The Stovebolt Site Forums The Shop Area Engine Shop 1956 235 PCV- What advantages?

I recently brought home all of the original 1956-vintage factory PCV system from a 235 engine at a salvage yard. I have similar engines retrofitted into 3 earlier trucks. The shop manual info about keeping the misc plumbing clean is pretty straight forward. But I'm unclear what the advantages of PCV vs. road draft tube were as viewed through the perspective of the time. Strictly to reduce pollution? Improve mileage? Increase power? Would I need to re-jet the carb to accomodate increased airflow if I went this route? I would appreciate the feedback of anyone who has done this to determine if this system is worth adding to an engine or not. Thanks! Dave

It was advertised as providing the following benefits especially in multiple stop/start applications like delivery vehicles:

A PCV valve can help extend the life of your car's engine by preventing the buildup of sludge in the oil and by keeping moisture out of the engine, the unburned gases which the draft tube needs air flow to evacuate have additives/contaminants which remain in the engine with the mositure leading to corrosion.

I have added a PCV system to a few customers engines that complained about the blow-by smell.

a PCV system ventilates the engine at all speeds, even idle. A road draft tube is ineffective below 30 MPH because it needs airflow past the end of the draft tube to create any flow through the crankcase. The biggest advantage is the introduction of clean air into the crankcase, if the inlet air is drawn from the "clean" side of the air filter. Road draft systems that rely on slots in the valve cover, or other unfiltered sources for the vent air introduce dust into the engine and increase abrasive wear of the engine parts. Even the vented oil fill caps had a very ineffective horsehair-type filter media in them.

When PCV systems became mandatory in the late 1960's, there was a dramatic improvement in engine life due to the reduced contamination of the lubricating oil.

When Califronia mandated that all engines should be equipped with "sealed" vent systems in the mid-1970's, the shop I owned retro-fitted hundreds of older engines with a special kit the Commiefornia legislature required. We plugged up the road draft tube, installed a PCV valve, and introduced clean air into the engine by installing a vent nipple from the filtered-air side of the air cleaner to the valve cover or crankcase. A sealed oil fill cap was part of the conversion to prevent any fumes from getting to the atmosphere. I can't recall ever having a problem when that was done.
Jerry

It works at idle, road draft doesn't.

Another advantage of having a controlled (somewhat) pressure in the crankcase is improved sealing of some piston rings.
I say some because even though many modern rings are expanded partially against the cyl walls by the pressure differential across the ring, older ring designs depend on the spring tension to expand them.
For gas expanded rings to work best there should not be much positive pressure below the ring and the PCV can evacuate the crankcase of blowby and keep a more uniform pressure
This is a delicate balance of having a system that can move just the proper amount of air to keep things right and has to take into consideration the pumping action of the pistons below the rings.
A 4 cyl with the pistons moving up and down in pairs but at opposite times tries to move a lot of air in and out of the crankcase while a 6 cyl or 3 cyl has less pumping action because of the piston movement being 120 degrees apart.
Most 1 cyl small engines pump so much air below the piston that their crankcase vent system has a check vlve in it that allows air out but none in--they have no fresh air let in.If the crankcase gets unsealed at dipstick or a missing bolt the pumping action will cause oil overheating and increased oil loss at many joints.
So a well designed crankcase vent system can help ring life, oil control, and compression control by letting the rings work more at a optimum.
The horsepower seekers have carried this to a high level with exotic design of gas operated rings and pump evacuated crankcases.
My seat-of-the-pants dyno can't register any difference in power output with or without PCV but the oil leaks sure do pickup when the crankcase is positive pressure.

Dave, You are right ontrack to be concerned about change in mixture when PCV is added because the engine is being fed air without the carb adding fuel.
Well, some fuel is added--the combustion blowby is fuel as is oil vapour that is carried over from the crankcase and there is the fresh air being inducted from the vent--but nothing is controlling all these variable sources.
And then there is the concern about where it is added to the induction tract.Is the inlet to the manifold on a runner to a cylinder pair or in the plenum below the carb? This can effect the mixture to a cylinder or pair.
So with an added on, not engineered PCV, we many times have compromises that can adversely effect engine life and performance.
I have many times questioned the wisdom of OEM placement of PCV entry into the inlet tract as some are notoriously horrible at plugging when the engine is older or the results of minimal maintenance of oil is overloading the air flow design.Ford 302 comes to mind.
Done lots of valve work with my eyes and mind open and I never saw the port and valve pluggage with coke (carbonized oil and ???)before PVC that we did after.Of course that changed again (for the better) with the coming of port FI because the valve area is sprayed w/fuel on each shot, and the inlet valves are much cleaner now.
Cleaning up for head work by manual methods can be a real lesson in carbon buildup in the engine.

I see a lot more pros than cons about adding the PCV. I suspected that there were advantages because I also have a GMC 270 engine yet to be installed in an upcoming 52 GMC project that has the PCV setup and it seemed that GMC didn't skimp on the best technology available. As long as I hang on to all the original road draft stuff, it wouldn't take much to change back if desired. But at only 1K to 2K miles per vehicle per year, it may take many years to determine any fouling problems. Another question: should I splice in a modern PCV or use the original, all metal GM PCV that looks like it will last forever? Thanks to all for the information. Dave

The original item can be disassembled and cleaned. If you've got one available, go for it. It will look a little more period-correct, also.
Jerry

Yes, I've actually got a couple of the originals. I prefer authenticity whenever possible. Thanks Jerry. Dave