model b -- P C V positive crankcase ventilation
- Thread starter Ron Engen
- Start date
BRENT in 10-uh-C
Well-known member
Ron, I am not sure exactly what you mean by "relieve the rear main seal", but there are some parameters that must happen to have an effective PCV system. First, you MUST have a rubberized-type seal in the rear main so as to seal off air in that area from being sucked into the crankcase. An original type slinger only will not work.
Next you will need an oil fill tube that can seal off with no venting into the crankcase but can allow excessive pressure to vent outward. This is needed at W.O.T. when manifold vacuum is lowest and cannot effectively create a negative pressure.
Finally if you get the above two things solved, you can drill the valve chamber cover and install a PCV rubber grommet from a Chrysler and then install a 90° PCV valve and then drill a hole on the back-side of the intake manifold about 2" above the carb. mount flange and thread it for a 90° pipe fitting. Plumb the two together and there you are.
Now, something to think about....
James Rogers (Dreamwerks Engines) and I have discussed this several times and one of the things that makes a good argument is that when the crankcase has a vacuum on it, the crankcase windage is severely limited. Based on the design of tappets and many other internal parts, oil that is being beat around from the crankshaft and carried in the windage is likely necessary to properly lubricate those items. If this oil mist is not there due to the vacuum, are you creating other issues? I am of the opinion that if you are racing your banger where parasitic drag from that oil is a problem, with limited amount of driving, it probably isn't an issue using a PCV but in a street engine, maybe a PCV isn't necessarily a good thing.
Next you will need an oil fill tube that can seal off with no venting into the crankcase but can allow excessive pressure to vent outward. This is needed at W.O.T. when manifold vacuum is lowest and cannot effectively create a negative pressure.
Finally if you get the above two things solved, you can drill the valve chamber cover and install a PCV rubber grommet from a Chrysler and then install a 90° PCV valve and then drill a hole on the back-side of the intake manifold about 2" above the carb. mount flange and thread it for a 90° pipe fitting. Plumb the two together and there you are.
Now, something to think about....
James Rogers (Dreamwerks Engines) and I have discussed this several times and one of the things that makes a good argument is that when the crankcase has a vacuum on it, the crankcase windage is severely limited. Based on the design of tappets and many other internal parts, oil that is being beat around from the crankshaft and carried in the windage is likely necessary to properly lubricate those items. If this oil mist is not there due to the vacuum, are you creating other issues? I am of the opinion that if you are racing your banger where parasitic drag from that oil is a problem, with limited amount of driving, it probably isn't an issue using a PCV but in a street engine, maybe a PCV isn't necessarily a good thing.
1931 flamingo
Well-known member
Sounds like you want to re-invent the wheel instead of finding a fix to "relieve the rear main seal". Does that mean you have a leak?
Paul in CT
Paul in CT
George Miller
Well-known member
But I would think a PCV would make your rings, pistons, and valves last longer, because of the oil fumes. Back in the 50ts when they started using PCV valves on truck engines, they started running more miles before overhaul.
BRENT in 10-uh-C
Well-known member
May be George, ...but IYO which one would have more lubricating properties, --an oil mist or oil fumes? Also, think about how the guides get oiled anyway.
I installed a PCV without the sealing Brent mentioned but could not get the carbs to work right. I took it out, but left in the drilled and tapped hole in the upper rear of the valve cover and put in a 90* brass fitting. You can feel the little puff-puffs of air coming out. It reduces pressure in the crankcase and I lke to think there is some cross ventilation from the oil fill tube, but that is probably minimal at best. One of teh accessory fill caps with the tube extending down into the slipstream might create a little vacuum effect for the cross ventilation.
George Miller
Well-known member
We may not agree on this one. I think the crank will be throwing plenty of oil around. Coming off the mains and rods, plus the dippers. But I will agree it is some thing to think about. Which I have not given it much thought.
Thank you guys for your thioughts and in put.I do not use a mechanical fuel pump,so I welded a fitting on the fuel pump block and added a second breather pipe.. My thoughts are if there is some way to come up with some type of vacum source to help take the pressure off of the rear main leak problem. I believe there is plenty of oil splashed and pumped around in the engine to lubricate adequetly.
MrTube
Well-known member
I think you'd want to leave the breather cap on the fill tube. Every PCV system I've seen always had a fresh air inlet as well as the suction end. For example on V8's you have the PCV valve on one valve cover and the fresh air inlet on the other valve cover.
On the A or B I would think you would install the PCV valve in the valve cover as high as possible and then just have a good filter on your fill cap to filter incoming air.
You don't want a vacuum in your crankcase either as it will cause stress on seals. There are warnings against this in any service manual for a modern car. The system was developed simply to keep fumes from venting into the atmosphere, I don't think it benefits the engine in any way.
You want your crankcase to remain as close to atmospheric pressure as possible.
All of the cars I worked on with PCV systems also had to be tuned with the PCV system in use. If you plug the valve the engine would run rich. Tune it without the PCV valve and it will run lean with it.
The fresh air vent I'm talking about on modern engines is the larger tube you usually see going to the air cleaner housing or throttle body. This is also where oil comes out of on older engines when the PCV system can no longer handle the amount of blowby.
I don't know if the system hinders lubrication in an A engine or not. Personally I would leave the breather system and forget PCV.
On a side note, I thought PCV systems did not show up on cars until the early 60s for CA and the late 60s for the rest of the US?
On the A or B I would think you would install the PCV valve in the valve cover as high as possible and then just have a good filter on your fill cap to filter incoming air.
