Originally Posted By: Shannow
Originally Posted By: Hujan
It sounds like both of you actually like the Castrol TWS in this car. If so, I'm curious what you think might be behind the widespread premature bearing wear in these engines (taking as true that it exists).
I've got no dog in the fight really, I just think that BMW specified it to mask an inherent problem. Thus I wouldn't really want to stray too far from what they recommend as a bandaid.
Checked the link that doodfood provided, and didn't like the shiny "bearing surfaces" on the sides of the big end. Clearly there is touching when running...and that's a no-no.
The oil has to escape this area and not be trapped. By trapping it, the oil that's escaping the high pressure areas (the areas of that are wiped) will be more likely to be recirculated back to the unloaded side where the fresh oil comes in.
End result is that the bearing thinks it's getting a higher supply oil temperature.
Here's what supply temperature and RPM do in a big end...popular misconception is that the oil "carries away heat", but it's actually largely generated within the bearing through viscous drag.
The number on the right axis is the big end oil supply temperature, the number on the left is bearing temperature...the oil exit temperature is higher. If the bearing was rained properly (adequate side clearance, the bearing temperature is the average of the supply and exit temperatures.
The "150C supply" temperature is missing the "full load" piece at the top end because it failed, and failed repeatedly in the paper (title at the top of the pic)...They survived at light/medium loads, even at high RPM...couldn't cut it when the full load was applied.
I think that the failures are a perfect storm of radial clearance, side clearance, and piston/bore alignment, causing overheating and failure of the oil film...
The oil with high HTHS is there to bandaid that if possible, but depending on the car, the day, and maybe a transition from light load high revs (I can see 135 oil temperatures just holding 100km/hr at 4,000RPM in my 3.8L V-6), and sudden application of power, the stars line up and bearings fail.
The pics aren't sudden failure, they are repeated minor rubs causing wear.
Thanks for your response.
It does appear that most if not all of the bearings pulled from these motors have shown wear particularly on the uppermost of the two bearing shells (what you call the "big side"). When you compare the photos to the photos of different types of bearing wear in, for example, Mahle/Clevite literature, the wear patterns are pretty consistent with what they term "insufficient lubrication" (as opposed to, for example, contaminants in the oil).
It does make sense that the oil for this car would want to maximize HTHS. Finding an oil that can "do it all" in the sense of having good cold flow and yet still offer high HTHS is what initially led me to ester-based oils and specifically Redline. Their 5w40 reports HTHS of 4.4, which is a tick below the stable 4.5 of the Castrol TWS, but with significantly better cold flow (97 cSt at 40 C instead of 160). Again, though, I am a touch skeptical of Redline's published data, though I'll allow that that's mostly based on a gut sense and not any hard data.
In the long term, I plan to do a dedicated engine build on this car. It will entail a slight increase in displacement (from 4.0L to 4.2L), but the primary goal will be to get the rod side clearance and bearing clearances (both rod and main) exactly where they need to be. In the meantime, I replaced the OE bearings with bearings that have a dry-film coating on them. The bearing thicknesses were adjusted to account for the coating. My hope is this combined with a good oil choice and proper maintenance will keep the engine intact until I'm read to do the build.
