Croustibat

Approx. none.

Serious drag racers use vacuum, but they also use 3+ bar boost, and put cement in the block cooling system to prevent it from cracking.

Having the crankcase under vacuum is good because the oil return line from the turbo has to be at lower pressure than around the turbo seals, otherwise oil goes where there is less pressure. Crankcase with pressure = oil in exhaust and intake. You dont want that.

Crank does create pressure from its pumping effect, and rings always leak some pressure when used with high boost. Using vacuum at the crank case allows for a more secure oiling system, as pressure will also be lower at the crank case than in the head, allowing oil to be sucked back from head to crankcase instead of just depending on gravity.

But for low boost on small 4pots (i'd say up to 2 bar boost), you can just stick a huge breather between cam covers and crankcase, and vent it to atmosphere. It will work enough. The vacuum system would be better, but power wise it should not change much. Not worth the money to do it if you are after HPs IMHO.

Manufacturers dont do it anymore for 2 reasons:

1/ emissions - it just is illegal now to vent to atm. Oil vapors are required to be recycled, one way or another. Most of the time they are just injected back at the intake and burned. On some cars, there is a closed catch tank and oil condensates back.

2/ fire hazards (oil vapors released under a hood may burn)

The problem with a catch tank that vent to atm is that it is often filled with water condensation, that goes in your breathers at night ... and in your cam covers, and in the crank case. Remember to empty it often, or to close it when not used... If you dont and it fills up, it will prevent any breathing, and you will get pressure in crankcase, thus oil going through turbo seals.