Type 4 Crankcase Venting

Type 4 Crankcase Breathing System Tests & Analysis

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.
We began this study of different systems because we initially raced with the conventional breathing system that included rocker vents. We were unhappy with the results. We were forced to either allow aerated, unfiltered oil to drain back into the engine, or to deal with a loss of oil to the breather can. We tried a breather can that de-aerated the oil, but still found that a significant amount of oil was kept in suspension in the breather system.
With open minds we decided to tackle this issue once and for all.

Setting Up the Test

Our base-line studies were done utilizing the most common venting arrangement for these engines which included vents of 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 and from where.

The test were conducted on Raby Engine Development’s fabulous chassis dyno. 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.
We then closed off the head vents and tested with a single 3/4″ vent from the chimney top. During the tests with this configuration we observed no significant volume of oil was pushed out of the chimney vent. We also observed no loss of torque that would indicate insuffecient crankcase ventilation.
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. We ran a simple clear plastic breather can that made it easy to see how much oil was accumulating. At the end of the 9 race season we drained the breather can for the first time. Here 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: In a typical breather arrangement that includes rocker vents a healthy % of oil that is pumped into the rocker chambers via the pushrods is aerated by combustion gasses (that have forced their way up the pushrod tubes) and pushed into the breather can. The oil then, as internal crankcase pressure allows, drains back into the engine. This aerated oil then eventually makes it way back to the sump.

Third: Since oil was pushing out of the rocker vents which were located at a high point in the rocker chambers we know that the rocker chambers were filled with oil. 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.
Finally, when no rocker vents were employed virtually no oil was pushed out of the crankcase and put in suspension in the breather system. Again this is especially significant for wet-sump engines.

Our Observations Explained

The drain back slots in the T4 case are not very large. On one side of them is the crankcase containing the pressure from combustion gasses that have leaked past the rings (does not matter how good the ring seal is there is always some of this), along with the windage which is not unlike a hurricane. Add to this the fact that the pressure in the crankcase is not at all constant, even when throttle position is. This has to do with the displacement of crankcase volume from the reciprocating pistons. They 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 incredible pulses.

The end result is a crankcase that is a veritable tempest in a teapot, filled with hurricane like winds, and rapid pressure pulses. Into this environment we are asking the oil from the heads to drain back through small passages that are sitting just above the lifters which spend a heathly amount of their time moving upward at a high rate of speed, further interfering with the oils return.
Now, if the rocker chambers are vented these narrow passages are also allowing pressure to move toward the heads. This is because regardless of the size of the vent on the chimney the pressure on the cylinderhead side of the drainback slots will be lower than the pressure on the crankcase side. This means the oil must share the drainback slots with combustion gasses that are moving in the opposite direction. The only motivation the oil has to return to the crankcase is gravity, and not a lot of it as the pushrod tubes have a modest angle.
When the rocker chambers are sealed the pressure differential at the drainback slots is neutralized. This eliminates what we found to be 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. But bear in mind there were other variables involved, though the final arrangement did yield lower oil temps.


H.A.M. recommends that no rocker chamber vents be used on the Type 4 engine. I also see no need for a special breather can, though one with a drain line back to the chimney is fine. It should be smaller than the primary vent line, though. For a typical wet-sump engine a simple breather can like those available from any speed shop will do, and any can with a filter will work fine. A single 5/8″ line from the chimney top should be plenty for most street applications. I do not believe that any more than a single 3/4″ hose from the chimney top will be required 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.