You don't want a vacuum in your crankcase either as it will cause stress on seals. There are warnings against this in any service manual for a modern car. The system was developed simply to keep fumes from venting into the atmosphere, I don't think it benefits the engine in any way.
You want your crankcase to remain as close to atmospheric pressure as possible.
All of the cars I worked on with PCV systems also had to be tuned with the PCV system in use. If you plug the valve the engine would run rich. Tune it without the PCV valve and it will run lean with it.
The fresh air vent I'm talking about on modern engines is the larger tube you usually see going to the air cleaner housing or throttle body. This is also where oil comes out of on older engines when the PCV system can no longer handle the amount of blowby.
I don't know if the system hinders lubrication in an A engine or not. Personally I would leave the breather system and forget PCV.
On a side note, I thought PCV systems did not show up on cars until the early 60s for CA and the late 60s for the rest of the US?
Last edited:
MikeK
Well-known member
I have great respect for Brent, but take exception with two points he mentioned: The need for a rear crank seal, and the need for a modified oil filler that blocks intake air.
I've posted on PCV stuff going back to Shelly's old Fordbarn, here's a repeat from a 2008 post. Take it as you wish, old info:
Positive Crankcase Ventilation
A Different Way to Say Goodbye to Model A Blow-By?
A treatise by Mike Keating
First let me start by saying I applaud many A’ers for taking charge and finding an alternative fix to the classic blow-by problem with a PCV valve. Traditionally Model A’s were adorned with aftermarket road draft tubes, or maybe just a pot scrubber and old sweat sock over the oil filler pipe. Tell me you haven’t seen the sock thing. Don’t tell me if it’s on your own A, even though you claim it as a period accessory! Some old car owners have taken to eliminate the problem rather than move it out of sight on their touring cars. I must, however, throw in my two-cents on the entire P.C.V. issue strictly from an engineering development point of view. I fear that long term use of an improperly adapted P.C.V. system may cause significant engine damage. I am not opposed to Positive Crankcase Ventilation systems. They are beneficial if properly designed and not just an uncontrolled suction system. But, are they a good thing just because modern cars have them? You decide.
I will start with what blow-by is, then blabber about some of the BAD THINGS that can happen if the PCV system is not engineered and calibrated to account for flathead valve characteristics, crankcase cross-ventilation, crankshaft bearings with or without modern seals or pressure lube, effects on fuel mixture during starting and running, and combustion temperature. I will also explain the GOOD THINGS. Finally, I will address the untold ‘why’ history of Positive Crankcase Ventilation (PCV) - if I haven’t put you to sleep by then. This is all presented I.M.H.O., please take it as such.
BLOW-BY, or gases leaking past rings or through ring end gaps and exhaust valve stems into the crankcase consists of two general types of unwanted substance and one good substance. Exhaust mixed with oil vapor, which everyone associates with the word ‘blow-by’ are the first two parts, and unburned fuel vapor passing the rings during the compression stroke is the third component. Few people think about the raw fuel vapor, but it is also part of blow-by. The exhaust, which contains water vapor, acids, and leftover, smaller hydrocarbons from incomplete combustion are bad, and so is the raw fuel blow-by during the compression stroke. The misty oil vapor is actually good in the engine, but makes the mess that everyone dislikes when it gets loose under the hood.
P.C.V. ENGINEERING: Automotive manufacturers spent research money to get rid of that road draft tube (your pot scrubber and sock) by sucking the crank case blow-by back into the engine intake manifold. Sound simple? There were plenty of variables. Some early PCV systems created quite high crankcase vacuums at times, occasionally 12” Hg (Mercury) or more. Later systems reduced this peak vacuum considerably. Keep this in mind as I discuss effects. Eventually, Detroit got it right. It took years. Every application is a little different.
ENGINE VALVES: With a PCV system, valves must be considered. Flatheads and OHV designs create entirely different problems. With any engine there is clearance between valve stems and the guides. The intake valves almost always have less pressure on the end of the valve stem that is exposed to manifold vacuum than the end that is inside the engine, even if the PCV system creates a crankcase vacuum. Lubricating oil naturally gets forced into the valve guide by the pressure difference. This is good, to a point. In an OHV you often need a stem seal to control how much oil flows down the intake guide because gravity floods oil down the stem to the guide. In a flathead without direct oil supply to the valve stems, like in a Model A or B, only the oil mist created by the running internals lubricates the valve stems where they enter the guides. Drop the crankcase pressure too much with an untested PCV system and the oil vapor falls out of suspension in the air inside the crankcase like rain from the sky when the barometer drops. Don’t believe me? Fill a flask with oil mist from a fogger and attach a refrigeration vacuum pump. Watch the mist fall. OK, maybe what little mist is left will suffice for the intake stems because the even higher intake manifold vacuum helps, but the exhaust valves are an entirely opposite story. Here exhaust pressure and heat blow down and through the stem clearance into the valve chamber/ crankcase. That pressure differential keeps oil OUT. In an OHV the exhaust stem is flooded by gravity with oil which cools and lubes it. In your flathead A with upside down valves, no such luck. Make this worse by pulling excessive vacuum in the crankcase with an unknown PCV calibration that causes the oil mist to diminish, and at the same time greatly increases the rate of exhaust flow down the stem clearance, and you now have exhaust valve stems/ guides starved for oil mist and cooling. Not good for long life or stick free valves. Several ways to fix this problem in a Model A/B, easiest is to keep the PCV regulation of crankcase vacuum in a flathead designed with no direct oil feed to the valve stems as close to zero as possible. You need just enough negative pressure to stop blow-by from exiting the engine and no more.
