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Mike Schmidt has written his explanation of how the stock breather system works. To read it, Click here.

After I burned my 2/6 rod bearings, I decided to investigate the crankcase oil control and breather system further to determine if this contributed to the oil starvation problem. I obtained a cam cover from Marc Thomas. I cut several holes in it and installed lexan windows. With a small video camera and light, I could see and record, what was happening to oil inside the cam cover while the engine was running.

These are pictures while the engine is running. The picture on the left is at high rpm while the car is moving in a straight line. The picture on the right is during a high speed, high rpm left turn. The engine is fitted with a GTS sump baffle and Accusump. The oil is Redline 20W 50.

In the left side picture, you can see the cam shaft bearing cap and the cam lobe is turning on the left side of the bearing cap. There is quite a lot of oil, but not so much that oil is trapped in the cam cover. It was freely running down the drain back holes with no "pooling" of excess oil. There is a thought that one of the reasons for oil starvation is that at high rpm too much oil is pumped into the upper cylinder head area which deprives the oil sump of sufficient oil. This is incorrect.

The right side picture shows what happens immediately after entering a left sweeping turn at high speed and high RPM. The oil turns to froth and there appeared to be a lot more of it although it was difficult to tell. The frothy oil was likely not coming directly through the pressure fed oiling system. It was probably being whipped by the crankshaft and was being blown upward through the oil drain back holes from the oil sump area.

I reached this conclusion after seeing it happen and doing research on how crankcase breather systems are designed for high performance engines. I found that the 928 has extremely limited crankcase breathing capacity. The 928 (32 valve motors) have two 12mm exits from the oil filler base into the breather system. One of those 12mm hoses is further restricted by a valve with a small opening. The recommended crankcase breather size for a 5.0 liter engine is an opening of 3/4" diameter or larger.

With insufficient breathing capacity to relieve crankcase pressure, the only route for the blow by gasses is up through the drain back holes in the cylinder head, out the breather connections in the cam cover, and eventually into the intake system. The crankshaft rotation throws oil to the right side (viewed from the rear) of the engine crankcase which causes oil to be blown up the drainback holes into the right side cam cover. The right side cam cover has the only breather connections on S4 & GT engines. This undoubtedly exacerbates the propensity for these engines to blow oil into the intake system when being run hard. The GTS engines have cam cover breather connections on both sides which may help. A sustained left turn is even worse with oil in the sump being forced to the right side.

We can make some changes to give better oil control with less oil lost to the intake, and provide better lubrication for the engine. I'll give examples of what I did, or plan to do. Other approaches may work as well, or better. Improvements should address three areas.

1. Both cam covers should have breathers.

2. Increase crankcase breathing capacity.

3. Provide effective oil control in the sump.

My plan was to supply fresh air to the cam cover breathers, install a larger crankcase breather outlet, and vent the blow by gasses into a vacuum source in the exhaust. I didn't use any of the stock breather system other than the cam cover breather fittings. The two 12mm outlets in the base of the oil filler were plugged.

1. Both cam covers should have breathers. A breather fitting was installed on the left cam cover in the rear hole. An oil control shroud was used under the fitting (inside the cam cover) the same as on the right rear breather fitting. The small passageway in the right rear breather fitting was drilled out to the same size as the front fitting. The front right cam cover breather was removed and the hole plugged using the plug from the left rear cam cover hole. The breathers were supplied with fresh filtered air from the air box below the air filter. This was accomplished by drilling holes in the air box and using a rubber hose to connect the breather fittings to the air box holes. Rubber grommets in the holes with the ID sized for a tight fit to the rubber hose insures a secure and dust free seal between the air box and hose. Now, the cam covers will receive clean air which will flow into the cam covers and down the oil drain back holes to the crankcase. This air will flush combustion byproducts out of the engine.

2. Increase crankcase breathing capacity. A vented oil filler cap was made using the stock cap as the interior to retain the threads and seal. An aluminum cover was constructed with a swiveling center to permit the cap to rotate around the vent line. The center part is made from brass and retained in the cover by an "O" ring. Illustrations are shown below.

Oil filler cap breather

Oil filler cap breather underside

3. Provide effective oil control in the sump. Nearly all the high performance engine building books I use recommend use of crankshaft oil scrapers, a windage tray, and a surge baffle in the oil sump. The books that don't recommend the surge baffle recommend instead that a dry sump oil system be used. I chose to stay with the 928 wet sump oil system to reduce cost, weight, and overall complexity. The design of the 928 oil sump is in some ways good, and some not so good. It's good in that the sump does have internal baffles and a flange of sorts around the sump edges to reduce the oil climbing the sides during high "G" cornering. Also, the oil pickup is in the center of the sump with a generous sized pickup tube and well designed inlet. It's not so good because it is much too shallow with insufficient distance between the crank counterweights and the oil. The pan floor is so close to the crank that a windage tray, or screen, can't easily be fitted.

To view crankshaft oil scrapers Click here.

To view oil sump surge baffles Click here.


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