Ranger wrote:
[quote]A couple months after I bought my car I had to get it’s conrod bearings replaced. Main bearings were ok. A year later when I had to pull the whole engine I noticed that the main bearings were now thrashed.
Over the weekend I disassembled a junkyard motor that I’d had in my car for a couple events before it got pulled for low oil pressure. It’s conrod bearings were thrashed. It’s main bearings were worn but still serviceable.
That got me to thinking that the conrods are the bearing weakpoint of our motor. I recall reading that piston and conrod mass is what really beats rod bearings to death at high rpms, but I don’t recall how high is “high”.
The first time my rod bearings went, my oil pressure seemed fine. Yet when the rod bearings were going bad in the second motor the oil pressure was 18psi at redline. Curious.
I don’t have any ideas for solutions, this was just musings.[/quote]
More musings, observations, and theories. I am not an engine builder but like you Scott, I am an engineer that has studied lubrication and observed a few engine failures. Fortunately, very few.
We are talking race engines now, not street engines.
I think that there is some point in the bearing wear equation where degradation goes from gradual (normal engine) to rapid (where we see high levels of iron and copper in the oil analysis results). Then, under race conditions the wear goes from rapid to toast in minutes. The increased wear creates clearance, the clearance allows the oil film to be squezzed out more easily, causing more metal to metal contact, casuing more wear causing more heat, causing more oil degradation, etc until BANG.
The rod bearings (it is usually only one) go first and the vibration takes out the mains.
Once the oil pressure drops, it is too late. Think about what causes an oil pressure drop. It is not the extra clearance in the bearings, it is the toasted bearing material that clogs the oil pick up. Clogged oil screen = rapid to toast in minutes.
In the one case where you replaced the rod bearings and the mains went. I think that there was probably just enough vibration, or maybe imbalance is the right term, that the new bearings were able to survive but the older bearings went from normal wear to rapid wear. It was probably only a matter of a session or two before the entire engine went bang.
These are the reasons (IMO) that when one replaces one bearing, one should replace them all.
Also, if you haven’t figured it out yet, the above phonomena (or at least my conviction that this is what happens in an engine failure) is one of the reasons why I am an advocate of synthetic oil. I believe that the synthetic oil significantly helps prolong the “gradual to rapid” wear phase.
My $0.02
Don
