Hey guys,
Still learning about engines. What is the difference between an open and closed crankcase? I know an open uses a PCV valve. But, are there advantages to running open vs. closed? How can you tell if your engine is supposed to be one or the other? Can you run an open crankcase closed? If so, what are the pros and cons of switching systems? Are engines unique, meaning some are built to have an open system, some are built to be closed? If so, how can you tell the difference? What is affected by the open/closed system? I have a PCV hooked up to my carb but if I plug everything, it seems to run alright either way. What's the deal with this stuff? Thanks in advance.

 

The crankcase of any engine needs to be open- allowing it to breathe. If the crankcase was closed- than it would build pressure and blow up throw the piston rings (hurting the seal), and would push on the bottoms of the pistons (hurting power and mileage).

On earlier engines, a simply open element breather located on the valve cover works fine. You should also have a PCV (positive crankcase ventilation) valve hooked up to the bottom of your carb. Newer engines starting using more complex tubing to prevent any HCs from getting into the atmosphere. So, instead of open type breathers- it is usually connected to the intake or air filter housing. Many still use a PCV valve as well.

 

Open ventilation is just vents on the valve covers, so when positive pressure in the crankcase occurs it depressurizes through the vents. Closed PCV uses a valve and a hose that connects to manifold vacuum, so when you're driving normally the vacuum pulls on the crank case and prevents gas from building up pressure. At WOT it just depressurizes through the vents. Running a closed PCV system prevents oil leaks.

 

I never thought it was referred to as 'closed'. Learned something new myself.

 

ON my car, I have a 302 engine with a carter 600 cfm carb, there is a pcv in the valve cover with the hose going to the big vacuum hook up in the front center of the carb. Would this be considered a closed system then? Also, on the other valve cover is a breather, which I remove to put the oil in. Does it matter what kind of breather is on that valve cover? Should that one be some kind of sealed cover instead of a breather? Will this cause any issues? Thanks.

 

That's how it's normally set up. The breather has a baffle and some kind of filter media in it to trap oil particles, but allows ventilation under crankcase pressure at WOT. It's also where fresh air is drawn in when the crankcase is under vacuum while you cruise.

 

id rather use a nice open breather setup with a catch can over a closed pcv system. The pcv system is for emissions...... 1000'th post!

 

No, the PCV system is NOT for emissions. The purpose of drawing a vacuum on the crankcase is so that when you're cruising normally(where you spend 95% of your time driving) you don't get pressure building up in the crankcase that forces it's way past seals, causing oil leaks. Or you constantly have a mess on your valve covers by the breather.