All modern cars will require a breather system of some form or other. In a petrol engine it is inevitable that some gases will ‘leak’ past the piston rings and into the crankcase, creating crankcase pressure which needs to be vented – through a breather system.
Standard systems are usually more than adequate for a mild increase in engine power, but start doubling power outputs and these systems will struggle to keep up with demand. And if the pressure cannot be vented from the crankcase efficiently enough it will cause problems. Leakages from around the sump gasket, oil filler cap (or the cap being blown off completely), cam and crank seals, and the dipstick are the usual areas the oily mixture will escape from when the pressure in the crankcase gets too much for the breather to
Obviously a modified engine will require something that is much more capable, which is why we fit aftermarket breather systems. These are fed with breather pipes from the crankcase (and usually one from the top of the engine too) and will have a separator box within them. This allows the air pressure to vent to atmosphere (or back to the inlet), but collects any oil in that mixture at the same time. This oil can then be returned to the sump or collected in the breather (or catch tank in these applications), and drained off later. Some high-end breather systems like the Spec-R items even use a two-piece design to further improve the breather system’s performance.
It is also worth noting the PCV (positive crankcase ventilation) valves that are fitted to many cars with a pipe connecting the breather to the inlet manifold. At low engine speeds this will actually draw any blow-by gases out of the crank and recycle them through the inlet – meaning no oily deposits are dumped on the road, but if the engine was to backfire the one-way valve within the PCV closes, and prevents the breather system being subjected to the high temperatures of a backfire and potentially raw fuel too. The PCV is also used in forced induction engines, where a pressurised inlet manifold (under boost conditions) would mean blowing boost directly into the crankcase if a one-way valve were not fitted.
To sum up, the standard breather system works fine on mildly modified engines, but if you’re going to ask that engine to work hard then treat it to a decent breather system.