CROSS VENTILATION: To get the benefit of scavenging water vapor and nasty corrosive combustion byproducts out of your crankcase before they contaminate your oil and make it acidic, you need this. A PCV system must have a clean air source at one end and the valve at the other end to positively VENTILATE the crankcase. If your house is airtight and you fill it with smoke, an exhaust fan will not clear it unless you open a window. Not a window in the same room, you must open a window on the opposite side of the house. On an OHV V8 this is easy. A hose from one valve cover, preferably at one end of that cover, to the air cleaner is the open window, or fresh air source. A second hose with a PCV regulation valve from the other end of the opposite valve cover to the intake manifold is the exhaust fan. Cross ventilation. NOT so easy in a Model A/B engine. Deep thinking (engineering!) required.
CYLINDER WASHDOWN and oil dilution from gas: If you want to increase the amount of raw fuel that goes past the rings during compression, pull a high vacuum in the crankcase with a poorly engineered PCV system, like one with no fresh air IN. Unlike a modern car with a 195° thermostat, a relatively cold A will not quickly boil the gas out of the oil.
LEAKING OR TOO-DRY REAR MAIN? The model A has neither oil pressure nor an actual rear crank seal like a modern engine. The crankshaft thrust bearing is a Babbitt or bronze (most insert conversions use bronze) piece OUTSIDE the crankcase. If you attempt to solve a rear main leak with a PCV setup that produces too much vacuum in the crankcase, you may end up with NO oil to the thrust. A PCV system was never intended to band-aid™ a leaking rear main bearing by sucking the oil forward. Of course everybody will point the finger at the engine builder when the thrust runs dry and fails. A modern engine can get away with several inches of vacuum in the crankcase. The model A engine is not so fortunate. If you have a modern rear seal like a Burtz, it rides on a thin oil film a few microns thick which allows just enough oil past it to lube the seal contact area without leaking. The A has no pump pressure, just gravity, pressing oil against the seal. If you subject it to excessive vacuum on the lip side that normally faces the oil, the lip will run dry and the seal will wear out. I have no idea what the upper limit would be, but anything more than an inch of crankcase vacuum in an A seems bad on all counts to me. I wonder how many times an engine builder using a modern seal or the seal maker has been blamed for failures actually caused by a poorly designed PCV system that creates too much crankcase vacuum ???
BACKUP INTO THE PCV SYSYEM AIR INTAKE (Not the intake manifold): Temporary full throttle operation at the peak of the torque curve, like up a steep grade in 2nd gear will generally produce more blow-by than a properly sized and calibrated PCV valve will pass, even with excellent rings. In a modern engine this temporary excess blow-by goes backwards through the hose to the air cleaner that normally supplies the fresh air to cross ventilate the crankcase. No big deal if it is just a very little and only occasional. If your PCV setup in an A is properly designed and calibrated and you are using the oil filler tube as the intake, when a little excess blow-by momentarily escapes, no big deal. If you have a system with NO AIR INTAKE and just a PCV valve to the intake manifold, on hard acceleration you may pressurize the crankcase and blow oil out the rear main like crazy – unless you pop the grommet or pan gasket or valve cover gasket or timing cover gasket or blow the hose your PCV valve is attached to first! Pick one.
AFFECT ON FUEL MIXTURE and performance: Manifold vacuum opens the PCV valve at some application specific point. The valve is closed during starting and W.O.T. under load because the vacuum is below that set point. During normal operation vacuum draws (OK, pressure pushes) blow-by past the PCV valve which also serves as an application specific flow control orifice. Just because a PCV valve fits your grommet hole doesn’t make it good. A 1975 Cadillac 500 cu. inch PCV valve will flow about five times as much as the valve from a four banger Pinto. Depending on the mix of fresh cross-ventilation air which has 21% oxygen and exhaust blow-by with no oxygen, at any given time while driving you’re feeding either almost straight air, or almost straight exhaust into the intake manifold, and at varying rates! Model A carbs are primitive fuel/ air controllers, that’s why you have the infamous G.A.V. knob. If you choose a PCV valve with the wrong calibration your mixture will severely swing back and forth from rich to lean as you drive. With a PCV valve you need just enough flow to prevent blow-by escape. Too much is trouble and will affect drivability.
COMBUSTION TEMPERATURE: This is lowered when blow-by is introduced through a PCV system. The benefit is reduced NOx emissions. The tradeoff is a shift in the heat range of the spark plugs you need. Your old plugs may now be too cold to burn off deposits. In a very low compression engine the reduced temperature also translates into less push on the pistons. It’s like replacing your stock head with one of even lower compression. That’s why in the ’60’s and ’70’s guys would disconnect the PCV at the drag strip. Two or three horsepower can make or break you in the bone-stock bracket. The horsepower is probably not much of a concern in a Model A unless you are running uphill which causes maximum power robbing blow-by feed from the PCV valve to the intake. Maybe 2 H.P. is important. Maybe not. I can’t answer that. I’m a flatlander and hills make me dizzy.
CALIBRATION of a PCV system: Actually there are three parameters. The first two, flow rate to the intake manifold and vacuum opening set-point are in the valve itself. The third came later in Detroit engineering. Ever wonder why you were always told to clean your breather cap which was stuffed with coarse steel wool (’60’s) or to replace the little plastic breather sponge element inside the air cleaner (’70’s)? Because it acts as a control orifice, too. That’s the third variable. The PCV SYSTEM is an application specific, engineered balancing act between drivability and benefits. Change a cam, change compression, change anything and the good Detroit engineers tweaked the calibration. That’s why there are 100 different PCV valves, not one universal replacement with 100 adapters.
