|The MGA With An Attitude
POSITIVE CRANKCASE VENTILATION - CV-102
Pulling a Vacuum, ala 1965-1969 MGB.
This is an odd topic for MGA discussion, but apparently a lot of people are interested in modifying their MGA to get it to stop dripping oil. What a strange idea! If you want to do this you need to install positive crankcase ventilation to route crankcase fumes into the intake system and draw a small vacuum on the crankcase in the process. There are many ways to screw this up, and only a few ways to make it work properly. So first you should understand how it works and why ventilation is necessary.
All internal combustion engines should have some form of fresh air ventilation through the crankcase to purge the blow-by gasses and reduce the accumulation of water vapor, acids and sludge in the crankcase. To do this the crankcase vent (front tappet cover tube) must either have the draft tube or be connected to a vacuum source. The original draft tube on MGA and early MGB works as a venturi when the car is in motion to draw a slight vacuum on the crankcase and produce air flow. Suction inside the air cleaner (pressure drop across the filter element) is very small compared to the vacuum level created by the draft tube. With the draft tube in place and the car moving air goes in through the air cleaner, then through the vent hose to the valve cover, through the crankcase, and out the draft tube. This is amazingly effective at road speed moving a lot of ventilation air through the crankcase.
Operation of the draft tube as a vacuum venturi is easy to demonstrate in your kitchen sink. Fill a tall glass with water and drop in a soda straw (not more than two inches from top of straw to water level). Cut the top end of the straw at 45 degree angle and turn it so the opening faces away from you. Blow across the tip of the straw at a right angle and the vacuum generated can draw water up the straw to produce an atomizing spray.
The bottom end of the draft tube is close to the exhaust down pipe, so it is not uncommon to notice a little warm oil smell under the right conditions. A bit of crankcase mist near the hot pipe can do that. It would be most likely to occur when you stop after a hard run and the fumes can waft around the car rather than blowing away in the breeze. Consider it to be one of the charms of an MGA.
The draft tube on the MG engine does not produce a very high vacuum level, but with the 1/2 inch bore pipes it does draw a moderately large volume of air. The manifold vacuum setup on the MGB produces crankcase ventilation any time the engine is running. The draft tube draws air through only when the car is moving. For the purpose of purging fumes and acids and water vapor from the crankcase it is sufficient to have the fresh air ventilation only while moving. With the draft tube setup, when standing still idling some blow-by gasses will be pushed gently from the valve cover vent into the air cleaner. If you spend a lot of time stopped and idling you may notice a little oiling of the front air cleaner canister, which should be very minor and of no consequence to the health of the engine. If the piston rings are badly worn producing copious amounts of blow-by, the air cleaner canister may accumulate a larger amount of oil. In this case you needn't worry much about it being detrimental to the engine, because it already needs a ring job. Anything you do with crankcase venting from that point on is a BandAid patch job while you procrastinate about doing the engine work.
You can usually tell if/when the crankcase ventilation is not working. The milky colored sludge inside the rocker cover is emulsified water in oil condensed inside the relatively cooler cover area.
#1 no-no: Do not install a valve cover with no air inlet vent. Blocking air inlet on the valve cover defeats crankcase ventilation. If you use a sealed valve cover with the draft tube you need to use a large unrestricted free flow ventilated oil filler cap with internal filter. If you use a sealed valve cover with positive crankcase ventilation you need to use an oil filler cap with inlet restrictor and internal filter.
#2 no-no: Do not connect the tappet cover vent to the air cleaner. When you connect both pipes to the air cleaner you have no pressure differential to drive the air flow, so you get no crankcase ventilation at all, only exhaust of the blow-by products and crankcase fumes into the carburetor intake. This results in stagnant vapor in the crankcase and all of the cylinder blow-by fumes being forced into the carburetor intake. This will oil up the front carburetor, possibly carbon up the front two cylinders, and allow accumulation of water vapor and other nasty acids and sludge in the crankcase. Frequent oil changes may keep cam and tappet and bearing wear to a minimum, but the engine still gets dirty inside. How well your engine fares with that arrangement depends on how much you drive, how many short cold-run trips you make, and how often you change the oil. With no crankcase ventilation you may expect to find some sludge accumulating inside, some in the oil and some adhering to all internal engine parts. With the crankcase exhaust directed into the carburetor intake you should also expect to have carbon deposits accumulating in the combustion chambers. Internal combustion engines need crankcase ventilation and do not like oil in the air intake.
Later model cars (MGB mid 1964 model production and later) have the tappet cover vent connected to the inlet manifold vacuum with a PCV valve, or to carburetor venturi vacuum without a PCV valve (starting with 1970 models). All of these cars have restricted fresh air intake to the valve cover.
#3 no-no: Do not connect the tappet cover vent directly to the inlet manifold. Doing this would draw a very strong vacuum in the crankcase, so strong that it could suck (dirty) air in past the normal seals to put too much raw air into the intake system. If you allow a free flow vent at the valve cover (like removing the oil filler cap) the direct manifold vacuum connection could pass so much air as to kill the engine for lack of fuel mixture.
MGB used two different methods to produce a low level vacuum in the crankcase (along with ventilation). I will cover the later method first because it is simpler to explain (and less likely to be used on the MGA). From 1970 models onward the tappet cover vent was connected to the venturi area of the carburetors. This provides a low level vacuum at idle, and slightly more suction at higher speed, but will never produce the very high vacuum levels present in the intake manifold. Because this is a low level vacuum source, no further pressure regulation is required. The plumbing is split to flow equal amounts of crankcase air into each of two carburetors, because the venturi porting effects fuel mixture. To use this method you need the carburetors that have the venturi vacuum port. 1970-1971 used SU HS carbs. 1972-1974 used SU HIF carbs. From 1975 on there was the single Z-S carb which also used venturi vacuum for this function. To install any of these carburetors you need the matching MGB intake manifold.
The 1964-1969 MGB connects the crankcase vent to the intake manifold, but the tappet cover hose is not connected directly to the high vacuum source.
In between there is a special PCV valve. This is the wide flat round thing with a spring and a rubber diaphragm inside. This device is actually a pressure regulated flow control valve. It uses the inlet pressure signal (crankcase absolute pressure) to regulate flow. This device will control the flow volume to accept passage of any amount of crankcase exhaust available. The inlet restrictor in the valve cover limits the fresh air inlet flow to a relatively small amount sufficient for crankcase ventilation (not to overly affect fuel mixture). The PCV valve will accept this flow plus any amount of cylinder blow-by gasses in variable volume, while all the time maintaining a narrow range of low level vacuum in the crankcase.
This type of PCV valve is a slick little device which I highly recommend using if you are going to port the crankcase vent into the intake manifold. You can set this up in the same configuration used for the 1964-1967 MGB. When using the MGA intake manifold you will need to install a 1/2 inch hose connector in the top of the manifold. You can use the original MGA valve cover with the original sealed oil filler cap by installing a flow restrictor orifice in the hose between the air cleaner and valve cover. Otherwise the rest of the setup is a bolt on installation using the MGB parts which are listed and diagrammed here on the Moss Motors web site.
For those with an overt curiosity to know how this PCV valve works, just turn the page.