Why Lubricant Choice Matters to your Porsche IMS Bearing

We all know oil is cheap and engines are expensive. At this point, most people know that we recommend oil changes every 6 months or 5,000 miles using a quality oil like Driven DT40 or DT50 full synthetic motor oil for their Porsche engines for the street and Driven’s XP full synthetic races oils for the track. 

Many wonder why we make specific recommendations for what oil to use in the M96 engine when fitted with our IMS Retrofit. To put it simply, lubricant quality and additive technology plays a huge part in reducing wear and extending component life. We already discovered through extensive research that oils containing the extreme pressure additive Moly helps reduce bore scoring in engines with Lokasil and Alusil bores. The same holds true to rolling bearing element bearings like ball bearings or even cylindrical roller bearings.

The purpose of a lubricant is to separate moving parts to minimize wear and reduce friction providing protection from moisture, corrosion, and suspended contaminants. The choice of what lubricant is used depends on many factors including operating environment, minimum and maximum temperatures, operating speeds, and loads. It is the base oil viscosity that determines the load bearing capacity for any lubricant with increases in load strength proportional to increases in viscosity. However, you don’t always have full film formation leading to hydrodynamic lubrication in a ball or roller bearings where moving components are separated by this film. It is this film that supports the loads without contact between surfaces.

If the bearing is not operating with full film lubrication, it is running in boundary or mixed-film lubrication. At low engine speeds with low viscosity oil and high loads, especially at startup and shutdown, the IMS bearing sees boundary lubrication where there is insufficient kinematic viscosity or bearing speed to create the required elastohydrodynamic (EHD) film to separate rolling bearing elements. Simply flooding the IMS bearing that is already partially submerged with more oil volume won’t prevent increased wear if there is insufficient lubricant viscosity and high loads and can actually cause a bearing to run hot. As bearing speeds increase, the amount of lubricant to support the applied load decreases. This is one reason Porsche redesigned the intermediate shaft starting with the 2006 model year to use a larger diameter ball bearing that in effect increases bearing surface speed, improving the load bearing capability of the IMS bearing at low engine speeds by allowing for full film lubrication at these lower engine speeds.

In fact, the American Bearing Manufacturers Association standard 9 provides a method to calculate effective bearing life with an adjustment factor a3 that when viscosity drops below 13 cSt or when there is very slow rotational speed. In those cases, bearings life can be 20-50% lower. Your IMS bearing regularly operates in these severe conditions where use of a quality oil with an exceptional additive package including moly and ZDDP should be used. Even the slight increase in HTHS viscosity of a 5w40 over that of a 0w40 can provide up to a 10% increase in oil film strength at extreme temperatures a modern engine can see. As an oil ages, viscosity modifiers in the oil break-down allowing for an oils viscosity index to shear out of grade. In the worst case scenario, a new fill of 0w40 with 13.5 cSt viscosity measured at 100 degrees C to shear down to a 30 weight with 10.0 cSt viscosity index at the same 100 degrees C in only a few thousand miles. It doesn’t help matters that these engines run much hotter than 100 degrees C (212F) and can often see temperatures of 240F or higher just sitting in traffic on a hot summer day. With the IMS bearing operating in extreme conditions most of the time with lubricant viscosities less than 13 cSt negatively affect bearing life, selecting the right oil and oil change intervals and deciding when to service your IMS bearing are hotly debated subjects.

Many think that because they are using a synthetic oil that they don’t need to change their oil as often and can observe long drain intervals recommended by auto makers. Although it’s true that modern synthetic lubricants don’t break down, however the additives do. Oils are formulated with corrosion inhibitors, anti-wear additives, anti-oxidants, viscosity modifiers, and other additives that do break down, reducing the lubricating oil’s performance. Besides viscosity loss, contamination from combustion byproducts including fuel and moisture in the engine further degrade the additive package and can reduce IMS bearing life. Lubricant quality and cleanliness is paramount, directly affecting bearing and overall engine health. This is why we recommend Driven DT40 or FR50 oils formulated with increased anti-wear and extreme pressure additives for M96 engines and why observing shorter oil change intervals of no more than 6 months or 5,000 miles as part of regular scheduled maintenance is essential. 

Sources:

Steven Sanchez. “Lubrication for Life”. Power Transmission Engineering, Oct 2020, pp. 20-27.

Lake Speed Jr. “What’s Tribology.” Total Seal