blow by is caused by differnent things ,not just piston rings or ring lands.

what it is is excessive oil or pressure is getting by the rings an getting to the exhaust valve.

normally there are 2 types:
Piston rings are going bad, or the ring gaps are stacked on top of each other , an oil gets by the comrpession ring an pushed out the exhaust valve causing the smoke to come out your tail pipe.

Valvestem seals can cause blow by as well, if they are worn out or not seling, oil will get by the seal an drop down past the stem an valve guide, causeing it to go out the exhaust.

usually, on turbo cars, excessive pressure from the turbo can actually push oil by the rings on the pistons an create blow by effect. If you run forged pistons, you must have a clearance of 2-3 thou more than a cast piston to make up for the amount of "expansion" the forged piston does, if your clearance is too big, youll get blow by.

same with ring gap, if its too wide, blow by.