Just wanted to start a seperate thread because the other one was getting crowded.

Dan, thanks for the deffinitions. My primary goal wasn't to get rid of the pcv, just giving the person a suggestion on what could possibly be done because he didn't have one. I deffinately now see the advantages of p(ositive) vs. p(assive) ventilation.

Here is another quesiton for you then. Why is it, at least on my year slant, chrysler used both a pcv and a breather? The breather has a line on the top that goes to the bottomside of the air cleaner, and on the bottom of the breather a smaller (5/16") to the gastank vents. Why couldn't they just take the tank vents directly to a carb or intake vaccum vent source?

Reposting this here for people who didn't see the other thread:

Some definitions-

Open ventillation: The crankcase is simply vented direct to atmosphere by means, usually, of an open-element breather cap. This ensures crankcase pressure doesn't build up and blow gaskets and seals or force oil up past the rings, but that's all it does. The crankcase fumes, which are smelly and toxic even on an engine in good condition, simply waft into the engine bay to make a mess (and usually into the passenger compartment via the cowl air intake to stink it up).

Passive crankcase ventillation: This system makes use of a road draft tube which extends from a point above the oil level (usually the valve cover) down to a point below the engine, such that the forward motion of the car generates a draft across the open bottom end of the draft tube, which creates suction in the tube and pulls crankcase vapors out. This system relieves crankcase pressure, too, but it also vents the vapors down and away from the passenger compartment and engine bay. As long as you always go over 30mph and never drive through snow deep enough to clog the draft tube, this system works OK. By circulating fresh air through the crankcase due to the draft (not present with a simple open vent), water vapor and other undesireable gases are flushed out of the crankcase, with the result being that the oil and engine internals stay cleaner, longer.

Positive crankcase ventillation: This system replaces the road draft tube with a hose running from the valve cover to the intake tract, *with a flow control valve in the hose*. This is an enormous improvement over passive or open ventillation, for it ensures a constant clean sweep of fresh air through the crankcase even at idle and low speeds when the open or passive systems do nothing (or next to nothing). Escape of crankcase gases into the engine bay and passenger compartment are virtually completely eliminated. Positive crankcase ventillation also helps combustion efficiency and reduces oil consumption by maintaining slight negative pressure (vacuum) in the crankcase, and keeps the engine oil and internals much cleaner, much dryer, much longer than an open or passive system.

The flow control valve (PCV valve) does several useful things: It reacts to changing engine vacuum to meter the optimal amount of crankcase gas into the intake tract so as to maintain the desireable negative crankcase pressure while not upsetting the fuel/air mixture. The one-way PCV valve also prevents a backfire from reaching the crankcase by slamming shut in response to a sudden rise in intake tract pressure.

Even racers, who as a rule used to run open breathers, have discovered the performance benefits of negative crankcase pressure. Racing PCV systems don't rely on intake vacuum, 'cause usually when racing, there is none! (wide open throttle). Instead, these elaborate systems rely on negative-pressure pulses very far upstream in the exhaust system, and vent the crankcase via special valves to the headers(!).

It is a common but mistaken notion that anything nominally for emission control and identified by a 3- or 4-letter abbreviation simply has to be bad and must be eliminated. A properly-implemented PCV system offers a huge list of real benefits and zero drawbacks.

All crankcase ventillation systems have a breather. The purpose of the crankcase ventillation system (of whatever type) is to take gases and air out of the crankcase, so there has to be an air inlet, and the air has to be filtered so as not to pull ambient grit into the crankcase. Air flows in through the breather, sweeps crankcase gases and water vapor along with it, and out via the PCV valve (or road draft tube).

The most basic type of breather is just a cap with oil-wetted mesh, keratin fibre or polyurethane foam very much like a lawnmower engine air cleaner. The problem with this type of breather is that under zero-vacuum conditions (wide open throttle, e.g. up a long hill), the crankcase gases flow right back out through the breather, i.e., the system reverts to a basic direct-vent mode. This causes objectionable odors in the passenger compartment and the release of unburned hydrocarbons into the atmosphere, where they create photochemical smog.

So, starting in 1964 in California and 1968 nationwide, the CLOSED positive crankcase ventilation system was introduced. On this system, the breather still has a filter mesh in it, but doesn't vent directly to atmosphere. Instead, it vents via a hose to the air cleaner. Most of the time, this is just exactly the same as venting to the underside of the breather with no hose. However, under those low/no vacuum conditions when an open system just vents off into the atmosphere, there's still suction above the throttle plate due to the influx of air through the air cleaner and into the carb, so the crankcase gases are still drawn off out of the crankcase and consumed in the engine.

On an engine in bad shape, with lots of blowby, these closed systems can "wet down" and ruin an air filter element with oil in short order. Other than that, they constitute an improvement.


