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  #21  
Old 08-26-2015, 11:08 PM
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Ironflight Ironflight is offline
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I am certainly not an expert, but the ones I have talked to (engine builders and engineers) all blame oil darkening to blow-by of combustion byproducts. Just a datapoint for you.
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Last edited by Ironflight : 08-27-2015 at 08:30 AM.
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  #22  
Old 08-26-2015, 11:17 PM
PCHunt PCHunt is offline
 
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Another datapoint maybe: The oil in my Pratt-Whitney R-1340 turns black within one or two hours after oil change. That's 10 gallons of oil, and roller-type lifters. Low compression engine, large cylinder diameter.
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  #23  
Old 08-27-2015, 04:55 AM
vic syracuse vic syracuse is offline
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Default Blow by

Yes it is usually caused by blow by. Interestingly enough, and much to my consternation, my Lycoming (Mattituck) IO-540 in the RV-10 turns the oil black and the Continental R-680 in my Stearman does not. Both have roller tappets.

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  #24  
Old 08-27-2015, 05:03 AM
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Steve Melton Steve Melton is offline
 
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well, there goes my hypothesis down the tubes.

hy?poth?e?sis
a supposition or proposed explanation made on the basis of limited evidence as a starting point for further investigation.
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  #25  
Old 08-27-2015, 08:04 AM
BillL BillL is offline
 
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That article should be considered entertainment, not something to make decisions on. Yes, ZDP is the additive for slipper follower cam-tappet systems. Push rod, not for bucket tappet, or for rocker followers. Automotive used chilled iron followers and phosphate, cast iron cams for low cost. Until 1971, a GM small block cost about $200 in production cost, complete with ignition and carb.

The aero design uses a nitrided steel cam for much better fatigue strength, but if abrasives are present (like diesel particulates) then the wear life is compromised. We don't have that, so the life is pretty good. Chilled lifters are another matter.

OIL: Oils serve a lot of functions as you know. Hydrodynamic lubrication, additives for boundary lubrication (too slow for hydrodynamic lifting), cooling, acid neutralization, particle suspension, and more.

Oil degradation is generally due to the same factors. Oxidation is typically the cause of blackening. This is from the areas where it gets hot. Around heads, exhaust ports zones, piston under crown, and the cycle of wiping off the cylinder wall from exposure to combustion. The bulk temperature of the oil also contributes to oxidation. Just take some and pour it into a frying pan and heat it up. It will begin to smoke as lower fractions evaporate then begin a slowly darkening. Time at temperature is a factor.

Lead is a suspended contaminant. If you take a used sample and centrifuge it, it will become more clear as the lead settles out. Dispersants and other additives keep it from plating out in the engine or settling into a gooey layer in the sump and low areas.

Sulfur in the fuel and exposure to combustion yields acids. The TotalBaseNumber, (TBN) is a measure of the ability of the oil to neutralize this. It is unseen and generally ignored, but can be reported in an oil analysis.

Particle contamination is from wear. Anything that slides. Everything that slides. This is reported in the oil analysis.

These are a few things about oil off the top of my head. Blackstone as much better explanations of the total oil picture, but not on the engine design related aspects. Oil formulations and engine design go hand in hand.

Guinness Book of World Records would like to see a pre 1985, push rod, engine that went 200k miles and did not have serious cam/follower degradation.
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  #26  
Old 07-08-2016, 01:08 PM
paleopete paleopete is offline
 
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http://www.economist.com/blogs/babbage/2010/11/tribology[/url]

This is an excellent article which explains what I have known for years and the reason I use 15-40 Diesel Engine Oil in my cars....living in California a slightly heavier viscosity oil is not as much of an issue as other areas and helps me to explain why I get such great mileage from my vehicles whether autos or aircraft.
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  #27  
Old 07-08-2016, 04:33 PM
8 ball 8 ball is offline
 
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Default Rv10inoz

Do you know any reason for rocket to rethink that? I have used Castrol GTX20w50. In my lycs for a good while.
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  #28  
Old 07-09-2016, 10:23 AM
F1R F1R is offline
 
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Default What Lycomings?

Quote:
Originally Posted by 8 ball View Post
I have used Castrol GTX20w50. In my lycs for a good while.
I'm just curious which lycomings, what comp. ratios and how many hours?
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