Last Updated: 11/15/2010
I know of few topics that elicit such dismissive and strongly held opinions as that of how to eliminate the cause of #2 rod bearing failure in 944 engines. Some builders claim that the aeration of the oil in the sump is at the root of the problem. Others claim that the fault lies in the design of the oil journals. There is yet another school of thought that holds that it is essentially a cavitation problem in sustained left-hand turns. One thing is certain; this is a topic that brings out some remarkably vehement opinions.
I recently replaced the engine in my 1985.1 944NA. I didn’t have any particular problem; I simply had 150,000 mi. on the engine (including a fair amount of track time) and was getting concerned about longevity. I had a variety of minor oil leaks (my floor pan anti-corrosion strategy) and was due to have the rod bearings replaced, along with the belts and rollers. I had an opportunity to acquire an engine with a warped head at a reasonable price so I started to do some research. I spoke with several engine builders about how to resolve the rod-bearing problem and how to improve the longevity of the engine in track usage.
Since I was the one paying for the rebuild, I tried to remove the rhetoric from the various arguments and reason it out for myself. I think that there is a valid claim to be made for all of these opinions. The conventional solution of adding an extra half quart of oil (to minimize cavitation in turns) only adds to the problem of foaming (due to crankshaft splash). There are some flow modifications that can be made to the girdle passages and the crankshaft passages. I had some of these done in the machine shop. We also did the best baffling job that we could. After some deliberation, I decided to add a 3 qt. Accusump, a Canton/Mecca spin-on oil filter housing and oversized external oil cooler. The Accusump is mounted in the spare tire well at the rear of the car. It is connected with an AN-10 stainless steel braided hose, via a manual shutoff valve mounted in the cockpit, to the top of the oil filter housing. The Canton/Mecca oil filter uses a replaceable 8 micron filter element. The oil cooler is a Setrab 625-10 unit mounted on the driver’s side of the car behind the left fog light opening. The lines for the oil cooler run from a 951 oil cooler adapter housing that includes an integral thermostat and pressure relief valve (this replaces the original oil/water cooler).
In terms of where I sit in the rod bearing debate, I think that I have reached a reasonable middle ground. I know that I now have an extra 3 ˝ to 4 quarts of oil available at system pressure, should it be needed. I can also pre-lube the oil system before every start (most track cars are driven relatively infrequently and the first fifty to one hundred revolutions are without oil pressure to all of the bearing surfaces). I have minimized the aeration problem by locating the oil cooler as far for the engine as possible (giving the foam an opportunity to precipitate out) and run as fine an oil filter as possible.
After all of my research, I disassembled the engine and sent the pieces to the machine shop. I used a machinist (Dean Palmer of Palmer Automotive Machine, West Haven, CT) who had worked on a number of engines for Fairfield County Motorsports (Fairfield, CT), the premier Porsche preparation shop in the area. I had Dean do all of the machining that he would normally do for one of their track engines. I then asked Jim Reilly of FCMS to complete the assembly and installation. Due to some interruptions (a family move from Connecticut to California) I added the Accusump after the engine was installed.
I mounted the Accusump on an aluminum shelf that I fabricated in the spare tire well at the rear of the car. This puts about 25lbs at the rearmost point in the chassis, giving a slight improvement to the weight distribution. I followed the suggestion of the engineers at Canton Racing Products (manufacturer of the Accusump) and ran AN-10 hose from the “oil” end of the Accusump to an adapter fitting in the top plate of the Canton/Mecca oil filter housing. This hose runs across the rear deck, along the right side of the shifter tunnel and through a bulkhead fitting (located just in front of the glove box liner) to the oil filter. After hearing a number of stories about failed electric valves, I mounted a manual shutoff valve just aft of the shifter, on the passenger side of the shifter tunnel. There is a pressure gauge on the “air” side of the Accusump to allow you to set the precharge. I replaced this with a 90 degree 1/8 NPT fitting and used an AN-3 line to connect it to an air pressure gauge mounted flat on the rear deck. The present sanctioning body that I run with (POC) requires that I cover the Accusump installation so I fabricated an aluminum cover for the spare tire well and mounted it with jet nuts to the top perimeter.
The recommended precharge for the Accusump is normal ambient air pressure (14.7 psi). This results in the air pressure gauge reading actual oil pressure any time the oil pressure is greater that 14.7 psi. When starting the engine for the first time with the Accusump, have an assistant hold the Accusump with the “oil” end aimed up. This will minimize any air pocket that might be captured in the cylinder. Once it has been cycled in this manner, it can be mounted horizontally. The mounting brackets offered by Canton work very nicely. They are worth the money.
So far, these changes seem to be working well. At my last event at Willow Springs, the ambient temperature was around 110 F. My oil pressure ran 4.5-5 bar and the oil temp remained at 225 F. I plan on checking the rod bearings at 40-50 hours for wear. The oil system that I now have may seem like overkill, but at least it gives me peace of mind.
Credits: Kevin Kehoe