Would I put a PCV system on a Model A touring car? Absolutely YES!! But it is not a band-aid™ for a tired smoking motor that needs a ring job. It needs to function as a ventilation SYSTEM not just a blow-by suction device. I’d start with a PCV valve from an application with a similar horsepower and torque curve, not one that looks like a nice fit. I would think about where to locate the valve based on what happens within the engine, not just where it is convenient or easy to connect it. Unless you want to blast new holes into the block, a Model A PCV system will always be a compromise, but still quite do-able. You can either use the oil filler tube as the fresh air ‘in’ or as the blow-by ‘out’. If you use it as the out with no air IN, you get no cross ventilation and high vacuum in the crankcase no matter what PCV valve you choose. Goodbye exhaust valve stems. If you use it as the fresh air IN, you need a hole somewhere else for the PCV to the intake manifold. But where? I would think the end of the valve chamber cover near the #4 exhaust valve, about an inch above the cover centerline. That would draw the blow-by with its one GOOD component, fine oil mist, past all eight valve stems. You are now also cross ventilating the forgotten blow-by that goes down the exhaust valve guides as well as the crankcase. The setup would not ventilate completely across the crankcase with the oil filler neck where it is, but it would be a compromise that gives you a lubrication benefit and also a short hose to the intake manifold. The result will be less valve chamber sludge and cleaner oil with less fuel dilution and acid breakdown.
The HISTORY of PCV engineering is also a story of marketing necessity. Well before the Model A, or even the T, it was known that blow-by diluted oil, corroded metal, created sludge, and was just plain stinky, nasty, and smelly. Nobody cared. At 4 years of age a car and its rings were pretty much toast and ready for a new owner. Oil was changed every 500 miles or whenever it leaked out or got burned up. Sludge was expected, and a road draft tube put the stink under and behind the car, unless you were cheap like Henry. But then, the A was a cheap car and in the third year when the blow-by got bad you bought a draft tube at the auto parts store to replace the oil filler cap. I found and paid $60 for a N.O.S. Fumaze still in the original box with the Western Auto $1.19 price sticker attached. Thought I died and went to heaven. God bless Hershey.
Before WWII there was no EPA, no word spelled ‘SMOG’, no real need to eliminate internal corrosion and sludge, and no need to maintain the extreme properties of modern oils. Engines were all very low performance and non-demanding. You might think PCV systems were the result of Federal air quality mandates in the early ’60’s, but this was only a secondary consequence. Vehicle sales and warranty terms in a competitive market after WWII was the primary force that lead to development of the PCV systems that replaced the road draft tubes.
Customers wanted higher engine performance, less maintenance, and longer maintenance intervals. Manufacturers were obliged to provide or die, and at the same time reduce both manufacturing costs and the dreaded expense of in-warranty service. High compression, hydraulic lifters, overhead valves, and yearly increasing H.P. and RPM made the cash register ring. All of this necessitated tighter ring tensions, improved alloys with better surface finishes and increased oil performance. Water, acid, and fuel dilution of the oil would spell disaster (warranty service costs) for the manufacturer of such an engine. “Change oil every 500” would circumvent this, but words like that kill sales when the competition says every 1000, claiming their maintenance is less frequent and less costly.
Manufacturers, or more correctly their marketing people, stepped up to the plate in the ’60’s and sold the public the tale that they were making “improvements” to clean up YOUR air quality, so the cars would cost a few hundred more. $3 worth of manufactured PCV parts replaced the $1 road-draft tube (net cost $2) and the engines with PCV now would generate less warranty problems from people who pushed the over stated oil change intervals to the max. A double edged sword here, if engines ran like new for more than three years and styling changes didn’t sell cars the industry would be in big trouble. Three year old non-clanking engines meant non-ringing sales registers. Magically, mysteriously, the oil change intervals increased. 2, 3, 5000 miles! Marketing ran this up the flagpole as another engineering marvel. So much for history!
Well, that’s my BIG OPINION story on Positive Crankcase Ventilation and I’m open to comments.
Bonus Question for Armchair Engineers:
In a turbo or super charged engine (intake manifold has pressure, not vacuum) how do you make a PCV system work properly?
I've posted on PCV stuff going back to Shelly's old Fordbarn, here's a repeat from a 2008 post. Take it as you wish, old info:
Positive Crankcase Ventilation
A Different Way to Say Goodbye to Model A Blow-By?
A treatise by Mike Keating
First let me start by saying I applaud many A’ers for taking charge and finding an alternative fix to the classic blow-by problem with a PCV valve. Traditionally Model A’s were adorned with aftermarket road draft tubes, or maybe just a pot scrubber and old sweat sock over the oil filler pipe. Tell me you haven’t seen the sock thing. Don’t tell me if it’s on your own A, even though you claim it as a period accessory! Some old car owners have taken to eliminate the problem rather than move it out of sight on their touring cars. I must, however, throw in my two-cents on the entire P.C.V. issue strictly from an engineering development point of view. I fear that long term use of an improperly adapted P.C.V. system may cause significant engine damage. I am not opposed to Positive Crankcase Ventilation systems. They are beneficial if properly designed and not just an uncontrolled suction system. But, are they a good thing just because modern cars have them? You decide.
I will start with what blow-by is, then blabber about some of the BAD THINGS that can happen if the PCV system is not engineered and calibrated to account for flathead valve characteristics, crankcase cross-ventilation, crankshaft bearings with or without modern seals or pressure lube, effects on fuel mixture during starting and running, and combustion temperature. I will also explain the GOOD THINGS. Finally, I will address the untold ‘why’ history of Positive Crankcase Ventilation (PCV) - if I haven’t put you to sleep by then. This is all presented I.M.H.O., please take it as such.
