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08-26-2015, 11:08 PM
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VAF Moderator / Line Boy
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Join Date: Jan 2005
Location: Dayton, NV
Posts: 12,256
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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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Paul F. Dye
Editor at Large - KITPLANES Magazine
RV-8 - N188PD - "Valkyrie"
RV-6 (By Marriage) - N164MS - "Mikey"
RV-3B - N13PL - "Tsamsiyu"
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Dayton Valley Airpark (A34)
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Last edited by Ironflight : 08-27-2015 at 08:30 AM.
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08-26-2015, 11:17 PM
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Join Date: Feb 2007
Location: San Diego, CA
Posts: 1,670
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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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Pete Hunt, [San Diego] VAF #1069
RV-6, RV-6A, T-6G
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2020 Donation+, Gladly Sent
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08-27-2015, 04:55 AM
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Super Moderator
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Join Date: May 2005
Location: Locust Grove, GA
Posts: 2,626
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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.
Vic
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 Vic Syracuse
Built RV-4, RV-6, 2-RV-10's, RV-7A, RV-8, Prescott Pusher, Kitfox Model II, Kitfox Speedster, Kitfox 7 Super Sport, Just Superstol, DAR, A&P/IA, EAA Tech Counselor/Flight Advisor, CFII-ASMEL/ASES
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Author "Pre-Buy Guide for Amateur-Built Aircraft"
www.Baselegaviation.com
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08-27-2015, 05:03 AM
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Join Date: Apr 2011
Location: Cincinnati, OH
Posts: 2,791
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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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Steve Melton
Cincinnati, OH
RV-9A, Tip-up, Superior O-320, roller lifters, 160HP, WW 200RV, dual impulse slick mags, oil pressure = 65 psi, EGT = 1300F, flight hours = 800+ for all
Simplicity is the art in design.
My Artwork is freely given and published and cannot be patented.
www.rvplasticparts.com
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08-27-2015, 08:04 AM
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Join Date: Sep 2007
Location: Central IL
Posts: 5,516
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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.
__________________
Bill
RV-7
Lord Kelvin:
“I often say that when you can measure what you are speaking about,
and express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meager and unsatisfactory kind.”
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07-08-2016, 01:08 PM
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Join Date: Jul 2016
Location: Visalia
Posts: 1
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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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07-08-2016, 04:33 PM
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Join Date: Dec 2014
Location: hilltop lakes' Texas
Posts: 135
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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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07-09-2016, 10:23 AM
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Join Date: Nov 2010
Location: ____
Posts: 829
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What Lycomings?
Quote:
Originally Posted by 8 ball
I have used Castrol GTX20w50. In my lycs for a good while.
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I'm just curious which lycomings, what comp. ratios and how many hours?
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