VW Type 4 Crankcase Breathing System Testing & Analysis

Should the heads be vented on a T4 engine as part of an effective crankcase ventilation system?
This question comes up regularly on the internet and to my surprise has sparked some pretty passionate arguments. My short answer is no. Venting the heads leads to oil pooling in the valve covers, lowering the oil level in the crankcase and in some spirited driving situations starving the pick-up causing a drop in oil pressure.  For this reason I recommend not venting the rocker chambers on Type 4 engines regardless of application, street or competition.

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

In the winter of 2008 and 2009 H.A.M. conducted exhaustive tests of several different oiling systems and crankcase ventilation systems using our 1.8l SCCA F-Production 914 as the test vehicle. The dry-sump oiling system that was employed for the vent testing, (and with which we competed for two successful seasons) was a two stage (one scavenge stage, one pressure stage), cam driven oil pump where the scavenge system operated at around twice the volume capacity of the pressure stage.
The tests were conducted on Raby Engine Development’s fabulous chassis dyno. Our base-line studies were done utilizing the most common venting arrangment for these engines, which included vents from each rocker chamber and a single vent from the top of the case chimney, all connected to a single breather can. The rocker vent lines were clear 5/8″ hoses and the chimney line was a 3/4″ hose. The breather can was not allowed to drain back to the crankcase so we could observe how much oil was actually being pushed out of the engine.
 We observed that at race speeds (5,000-8,000 RPMs) after approximately two minutes a significant volume of aerated oil began pulsing out of the head vents. To combat this we increased the size of the chimney vent. The condition persisted and eventually we tested with a 1.5″ hose running from the chimney! Despite this large chimney vent we still observed oil pushing out of the head vents, yet none escaped through the chimney vent, despite its enormous size.
We then closed off the head vents and tested with a single 3/4″ vent from the chimney top. And just as before when the heads were vented, no oil escaped through the chimney vent. We also observed no change in torque that would indicate insufficient crankcase ventilation and we had no leaks from seals or pushrod tubs or any of the places you see oil pushing out due to high crankcase pressure.
In 2009 we hit the tracks with this simple arrangement, running a full season that had us competing for the SCCA SARRC points title in what was then the super competitive F-Production class. We raced at Carolina Motorsports Park, Road Atlanta, Barber Motorsports Park, and Roebling Road. We gathered 5 wins and 2nd in points (congratulations to Paul Kullman, the champ, driving a Miata). We suffered no engine failures. Being a dry sump engine we vented the 3/4 chimney vent into the dry sump tank and vented the dry sump tank into a simple clear plastic breather can that made it easy to see how much aerated oil was accumulating.  There was damn near none and I went a full season without bothering with the plastic breather bottle. After 9 races,  I drained it for the first time. Below is the season total accumulation. Approximately one cup of very aerated oil. That’s it. Engine oil analysis at the end of the season showed no significant bearing or valve guide material. So we ran another season on the same engine with nothing more than a head freshening and the results were the same.

Breathing System Test Data Analysis

Type 4 Crankcase Breathing System Tests
Type 4 Crankcase Tests
First: A single -12 (3/4″) line running from the chimney is adequate ventilation for a high revving 1.8 l Type 4 engine. This is significant as the 1.8l engine has a ~2:1 rod/stroke ratio. Higher rod ratio engines require more breathing capacity than lower rod ratio engines. Higher rev engines, especially engines that see sustained high revs like race engines, also require more breathing capacity than lower revving engines.

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.

When the rocker chambers are sealed the pressure differential at the drainback slots is neutralized. This eliminates the biggest hurdle in the oils effort to return from the heads to the crankcase.
It should be noted that, while not as scientific as the other data we gathered, we found that on average throughout the race season our oil temps were 5-10° cooler than we experienced when we ran vented rocker chambers. We attributed this to oil spending less time in the heads. But bear in mind there were other variables involved, though the final arrangement did yield considerably lower oil temps than we observed before making the change.


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. 

Leave a Reply

Please log in using one of these methods to post your comment:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s