BLOW-BY, or gases leaking past rings or through ring end gaps and exhaust valve stems into the crankcase consists of two general types of unwanted substance and one good substance. Exhaust mixed with oil vapor, which everyone associates with the word ‘blow-by’ are the first two parts, and unburned fuel vapor passing the rings during the compression stroke is the third component. Few people think about the raw fuel vapor, but it is also part of blow-by. The exhaust, which contains water vapor, acids, and leftover, smaller hydrocarbons from incomplete combustion are bad, and so is the raw fuel blow-by during the compression stroke. The misty oil vapor is actually good in the engine, but makes the mess that everyone dislikes when it gets loose under the hood.
P.C.V. ENGINEERING: Automotive manufacturers spent research money to get rid of that road draft tube (your pot scrubber and sock) by sucking the crank case blow-by back into the engine intake manifold. Sound simple? There were plenty of variables. Some early PCV systems created quite high crankcase vacuums at times, occasionally 12” Hg (Mercury) or more. Later systems reduced this peak vacuum considerably. Keep this in mind as I discuss effects. Eventually, Detroit got it right. It took years. Every application is a little different.
ENGINE VALVES: With a PCV system, valves must be considered. Flatheads and OHV designs create entirely different problems. With any engine there is clearance between valve stems and the guides. The intake valves almost always have less pressure on the end of the valve stem that is exposed to manifold vacuum than the end that is inside the engine, even if the PCV system creates a crankcase vacuum. Lubricating oil naturally gets forced into the valve guide by the pressure difference. This is good, to a point. In an OHV you often need a stem seal to control how much oil flows down the intake guide because gravity floods oil down the stem to the guide. In a flathead without direct oil supply to the valve stems, like in a Model A or B, only the oil mist created by the running internals lubricates the valve stems where they enter the guides. Drop the crankcase pressure too much with an untested PCV system and the oil vapor falls out of suspension in the air inside the crankcase like rain from the sky when the barometer drops. Don’t believe me? Fill a flask with oil mist from a fogger and attach a refrigeration vacuum pump. Watch the mist fall. OK, maybe what little mist is left will suffice for the intake stems because the even higher intake manifold vacuum helps, but the exhaust valves are an entirely opposite story. Here exhaust pressure and heat blow down and through the stem clearance into the valve chamber/ crankcase. That pressure differential keeps oil OUT. In an OHV the exhaust stem is flooded by gravity with oil which cools and lubes it. In your flathead A with upside down valves, no such luck. Make this worse by pulling excessive vacuum in the crankcase with an unknown PCV calibration that causes the oil mist to diminish, and at the same time greatly increases the rate of exhaust flow down the stem clearance, and you now have exhaust valve stems/ guides starved for oil mist and cooling. Not good for long life or stick free valves. Several ways to fix this problem in a Model A/B, easiest is to keep the PCV regulation of crankcase vacuum in a flathead designed with no direct oil feed to the valve stems as close to zero as possible. You need just enough negative pressure to stop blow-by from exiting the engine and no more.
CROSS VENTILATION: To get the benefit of scavenging water vapor and nasty corrosive combustion byproducts out of your crankcase before they contaminate your oil and make it acidic, you need this. A PCV system must have a clean air source at one end and the valve at the other end to positively VENTILATE the crankcase. If your house is airtight and you fill it with smoke, an exhaust fan will not clear it unless you open a window. Not a window in the same room, you must open a window on the opposite side of the house. On an OHV V8 this is easy. A hose from one valve cover, preferably at one end of that cover, to the air cleaner is the open window, or fresh air source. A second hose with a PCV regulation valve from the other end of the opposite valve cover to the intake manifold is the exhaust fan. Cross ventilation. NOT so easy in a Model A/B engine. Deep thinking (engineering!) required.
CYLINDER WASHDOWN and oil dilution from gas: If you want to increase the amount of raw fuel that goes past the rings during compression, pull a high vacuum in the crankcase with a poorly engineered PCV system, like one with no fresh air IN. Unlike a modern car with a 195° thermostat, a relatively cold A will not quickly boil the gas out of the oil.
LEAKING OR TOO-DRY REAR MAIN? The model A has neither oil pressure nor an actual rear crank seal like a modern engine. The crankshaft thrust bearing is a Babbitt or bronze (most insert conversions use bronze) piece OUTSIDE the crankcase. If you attempt to solve a rear main leak with a PCV setup that produces too much vacuum in the crankcase, you may end up with NO oil to the thrust. A PCV system was never intended to band-aid™ a leaking rear main bearing by sucking the oil forward. Of course everybody will point the finger at the engine builder when the thrust runs dry and fails. A modern engine can get away with several inches of vacuum in the crankcase. The model A engine is not so fortunate. If you have a modern rear seal like a Burtz, it rides on a thin oil film a few microns thick which allows just enough oil past it to lube the seal contact area without leaking. The A has no pump pressure, just gravity, pressing oil against the seal. If you subject it to excessive vacuum on the lip side that normally faces the oil, the lip will run dry and the seal will wear out. I have no idea what the upper limit would be, but anything more than an inch of crankcase vacuum in an A seems bad on all counts to me. I wonder how many times an engine builder using a modern seal or the seal maker has been blamed for failures actually caused by a poorly designed PCV system that creates too much crankcase vacuum ???
