Quote:
Originally Posted by regular guy
Here's the response from Clevite. Black text will be the response verbatim. Where enecessary, I collated answers from multiple emails together into the same topic. Clevite granted permission to post this in public.
Q: Are there any signs of detonation?
A: Probably, but certainly signs of very high cylinder pressure on the supercharged applications. Detonation is uncontrolled combustion of the fuel in an engine. Normal combustion in an engine starts with a fuel burn at the sparkplug then burns on an ever expanding flame front until the fuel is consumed. It is similar to a grass fire starting at one spot and burning in an ever expanding flame front. In detonation the spark ignites the fuel then fuel on the back side of the combustion chamber explodes before the flame front has reached it.
When that occurs, cylinder pressure spikes quickly and sometimes reaches 2-3 times normal cylinder pressure. The duration of the spike is short but that pressure spike is really hard on the oil film. Those dark, worn, some what round spots of wear on a number of upper rod bearings may be and probably are indications of that happening. What can cause detonation? Lean fuel conditions, poor octane quality fuel, too much ignition timing, too much engine compression for the fuel being used - these are the most common causes.
Q: Do you recall if the signs of detonation were predominantly on supercharged over NA bearings? Or did you see signs on each?
A: (Still waiting for the answer to this question.)
Q: Are there any signs of bearing clearance deficiencies. The nominal bearing clearance is 0.00140 inch on a 2.04655 inch journal.
A: I know you are all hot on this, but I just don't see it. The upper shells in most of the samples show a wear pattern over 2/3 of the surface which we'd consider normal.
Q: Are there any signs of oil starvation (might be related to (2) above?
A: Lots of indications here. The bearings that show delamination are called a hot-short failure and this is almost always a lack of oil film issue. I think you told me the factory oil recommendation was somewhat thick for the clearance being run. This is also related to oil temp and climate conditions.
Q: Is this normal wear given the mileage for each (I will provide mileage for each)?
A: I believe these were all non-supercharged applications from the factory, correct? If so, the N/A sets looked pretty normal.
Q: How many more miles estimated until break down and failure?
A: All the supercharged sets looked pretty poor. Those in the copper were close to total failure.
Additional comments:
Considerations: It's not my position to second guess BMW engineering. If the cars/engines were all stock that's one thing, but alterations to the OE condition are always made at some risk. We do know that as cylinder pressures increase, oil film thickness decreases. When the thickness reaches what we call boundary lubrication, head builds rapidly and failures result because the shaft and bearing actually start touching. Mixed lubrication, a condition where we move back and forth between boundary and hydrodynamic( seen in many of your samples) results in accelerated wear.
Q: Tell me more about #3. BMW-AG only allows 10W60 oil in this engine. BMW-NA seems to have broken ranks with BMW-AG as of August 2013 and now allows 0W30 - 5W40. No other region seems to be allowing thinner oil. Your comments on #3 seem to indicate this is a separate issue than too little clearance on #2. I always thought they were linked and related. Maybe a little clarification on there for my benefit. Tell me more about oil temp and climate conditions as they relate to these samples.
A:
1) We know now that the tighter the bearing clearances in engines, the thinner the oil needs to be to flow adequately. We like tight clearances because they; 1) spread the load on the oil film out over a wider area, reducing the psi load on the film and reducing the chances for boundary or mixed lubrication conditions and 2) allow longer engine/bearing life. We also favor full synthetic oil because of it's improved lubrication properties and it's cold weather and hot weather viscosity stability.
2) Bearings that have been hot will typically exhibit wiping or smearing if you will, of the overlay. Next comes discoloration on the bearing material indicating more heat and finally delamination of the bearing material. All three of these conditions were seen in the samples you sent in. The #1 cause of the heat seen in the bearings is loss of oil film thickness resulting in bearing to crankshaft contact. This in turn increases the friction coefficent, resulting in heat. Babbitt overlays melt in the 500F range. Oil is an excellent tool for cooling the engine, but in this case, we have hardly any oil flowing across the surfaces to carry the heat away!
Also, please remember, bearings are not self-healing! Damage done during perhaps a cold start or hard pull or a bunch of boost is there for good!
3) Supercharging: I believe, based upon the samples you sent me, that supercharging increases cylinder pressure and consequently, the load upon the conrod, the bearing and ultimately the oil film between the bearing and the crank. We've known, since the day the first engine had a power adder installed, that there was a detriment to the life of the engine as the power is increased. It's pretty much a direct but inverse relationship.
More power= less life. I have customers making 10,000 HP in a 490 CID engine but bearing life is less than 1 minute at full throttle/ max boost.
4) We're a bearing manufacturer, suppling both OEM parts and performance parts to the automotive industry. In the performance/ racing venue, we can optimize both materials and designs with just one thing in mind; surviving high power output. We offer a line of performance/race bearings in the aftermarket but not for the BMW engines you have. We also have the capability of manufacturing bearings for companies with very specific needs and applications. As I mentioned, we are quoting on a BMW bearing now for a performance engine builder/supplier specializing in BMW.
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My interpretation is that Clevite doesn't think clearance is an issue so much as too thick oil. I was perplexed by some of the answers, specifically about the supercharging. Most of the samples I sent had very low supercharged miles: one as few as 1800 supercharged miles, two at 5000 supercharged miles, and only one at 14000 supercharged miles. Clevite noted the superchaged samples were much worse than the NA samples. Since most of the samples had very low supercharged miles, I asked how much a role the supercharging played in these cases. Unfortunately I don't yet have an answer to that question.