Crankcase pressure


Mar 28, 2006
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Just wanted to say hello to everyone on this board. This is my first post. Ok, I have a 71 Duster with a 360 in it. It seems to build up a lot of pressure in the crankcase. I have a PCV valve on one side and a breather on the other valve cove. What happens when I run the car at the track is I get a lot of oil coming from the breather and PCV. It spray a long the valve covers. I have a 750 DP carb, Mopar single plain intake, 533 lift cam Eddy heads, TTI headers, Harland Sharp rockers. Full MSD ignition, Steel crank, rods and pistons from Mopar. The rebuild is not old. I have heard with a high lift cam the engine is not making much vacuum. Should I put a vacuum pump or evac systems on the headers? I just had to replace the oil pan gasket because I think it blew because of this problem. I don't know for sure. My be this has happened to someone else? :silent:
Are you sure there are no broken rings? Cylinders not scored? I know on the diesel engines that I work on if there are any rings broken or scored liners there will be major blowby. However, some engines just have a lot of blowby by design. I will be interested in if anyone has this problem on other 360 engines and how to solve it.
Good compression check on each cylinder. Two 440's that I built did the same thing. One would blow out the oil dipstick and the other actullay bent the intake valley pan up? :wack:
At wide open throttle (WOT) you have no vacuum that is why your pcv valve doesn't not work. Theres nothing wrong with your car it's just simple physics. If you will notice drag car motors do not use a pcv valve because of this problem of no vacuum at WOT. They use breathers in both valve covers with valves in the headers. The exhaust passing by the valve in the header creates vacuum which draws out the positive pressure in the crank case. This will not work on the street because of the back pressure of closed exhaust. There are belt driven vacuum pump systems that can be purchased that will work for a street application. These work much better than the header systems and are the best thing going right now. It will set you back about $500 to $600 though.
a vacupan setup ,with mufflers is not good. they will back pressure the motor and cause oil to leak from breathers and push dipstick out also.
I know of two racers with same problem, one disconnected his vacupan system, no more problems, other guy spent $1100.00 on a vacuum pump setup. just run breathers. if you run open headers, then use a vacupan setup.
340mopar said:
At wide open throttle (WOT) you have no vacuum that is why your pcv valve doesn't not work.
If you had no vacuum, then your carburetor would not work either.

When the PCV is functioning properly, it should be fully open during high engine loads (i.e. hard acceleration) when blowby production is high. During light engine loads (idle and part throttle cruise) where manifold vacuum is high, the PCV actually closes partway to limit the amount of blowby into the combustion chamber.

PCV system operation
At WOT vacuum does not make a carburator work, air flow thru the venturis and boosters does. The only way to have vacuum in an intake at WOT is if the carburator flows less cfm that the engine is pulling, for example Nextel Cup cars running a restrictor plate race. These engines are trying to pull more air thru the motor than the restrictor plate will allow. This creates a vacuum below the restrictor plate. Put a vacuum gauge on your manifold and stand on the throttle, it will read 0.

Basically 70440duster has three choices:

#1 - run a header evac system to use with open headers (racing only), then hook up the pcv valve system for street driving.
#2 - an expensive belt driven vacuum system.
#3 - enough breathers installed on the motor to offset the positive pressure.
Hello. I'm new to this forum. Hope I can help.

The venting of crankcase pressure under WOT through the breather is normal. Blowing oil out the breather is not normal.

Check your oil level. Confirm it's not overfilled. If it's an aftermarket pan with a larger sump you may want to try lowering the oil level a quart or so.

Confirm you have an oil baffle under the breather and the PCV valve.

If you still experience excessive blowby, perform a dry and wet compression test or a leakdown test to confirm wear or damage on the rings or cylinder walls. Hope this helps.

340mopar said:
At WOT vacuum does not make a carburator work, air flow thru the venturis and boosters does.

And tell me again, WHY is there 'air flow thru the venturis and boosters'???? Just because your vacuum gauge reads 0 does not mean there isn't a pressure differential. Maybe vacuum is not the right word, but if the air pressure inside the intake manifold were exactly the same as the air pressure just above the carb, then there would be no reason for air to flow through the carb. Agree?
A motor is just an air pump. It moves air thru the inlet and the outlet. Throw some fuel at it and an ignition source and it will spin on its own. A pump does not create pressure or vacuum. It moves a certain amount of air or fluid depending on how big it is, if you restrict the out flow the restriction creates pressure in front of the restriction. If you restrict the intake flow it will create a vacuum behind the restriction. When the piston travels down in the cylinder with the valve open it creates a low pressure area. Being the atmospheric pressure is about 14psi at sea level it travels from the higher pressure to the lower pressure. This is why engines at sea level produce more power than engines at higher elevations. Higher altitudes have lower atmospheric pressure to move for the high pressure to the lower pressure.

So yes, you have right idea that it is because of the pressure differential that the air moves thru the engine but the low pressure lies in the cylinder not the intake that is why there is no vacuum at WOT.
So anyway... If you're pushing oil out, then the rings arent sealing for soem reason. If it's new, that may be it. What were the ring gaps set to? What kinds of rings? What was the honing procedure? I've seen quite a few seating issues with too rough a finish for today's rings coupled with the ethanol pump gas that washes the oil of the walls quickly. Have you done a leakdown test? That will tell you how bad the rings are. How does the car run at the track? Give us a rundown of the whole package...What is your trap rpm?
I will tear it back down and look at the rings. Thank all of you for the good advice. Will keep you posted. :sad2:
340 mopar and moparteacher have it right, the motor should not be
expelling oil throught the breathers on a tight motor.That would indicate
ring issues or perhaps exhaust valve guides ( does the engine smoke on heavy
accell ? ).

As for the vacuum issue, 340mopar hits it on the head, an engine does not suck air in. It opens a void that is filled by atmospheric pressure. Unless
it is supercharged, turbo, or more than 100% volumetric efficient due to
a proper build and great exhaust scavenging. The same goes for your body,
you dont really inhale ,you open your lungs and they fill . The lower the
elevation the bigger the column of air above you. Thus at high elevation
you get less O2 because your getting less air shoved into your lungs.
Are you seeing oil vapors at your breather, or is it just oily. I get oil around the breathers because I dont have a baffle. If you have blown rings you will see a huge vapor plume coming from breathers or any other place they can come from. Do a leakdown test before a tear down.