BACKUP INTO THE PCV SYSYEM AIR INTAKE (Not the intake manifold): Temporary full throttle operation at the peak of the torque curve, like up a steep grade in 2nd gear will generally produce more blow-by than a properly sized and calibrated PCV valve will pass, even with excellent rings. In a modern engine this temporary excess blow-by goes backwards through the hose to the air cleaner that normally supplies the fresh air to cross ventilate the crankcase. No big deal if it is just a very little and only occasional. If your PCV setup in an A is properly designed and calibrated and you are using the oil filler tube as the intake, when a little excess blow-by momentarily escapes, no big deal. If you have a system with NO AIR INTAKE and just a PCV valve to the intake manifold, on hard acceleration you may pressurize the crankcase and blow oil out the rear main like crazy – unless you pop the grommet or pan gasket or valve cover gasket or timing cover gasket or blow the hose your PCV valve is attached to first! Pick one.
AFFECT ON FUEL MIXTURE and performance: Manifold vacuum opens the PCV valve at some application specific point. The valve is closed during starting and W.O.T. under load because the vacuum is below that set point. During normal operation vacuum draws (OK, pressure pushes) blow-by past the PCV valve which also serves as an application specific flow control orifice. Just because a PCV valve fits your grommet hole doesn’t make it good. A 1975 Cadillac 500 cu. inch PCV valve will flow about five times as much as the valve from a four banger Pinto. Depending on the mix of fresh cross-ventilation air which has 21% oxygen and exhaust blow-by with no oxygen, at any given time while driving you’re feeding either almost straight air, or almost straight exhaust into the intake manifold, and at varying rates! Model A carbs are primitive fuel/ air controllers, that’s why you have the infamous G.A.V. knob. If you choose a PCV valve with the wrong calibration your mixture will severely swing back and forth from rich to lean as you drive. With a PCV valve you need just enough flow to prevent blow-by escape. Too much is trouble and will affect drivability.
COMBUSTION TEMPERATURE: This is lowered when blow-by is introduced through a PCV system. The benefit is reduced NOx emissions. The tradeoff is a shift in the heat range of the spark plugs you need. Your old plugs may now be too cold to burn off deposits. In a very low compression engine the reduced temperature also translates into less push on the pistons. It’s like replacing your stock head with one of even lower compression. That’s why in the ’60’s and ’70’s guys would disconnect the PCV at the drag strip. Two or three horsepower can make or break you in the bone-stock bracket. The horsepower is probably not much of a concern in a Model A unless you are running uphill which causes maximum power robbing blow-by feed from the PCV valve to the intake. Maybe 2 H.P. is important. Maybe not. I can’t answer that. I’m a flatlander and hills make me dizzy.
CALIBRATION of a PCV system: Actually there are three parameters. The first two, flow rate to the intake manifold and vacuum opening set-point are in the valve itself. The third came later in Detroit engineering. Ever wonder why you were always told to clean your breather cap which was stuffed with coarse steel wool (’60’s) or to replace the little plastic breather sponge element inside the air cleaner (’70’s)? Because it acts as a control orifice, too. That’s the third variable. The PCV SYSTEM is an application specific, engineered balancing act between drivability and benefits. Change a cam, change compression, change anything and the good Detroit engineers tweaked the calibration. That’s why there are 100 different PCV valves, not one universal replacement with 100 adapters.
Would I put a PCV system on a Model A touring car? Absolutely YES!! But it is not a band-aid™ for a tired smoking motor that needs a ring job. It needs to function as a ventilation SYSTEM not just a blow-by suction device. I’d start with a PCV valve from an application with a similar horsepower and torque curve, not one that looks like a nice fit. I would think about where to locate the valve based on what happens within the engine, not just where it is convenient or easy to connect it. Unless you want to blast new holes into the block, a Model A PCV system will always be a compromise, but still quite do-able. You can either use the oil filler tube as the fresh air ‘in’ or as the blow-by ‘out’. If you use it as the out with no air IN, you get no cross ventilation and high vacuum in the crankcase no matter what PCV valve you choose. Goodbye exhaust valve stems. If you use it as the fresh air IN, you need a hole somewhere else for the PCV to the intake manifold. But where? I would think the end of the valve chamber cover near the #4 exhaust valve, about an inch above the cover centerline. That would draw the blow-by with its one GOOD component, fine oil mist, past all eight valve stems. You are now also cross ventilating the forgotten blow-by that goes down the exhaust valve guides as well as the crankcase. The setup would not ventilate completely across the crankcase with the oil filler neck where it is, but it would be a compromise that gives you a lubrication benefit and also a short hose to the intake manifold. The result will be less valve chamber sludge and cleaner oil with less fuel dilution and acid breakdown.
The HISTORY of PCV engineering is also a story of marketing necessity. Well before the Model A, or even the T, it was known that blow-by diluted oil, corroded metal, created sludge, and was just plain stinky, nasty, and smelly. Nobody cared. At 4 years of age a car and its rings were pretty much toast and ready for a new owner. Oil was changed every 500 miles or whenever it leaked out or got burned up. Sludge was expected, and a road draft tube put the stink under and behind the car, unless you were cheap like Henry. But then, the A was a cheap car and in the third year when the blow-by got bad you bought a draft tube at the auto parts store to replace the oil filler cap. I found and paid $60 for a N.O.S. Fumaze still in the original box with the Western Auto $1.19 price sticker attached. Thought I died and went to heaven. God bless Hershey.