Ah, you've got a '70 California or '71 50-state car. On those, the crankcase was used to store fuel vapors evaporating from the fuel tank and carburetor float bowl with the engine off so that they wouldn't be released to the atmosphere to cause photochemical smog. The fuel tank vent was plumbed to the crankcase breather, and the carburetor bowl vent was plumbed to a fitting on the fuel pump housing (which led into the crankcase). When the engine was started, these fuel vapors were drawn off via the PCV system and burned. This early evaporative emission control system was notorious for causing poor hot starts and hot idling problems. In '72, the crankcase was no longer used to store fuel vapors. Instead, a canister filled with activated charcoal granules, mounted at the right front corner of the engine bay, was used for this purpose (and still is, to this very day). The canister system had none of the starting and idling problems of the earlier system, and retrofitting a canister to a '70-'71 car is not very difficult.

Connecting the gas tank or float bowl vents to a vacuum source, or to each other, would be a VERY poor idea.

Wow, your a few bookloads of knowledge!

Good call, I do have a 71 car. But there was nothing plumbed to the fuel pump.

Dan,
That was the best explanation that I've read. Excellent posts! You should turn them into a short article for the article section.
Bob D

Bob -- I'll work up an article as soon as I get some time.

Pierre -- originally there would've been a fitting on the fuel pump body "inboard" of the round section. These pumps are not as widely stocked as the pre-'70, post '71 pumps and so it's very common to find these '70-'71 cars with the earlier/later fuel pump without the bowl vent fitting. Where does the line lead from your carb float bowl fitting, if there's still a line on it?

The original 1920 is long gone. I'm running a GM TBI system now with a PCV port. The opening on the block for the fuel pump is blocked off, with the electric pump strapped to the frame rail near the right rear tire.

Oh, cool! How's it run with the TBI? What'd you use for a computer? Got a writeup?

It runs.... well, lets just leave it at decent.

I bought the kit from howell. It came with practically everything except the fuel line. 2 injector throttle body. The computer is a GM 1227747. It has the capability of controlling spark but I'm not using that. I'm running a magnetic pickup distributor with msd-6a.

It has some drivability issues. Overall it runs rich, and in the summer months usually sets the too rich code. I haven't noticed it lately now that it starts getting cold, as I guess the colder, denser air charge makes up for the excess fuel. There is still a bog when stomping it, but lots less noticable after it warms up. The deceleration cutoff is also too steep, as you can feel the rpms drop too low when coming to a stop. This still remains even after 5 or 6 'custom programmed' chips from howell. I learned my lesson here this time, next setup will be programmable.

I don't have a formal writeup, but I do have a post in the slixers gallery section of this forum detailing my installation.

Dan,

I have seen cars where the fuel bowl vent was connected to the gas tank return line.

What are your thoughts on that?

Another thing I have seen is the fuel bowl vent, vented into the bottom of the air cleaner base so vapors are burned.

Thoughts?

Looking through the junkyard you see all kinds of strange things but
I have never seen any of these lines hooked to a vacuum source on a MOPAR.

Thanks!

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

This makes problems because both the carb bowl and the fuel tank need to be vented to atmospheric or slightly-lower-than-atmospheric pressure. Venting them to each other means neither is vented correctly and it can cause weird driveability and economy problems.

Really, really bad idea, this. It will not only spoil fuel economy and driveability, but create a very large fire hazard (that is, a very large hazard of a very large fire).
It's amazing more people don't blow themselves up trying knotheaded tricks like this what you're describing.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Dan,

Please explain why you would get weird driveablity problems and economy problems.

I know several folks that have done that when the charcoal canister blows up. They tell me it still vents to the top of the carb neck where the air comes in so vapors are drawn in.

What are some options to fix this? other than buying a new canister from Year One for $80.00.

Thanks,

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

Because the pressure above the liquid fuel in the carb bowl has a significant influence on the carb's metering.
Buying a new canister from a parts store for considerably less. They are all rather universal, as long as they have the connections you need and you can figure out how to mount them. Just because the parts store hasn't got a listing for a canister for your particular Mopar doesn't mean you need to go to Year One.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

http://tinyurl.com/3hvolx

The top of this carb has 2 tubes pointing straight up just inside the air cleaner mounting base. Aren't this tubes fuel bowl vents and aren't they being vented into the air cleaner?

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67Cuda,FAST EZEFI,340cu,CR=10.25,RollerCam&Rocker (XR268HR,#20-810-9)(#1622-16)(EddyRPM#60779,#7576), (MSD6AL,#6425) A904, GearVendorsOD, 8 1/4,3.55:1, ClassicAir

There is enoughfuel in your bowl to cause some real damage....but the amount of fuel venting is low...

The amount of fuel venting from your tank as it warms up on a hot day or after filling can be considerable.

.....so you have the tank venting into your aircleaner....and the motor coughs a little on startup...and you get a cool power bulge in your hood.....

The canister is a way to store fumes in a safe fashion.....till they are drawn in and burned.

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Yeah....Im the one who destroyed this rare, vintage automobile.....