Ok so, which crankcase pressure is best, keep the crank case pressurized to stop blowby, or vacuum it out?

 

Normally you'd want the crankcase to be under vacuum to stop blowby, not under pressure.

 

and how exactly does vacuum stop blowby? sucks the rings to the piston walls?

 

Well, think of it this way.... Cyl pressure is on the order of 1000-1500 psi. So, how much pressure do you think you can get in that crankcase, anyway? I don't think the difference between hard outer space vacuum (0 vs 15 psi absolute ), or even between 1 atmosphere and 2 atmospheres (15 vs 30 psi absolute), is going to make much difference to blowby. In fact, even if you had a typical air compressor in your car and could put 120 psia in it (135 absolute), you wouldn't significantly affect blowby.

On the other hand, when a piston moves, air is displaced below the piston, in the crankcase, as well as above the piston where we're really concerned with. This is a loss to the pumping action above the piston. Pulling a vacuum in the crankcase reduces the loss.

 

Idealy you want a small negitive pressure (vacuum) in the crankcase. The PCV creates a small vacuum and the breather helps vent the rest of the pressure and heat built up in the crankcase. A large vacuum in the crankcase wont help reduce blowby, only new rings will.

 

Some racing engines use a vacuum pump to put a negative pressure in the crankcase. Good horsepower gains can be had depending on the combo. We are talking 30 to 40 horsepower with high output motors.

 

Very high output engines. I run an electric vacuum pump to compliment the header evac system. Mainly I use it to pull the condensation out of the crankcase after a run. While the engine is running at an idle, I only get 2-3 inches of vacuum. Under power, the crankcase goes into pressure. That's why I still use a header evac system. A good belt driven vacuum pump will increase the flow of air as the engine rpms change. I've never run this engine without the vacuum pump so I don't know if it makes a difference in power.

The best way to find oil leaks is to pressurize the system. Plug the breathers/PCV for 30 seconds or so then if you unplug them, there should be pressure blowing out briefly. Any leaking gaskets/seals will have oil leaks once the crankcase is pressurized. If you can keep the crankcase in a vacuum, you can eliminate those leaks. The rings will also seat a bit better but you really need low tension rings to see an improvement. I use total seal rings and have 2% leakdown so the vacuum pump won't help there.

Belt driven vacuum pumps can pull a lot of air. Vacuum relief valves are installed to keep the vacuum below 12". By about 15-17" of vacuum, you can start pulling oil from the bearings. Any good vacuum pump can easily pull 28" but when the engine is running, the pump also needs to move a lot of air to maintain that kind of vacuum.

As mentioned above, the PCV valve pulls the crankcase into a bit of a vacuum but it's using manifold vacuum. Under WOT, manifold vacuum drops to almost zero. You're also pulling and moisture/dirt from the crankcase oil back into the intake. Water doesn't burn. If your rings have a lot of blowby, you get a lot of contamination going back into the intake.