Before WWII there was no EPA, no word spelled ‘SMOG’, no real need to eliminate internal corrosion and sludge, and no need to maintain the extreme properties of modern oils. Engines were all very low performance and non-demanding. You might think PCV systems were the result of Federal air quality mandates in the early ’60’s, but this was only a secondary consequence. Vehicle sales and warranty terms in a competitive market after WWII was the primary force that lead to development of the PCV systems that replaced the road draft tubes.
Customers wanted higher engine performance, less maintenance, and longer maintenance intervals. Manufacturers were obliged to provide or die, and at the same time reduce both manufacturing costs and the dreaded expense of in-warranty service. High compression, hydraulic lifters, overhead valves, and yearly increasing H.P. and RPM made the cash register ring. All of this necessitated tighter ring tensions, improved alloys with better surface finishes and increased oil performance. Water, acid, and fuel dilution of the oil would spell disaster (warranty service costs) for the manufacturer of such an engine. “Change oil every 500” would circumvent this, but words like that kill sales when the competition says every 1000, claiming their maintenance is less frequent and less costly.
Manufacturers, or more correctly their marketing people, stepped up to the plate in the ’60’s and sold the public the tale that they were making “improvements” to clean up YOUR air quality, so the cars would cost a few hundred more. $3 worth of manufactured PCV parts replaced the $1 road-draft tube (net cost $2) and the engines with PCV now would generate less warranty problems from people who pushed the over stated oil change intervals to the max. A double edged sword here, if engines ran like new for more than three years and styling changes didn’t sell cars the industry would be in big trouble. Three year old non-clanking engines meant non-ringing sales registers. Magically, mysteriously, the oil change intervals increased. 2, 3, 5000 miles! Marketing ran this up the flagpole as another engineering marvel. So much for history!
Well, that’s my BIG OPINION story on Positive Crankcase Ventilation and I’m open to comments.
Bonus Question for Armchair Engineers:
In a turbo or super charged engine (intake manifold has pressure, not vacuum) how do you make a PCV system work properly?
BRENT in 10-uh-C
Well-known member
First off, I have no problems whatsoever with you'all disagreeing with me on this because I have not done actual research to know exactly why my way OR your way would --or would not work.
Lets start off by the PCV name. With a Model A crankcase, why is, --or isn't its present configuration "Positive"? Since the pistons moving and compressed air is leaking past the piston rings, the crankcase likely has a slight "Positive" pressure where vapors can/could easily escape the oil filling tube by that force. So "what" is the PCV system doing that overcomes this?
I am of the opinion to control windage, you must create negative pressure in the crankcase. To use someone's "house" illustration, with the front door left standing open (oil fill tube) and drafty windows (rear main seal, dipstick, front seal), I just cannot see how there is enough manifold vacuum volume to overcome these "leaks" to create negative pressure in the crankcase. As far as removing corrosive gasses, think about how those gasses are getting churned in the rotating air from the crankshaft, ...and think about the path those gasses would need to travel to get into the valve chamber area where they can be vacuumed out.
From my experience with racing engines, a scavenging effect can be made by mounting a tube into the exhaust collector at an angle which will create a siphon effect. Also, some racing applications use a pump tr create a positive vacuum. With regard to un-naturally aspirated engines, usually there is still an air inlet somewhere in the induction system that can create negative pressure.
.
Kurt in NJ
Well-known member
Many of the european cars use an orfice ---for a model A all you need is worn intake guides, the other part of the system in a european car was the breather is hooked to the aircleaner ---so excess fumes are fed into the intake under hard acceleration.
the orfice is about .035
The Mercedes compressor engine still has an orfice --they are in the intake ports, they attach to what looks like a PCV valve, but it is just a check valve, that is attached to an oil seperator with a passage to the crankcase, on the valve cover there is a chamber that attaches to another oil seperator, 1 hose is a drain to the crankcase, the other goes to the intake before the supercharger ---it is not perfect, after 100k or so (depending on use and oil changes) the orfice will get totally or partially clogged --then the other seperator gets overwelmed and will put too much oil into the intake ---it collects in the intercooler, covers the mass air sensor and makes the car run bad (sometimes)--turns on the check engine light ---
baisically the supercharged car has 2 pcv systems, most times the supercharger is not pumping, the vacuum system does the job.
Another system I have seen is a venturi orfice to generate vacuum with airflow, with that the pcv can be intergrated with the vacuum brake booster system.
the orfice is about .035
The Mercedes compressor engine still has an orfice --they are in the intake ports, they attach to what looks like a PCV valve, but it is just a check valve, that is attached to an oil seperator with a passage to the crankcase, on the valve cover there is a chamber that attaches to another oil seperator, 1 hose is a drain to the crankcase, the other goes to the intake before the supercharger ---it is not perfect, after 100k or so (depending on use and oil changes) the orfice will get totally or partially clogged --then the other seperator gets overwelmed and will put too much oil into the intake ---it collects in the intercooler, covers the mass air sensor and makes the car run bad (sometimes)--turns on the check engine light ---
baisically the supercharged car has 2 pcv systems, most times the supercharger is not pumping, the vacuum system does the job.
Another system I have seen is a venturi orfice to generate vacuum with airflow, with that the pcv can be intergrated with the vacuum brake booster system.
James Rogers
Well-known member
If you are trying to cure a rear main oil leak by introducing a vacuum into the crankcase, you will probably succeed as long as the engine is running. Once the engine stops and no more vacuum the crank sits down in the bearing saddle the oil draining back into the crankcase will overwhelm the slinger channel and the leak will return. This is if the problem is bearing clearance, if it is thrust clearance it won't help and the leak will just continue. The best way to stop a leak in a Model A or B is to just bite the bullet and have the mains repoured and align bored.
