VW Type 4 Crankcase Breathing System Testing & Analysis

The chimney is baffled by design to make it hard for liquid oil to exit out the top of the engine, which is the appropriate place to vent a crankcase, not low and outboard, like the heads. 

It’s worth noting in this short summation that the higher the crankcase pressure the more oil will tend to pool in the valve covers of vented heads. Venting valve covers as a way to deal with excessive blow-by is not a solution to the problem and may very well lead to lower oil pressure in even mildly driven vehicles as the lower the oil level is in the sump the lower the oil pressure will be. 

If you want to learn more on the subject, read on. But be forewarned, the content must be highly inflammatory as in the past it has driven some folks on the internet to such a state of madness that they questioned my sanity my motives and my overall good judgement. I take no responsibility for your reaction to the controversial material covered in this study. Read at your own risk.  Before you dive in, let me say that I had no financial incentive pegged to the outcome of this testing. H.A.M. doesn’t sell crankcase breather systems, elaborate or otherwise. When Jake and I conducted this study our only motivation was to settle this issue (as far as we were concerned) once and for all to the benefit of our race program, and our Type 4 customers.

I know there are a variety of approaches to ventilating flat 4 engines. To that end, if you like your current crankcase vent system you can keep your current crankcase vent system. It’s no skin off my nose either way.  

Len Hoffman

Breathing System Test Data Analysis

Second: With a typical breather arrangement that includes rocker vents, a wetsump engine being driven in a sustained, spirited fashion will soon have enough oil suspended in the valve covers and breather can to cause a significant drop in sump oil, lowering oil pressure and increasing the possibility of starvation. And it’s no wonder; oil is pumped into the heads through the pushrods where crankcase pressure causes it to accumulate until the valve covers are full, then it pushes out of the heads into the hoses on its way to the breather can before it eventually makes its way back to the sump, (if a drain hose is routed to the chimney), but not until revs are reduced. A potentially harmful and unnecessarily circuitous route, IMO.  This is especially significant for wet-sump engines used for track days, Auto-Crossing or just spirited driving since wet-sump engines have a very limited supply of oil and can ill-afford to have a major percentage of it gathering in the rocker chambers and in suspension in the breather system.

Our Observations Explained

The drain back slots in the T4 case are not very large. As a matter of fact, they are quite small. And on one side of them is the crankcase containing the pressure from combustion gasses that have leaked past the rings (even the best ring seal will have some of this), along with windage which is not unlike a hurricane. Add to this the fact that the crankcase pressure oscillates between negative and postive with every 180* of crank rotation. This has to do with the displacement of crankcase volume from the reciprocating pistons. The pistons do not approach TDC at the same rate that they approach BDC. This differential in piston speed is more significant the higher the rod/stroke ratio and creates some incredibly powerful pulses. These pulses are why crank seals are designed with two lips. One to keep oil in, the other to keep atmospheric pressure out. These pulses can be observed on an idling engine by holding a piece of paper over a vent opening. The paper will flutter up and down. At speed, these powerful pulses combine with windage to create an atmosphere in the crankcase that’s a veritable tempest in a teapot. Into this environment we are asking the oil from the heads to drain back through small passages that are already dealing with positive pressure pulses and are sitting just above the lifters, which spend a healthy amount of their time moving upward at a high rate of speed, further interfering with the oils return. 

I can not overstate how hostile the conditions are inside these engines when they are operating at high revs.

The negative pressure pulses can be strong enough on even modest high performance engines to suck valve cover gaskets inward, causing a massive oil leak. This is why valve cover gaskets should be glued in place, and track cars should have retaining tabs welded in place to hold the gasket captive.  

The positive pulses push accumulated oil out of the heads faster than the negative pulses can pull oil down to the sump because the resistance opposing the positive pressure pulses (atmospheric pressure and free flowing breather hoses) is not as great as the resistance offered by the diminutive drain back slots and the extreme conditions below them. This is ultimately why vented heads are  very problematic for high revving engines.

Conclusion

In case you haven’t figured it out yet, H.A.M. recommends that no rocker chamber vents be used on Type 4 engines. I also see no need for a special breather can for a track car, though one with a drain line back to the chimney is fine. For a healthy wet-sump engine a simple breather can with a filter like those available from any speed shop will do. A single line with a 1/2″ or larger opening from the chimney top should be plenty for most street applications. I do not see the need for any more than a single 3/4″ hose from the chimney top for any N.A. application. That’s what worked in our specific application, which I believe to be about as demanding a scenario as a N.A. Type 4 engine is likely to be faced with. Sophisticated cans with oil-air separation that vent to an aircleaner are worth consideration for street cars. Those systems will definitely benefit from the use of a single line from the chimney as the breather-can will have less aerated oil than if the heads were tied into the system, meaning less oil intrusion into the induction system.