a hose from the oil fill tube back to the intake. Essentially, this is what a PCV ssystem does - capture excess escaped compression from the crankcase and reburn it, only a PCV system does it much more completely.

First, PCV systems work with positive pressure (hence the name) that circulates in, around and back out of the crankcase all the time the engine is running. This is done by making a source for clean, filtered air - usually the air cleaner - and pulling fresh air from here into and though the crankcase. The home-made scavenging systems just diverted the excess crankcase pressure back to the intake, with no source for clean air to enter somewhere else.

I wish it was knowable to me at what temperature crankcase oil must rise before it can start expelling water vapor from the oil. In these days of $69 digital lazer touchless thermometers from Harbor Freight, it would take only a few minutes to find out how warm the oil pan is in relationship to the water jacket. I say this because without a PCV system, the only way whereby engine oil can begin to expel condensation from the last run is for that oil to reach whatever temperature it is that it takes to make this happen.

I can imagine an antique car nut going out to his garage in this February weather, firing up old faithful, and letting it run till the dash temp gauge says normal - whatever that is - maybe 160 or 170 degrees F. Then he thinks he's warmed her up good, and shuts it back off and goes back to his popcorn. Had he pulled his dip stick and dropped one drop of motor oil on his hand, it would still have been below body temperature - way too cold for any condensation to begin to boil off. In point of fact, what he just did with that short engine run actually put MORE condensation in the oil than it removed.

Our engines weigh a minimum of 200 pounds more than belly button motors (SBChevies), maybe even 300 pounds more. It takes time to heat up all this heavier cast iron - much more time thank is required for smaller and lighter engines. My own guestimate is that a flathead must run a minimum of 30 minutes winter idling before the engine oil reaches 125 degrees. If you could put the engine to work quickly - make it climb a mountain while it warmed up, it would get hotter much quicker. My point is that until motor oil reaches a temperature where water vapor begins to evaporate or boil off from that temperaure, no condensation can possibly leave a road draft tube vented crankcase. There's just nothing to make it happen, so it doesn't. This is why so many older engines were so badly crudded up in the valve chambers when we got to tear them apart. They seldom really warmed up far enough to clean themselves up - UNLESS there was a PCV system in place to go to work as soon as the engine started up.

It is for this reason that i xtrongly recommend a 180 degree thermostat in any good flathead, if you hope to keep the motor clean inside. This helps the enire motor block warm up more quickly to help get this evaporation process started. JMHO

[MBF]

Thanks, but I do have a source for clean air on the oil fill tube. The filtered &vented cap has a fitting for a hose that I plan to run up to the air cleaner. The air cleaner would be the primary filter, and the mesh screening inside the oil fill cap would act as a secondary filter. My question is whether or not an actual PCV valve is needed on the tube going between the road tube fitting and the intake manifold, or will this vacuum line work fine w/o a PCV/check valve? I have it running about 180, but in this cold weather here in NY it isn't for a long period of time, and only when the roads are clean. I'm trying to get it ready for show / cruise season that will be starting up in a couple of months. Can't wait for warmer weather. Thanks GB-Mike

[billwillard]

That would depend on how big the oppening in your line is. I would run a pvc valve. A pvc valve does two things. IT restricts the amount of air drawn by having a very small oppening. #2 it has a valve in it that closes if vacum falls too low. fram fv333 or a/c cv1007c are part #s for pvcs. Be sure you install it so it opens. Some are spring loaded so it can be laid on its side but others just have a small ball that vacum pulls open and these have to be installed vertal.

[MBF]

That's what I did. Now if the weather would only cooperate here I could take her out for a ride and get it hot. Maybe next weekend-every day is a day closer to Spring. Thanks for the help. Mike

[MBF]

I put a PCV valve in the vacuum line from the road tube to the intake manifold. I bought a new 60 gallon compressor @ LOWES (their web guy made a mistake and had it listed for $89.00 instead of the usual $499.00) so how could I pass that up when I needed to upgrade anyway-and its American made or at least assembled! Anyway, Monday it was colder than hell outside but I needed the rack body to bring my new purchase home. Ran ok, but even w the radiator blocked off-couldn't get any heat showing on the temp guage-usually runs around 180 when its above freezing out. I'll finish off the plumbing this weekend and maybe post some pics. Mike