BRENT in 10-uh-C
Well-known member
82ndscotty
Member
ww2 jeeps had a pvc system. i run one on my A using a 2000 pinto oil filler cap and pcv,drilled the valve side plate to drop a hose down for a road draft tube. had to richen the carb some. no more rusty dist or drive. Scotty
BRENT in 10-uh-C
Well-known member
What is the difference between a Crankcase Ventilation system, and a Positive Crankcase Ventilation system?? Do we think the terms are, --or can be used interchangably?
MikeK
Well-known member
Definitions of "ventilate"- 1. Fresh air exchange; 2. Admit fresh air..; 3. Replace stale or noxious..
Definitions of "positive"- 1.with certainty 2. possessing affirmation 3. forward 4. directly
Shooting in the dark here, I would think based on that definition a "Crankcase Ventilation System" would be more than a simple road draft tube, because you would need some opening or source to "exchange, admit, replace." I suppose that could be as simple as having an open air entrance vent in addition to a rear-facing road draft tube that would only produce a slight suction while the vehicle was in forward motion. A road draft tube by itself would not satisfy "ventilate", although the root word, "vent" implies other, broader meanings unrelated to air. EG: 'steam vent'; 'exhaust vent'.
Now add "positive" to the term and you imply the need to have a constant or mechanical means of "exchange, admit, replace." Even conventional modern PCV systems are not entirely "positive", they fall flat at WOT and dump blowby like a road draft into the air cleaner box.
If you replaced the Model A oil filler cap with an elbow and a forward facing funnel (like a manifold heater intake works) the running fan would put slight pressure in the crankcase. A simple road draft tube from the upper corner of the valve chamber cover would make it work as a "PCV" relieving moisture, acid and sludge buildup in the oil, but would not, of course, fly with modern car emissions standards. You would probably also need a Burtz rear seal to avoid a gusher.
Another scenario for a truly "positive" system would be to leave the oil filler tube and cap alone [use as an intake] and run a draft tube from the upper rear of the valve cover to a venturi attached to the end of the tailpipe, or perhaps within a slightly enlarged pipe somewhere after the muffler. The slight suction produced by exhaust past the venturi would do the work, but again, wouldn't pass modern emissions. You would have no rear main problem with this one. Not a new idea, just a forgotten one.
Hmm, I like that last one, and no ugly valve and hose to the intake! It could be almost invisible if it followed the trans/drive side of the exhaust manifold and pipe. If I keep shoveling and find the garage maybe I'll see if I can come up with a tailpipe adapter that would be easy to fabricate. I'll have to dig out my Dwyer magnehelic differential air pressure gauge and experiment. Running the car in a closed garage should take the chill off the air as I work.
Milton
Well-known member
True, however, in Cal. the law was made retro active to include cars back to 1955.
James Rogers
Well-known member
Mike, if you will study the crankcase ventilation system of the 8BA flathead you will see the exact system you describe. The filler cap has a shield that acts as a catch for the wash of the fan. This air is directed into the filler and down to the crankcase. In the crankcase a tube leads back up to the intake and into a tube that leads around the motor and along the pan rail about half way to the bellhousing where it angles to the road and is slash cut on the rear so the passing air creates a vacuum and pulls the fumes out to the atmosphere, road draft. No positive vacuum, just air directed by passive vacuum from the oil fill to the road. I can take pictures of this tomorrow if needed.Definitions of "ventilate"- 1. Fresh air exchange; 2. Admit fresh air..; 3. Replace stale or noxious..
Definitions of "positive"- 1.with certainty 2. possessing affirmation 3. forward 4. directly
Shooting in the dark here, I would think based on that definition a "Crankcase Ventilation System" would be more than a simple road draft tube, because you would need some opening or source to "exchange, admit, replace." I suppose that could be as simple as having an open air entrance vent in addition to a rear-facing road draft tube that would only produce a slight suction while the vehicle was in forward motion. A road draft tube by itself would not satisfy "ventilate", although the root word, "vent" implies other, broader meanings unrelated to air. EG: 'steam vent'; 'exhaust vent'.
Now add "positive" to the term and you imply the need to have a constant or mechanical means of "exchange, admit, replace." Even conventional modern PCV systems are not entirely "positive", they fall flat at WOT and dump blowby like a road draft into the air cleaner box.
If you replaced the Model A oil filler cap with an elbow and a forward facing funnel (like a manifold heater intake works) the running fan would put slight pressure in the crankcase. A simple road draft tube from the upper corner of the valve chamber cover would make it work as a "PCV" relieving moisture, acid and sludge buildup in the oil, but would not, of course, fly with modern car emissions standards. You would probably also need a Burtz rear seal to avoid a gusher.
Another scenario for a truly "positive" system would be to leave the oil filler tube and cap alone [use as an intake] and run a draft tube from the upper rear of the valve cover to a venturi attached to the end of the tailpipe, or perhaps within a slightly enlarged pipe somewhere after the muffler. The slight suction produced by exhaust past the venturi would do the work, but again, wouldn't pass modern emissions. You would have no rear main problem with this one. Not a new idea, just a forgotten one.
Hmm, I like that last one, and no ugly valve and hose to the intake! It could be almost invisible if it followed the trans/drive side of the exhaust manifold and pipe. If I keep shoveling and find the garage maybe I'll see if I can come up with a tailpipe adapter that would be easy to fabricate. I'll have to dig out my Dwyer magnehelic differential air pressure gauge and experiment. Running the car in a closed garage should take the chill off the air as I work.