Crankcase Ventilation

All engines have to have some means of getting rid of the blowby fumes from the bottom end - it helps reduce oil dilution and contamination and generally helps keep the engine internals clean. For street engines the ventilation system is part of the emissions controls so you'll probably have to have some sort of system to recycle the gases.

Stock engines draw the gases back into the intake tract via the PCV valve. At high vacuum/low load conditions the vacuum pulls the poppet closed and the flow has to pass through a small orifice in the valve. This stops too much air being pulled into the manifold when there is little blowby anyway. When the throttle is opened and the vacuum drops off the poppet in the pcv opens and a much larger volume of gas is pulled into the manifold. Ideally the volume passing through the pcv is greater than the volume of blowby, the shortfall being made up by fresh air that enters the engine either via a vented filler cap in the rocker cover or a line connecting the rocker cover to the air cleaner. As the engine wears and the blowby increases though, the blowby may exceed the flow through the pcv and if a vented rocker cover is used the fumes will escape, producing that distinctive clapped-out-motor smell in the car.

The pcv system works quite well in a stock engine, but the low vacuum produced by a long overlap cam will confuse the valve and it will often be wide open when it shouldn't be, acting like a major vacuum leak. An alternative is to simply vent the gases to atmosphere, like the walking stick vent pipe on the old grey motors. Usually this line runs from the top of the engine down to the bottom, to the low pressure area that exists under a moving vehicle. This gets rid of the blowby, but is limited in its ability to flush it out with fresh air, especially if the car is stationary or moving slowly. Still it's a simple system and requires nothing more complex than a hose and an effective oil separator to prevent oil being carried out with the blowby gas, plus a vented filler cap to let fresh air in.

A slightly better system might be to run the hose into the air cleaner to recycle the gases instead of simply dumping them, but again an effective oil separator is essential to prevent drawing oil into the intake. A further refinement is to add another small line from the rocker cover to the manifold, restricting the flow with a small (say 1 to 2mm) orifice. At least this will get some fresh air circulating through the engine at idle and on the overun. This line too will need an oil separator, and in some rocker covers this is nothing more than a small tin baffle. Don't count on these to work effectively, and with the added blowby sometimes encountered with a highly loaded, high output engine they won't do the job. Look at the separators used on modern diesel engines, you may even be able to adapt one. Often they are nothing more than a canister filled with wire mesh similar to what is used in the old oil bath oil cleaners, with the outlet at the top. Generally bigger is better, so make the flow paths big enough to avoid oil being swept along with the air. When it's all working properly the engine will be clean (and stay clean inside) and fume-free.

You've probably seen systems that draw blowby gases into the exhaust via a small checkvalve. This looks like an excellent setup for a competition engine, though I'm not sure how effective it would be with a muffler and full exhaust system like on a street car.

One more thing to look at is the path taken by the gases as they travel from the bottom of the cylinders up to the rocker box. Often, and this is the case with the Holden 6, this is the same path taken by the oil as it drains back from the head. If the volume of gas flowing up is high enough, it can slow or even stop the oil drainback, and I've seen the rocker box completely full of oil and spewing from the breather on one V6 engine that ran continuously at full load. So if you are having problems with oil control in the top end consider using an alternative exit point for the blowby, perhaps from a timing cover or similar, but again you will need an effective oil separator.

Aftermarket rocker covers should be checked out carefully before being fitted. It's not unusual to find the car has suddenly become quite smokey after doing nothing but changing the rocker cover. Unfortunately many of these covers are impressively shiny but half-baked from an engineering viewpoint. The problem is that the baffling around the pcv port is inneffective and allows oil to be drawn into the engine, hence the smoke. If you absolutely must use an aftermarket cover (perhaps for rocker clearance reasons) be prepared to spend a little time fixing the baffling first.