oil thickness and bearing clearances

Messages
2,324
Location
Baltimore, MD
i have heard that an oil can be too thick for certain bearing clearances and that thick oils will lead to bearing starvation if the oil is thick enough. but is this really true? i have been throwing it around in my brain for a couple of days and... ...this is how i see it. for example 5w30 vs 15w40 5w30 is thinner across the board when temps are equal but 15w40 is thinner hot than 5w30 when cold.
 
Messages
4,009
Location
Calgary Canada
I have considered this issue for my '98 K1500 with a 5.7L vortech that at some time in it's life had the intake manifold issue. It has 185K miles on it now and has a hot oil pressure of around 40 psi at 1500 - 4000 rpm with a 12.2 Cst 0w-30 oil. This is a little lower than it used to be. My thought is to continue running that 12.2 Cst XD3 Ow-30 winter, and switch to a 15w-40 dino XD3 in the summer. I'm hoping this will bring the oil pressure up by about 10 psi when hot. If this were the case, and at the risk of asking a stupid question...does this seem like a sensible plan?
 
Messages
4,009
Location
Calgary Canada
Chevron Oil is really tough to come by here in Canada...and if you can find it, you have to pay gas station prices for it (because that's the only place it's sold). I'd love to use the delo, and would love to have a source for reasonably priced delo 5w-40, M1 T&SUV or Delvac 1 5w-40...but it's the same story all around in Canada....can't get it. Plus...the Esso stuff is soooo cost effective in certain applications.
 
Messages
1,183
Location
Vermont
Don't [Bang Head] trying to find what isn't conviently availible, though you could certainly keep an eye out for it and purchase some when found. Though I have no experience with the oils you mention, they are of the same line-up, and in theory should continue to serve you well (as I'm lead to believe). If it is Esso that you speak of, many in the US envy your position in having it readily availible.
 
Messages
4,009
Location
Calgary Canada
Rotella T is easy to come by. The Rotella Synthetic we get here is Rotella T-SB (Synthetic Basestock) 0w-40. But it's expensive. XD3 0w40 is close to half the price. I'm going to do a UOA on the XD3 0w-30 @ 3k miles and then probably add a full bottle of VSOT in May (@ 3k miles) to bring it to a 40 wt. Assuming the UOA looks good, I'll keep running the XD3...and see how the increased viscosity affects things.
 
Messages
651
Location
Punxsutawney, PA.
That's kinda like comparing apples to oranges. The 40 weight will appear thinner while it's hot than a 5w-30 does when it's cold. Take a cold bottle of each and shake them. I'll bet the 15w-40 feels thicker. At 100*C for both, I guarantee the 15w-40 is thicker.The 5w-30 would have a cold start advantage as it will act like a 5 weight when cold and the other will act like a 15 weight when cold. On to your question. A tightly clearanced engine with a heavy oil could cause oil starvation in the bearing or at the least have a higher temperature reading at the bearing. On the other hand, a loose engine and a thin oil will probably show low oil pressure when hot. The oil is still doing its job but not under ideal conditions. Bottom line is use a 5-30 or 10-30 for winter and save the 15-40 for summer.
 
Messages
24
Location
Loma Linda, CA
I am a long time devotee of Ester Synthetics. I run Redline in my 2 trucks, and Maxima Extra in my 7 dirt bikes and ATV's. (If I were a racer looking to milk the most horsepower out of my engine, I would run the extremely slick Maxima Ultra.) http://www.redlineoil.com/pdf/4.pdf http://www.maximausa.com/technical/lubenews/LubeNews2002.pdf For example, Redline 5W30 has a cSt @ 40 degrees C of 67, and a cSt at 100 degrees C of 10.9. That will give great cold starts, the oil is very "pumpable", the oil has a naturally high Viscolity Index (so not many if any Viscosity Index Improvers are needed), and it is 30 weight for tighter bearing tolerances. OK, I know that sounds thin for a hot summer day, but get this, it has a High Temperature/High Shear (cP @ 150 degrees C) of 3.8! That's high enough to make "heavy" 40 weight. (30 weight and "light" 40 weight oils require a minimum HT/HS of only 2.9. "Heavy" 40 to 60 weight oils, require a minimum HT/HS of 3.7.) So, if you are getting the picture I'm trying to pain here, with ester synthetics you can run a thinner oil that is very pumpable and won't bog your crank, yet in the bearings and on the hot wall of your cylinder, you are getting the protection of a heavy 40 weight. BEST OF BOTH WORLDS! What more could we ask for??? [ February 03, 2006, 06:20 AM: Message edited by: Wayne Kelln ]
 
Messages
13,132
Location
By Detroit
It really seems like an oil thickness should be tailored to the bearing clearance. Don't some high pefromance engines use wider clearances and a thicker oil? Or is the range of practicable bearing clearances inconsequential with respect to oil thickness within the range of typical oils viscosities. Also, what about an old engine that has widened its clearances over time and miles? It seems one should step up to a thicker oil in that case. One could look at oil pressure and if it drops over the long haul, correspondingly increase the oil thickness. Is there any reason if the manufacturer specifies a pressure of say 45 psi at 2000 rpm, that when the engine is old 30 or 35 psi is acceptable when a thicker grade will get you back to spec, or is the spec no longer relevant since the clearance is no longer what the manufacturer originally spec'ed the oil on? [I dont know] [Confused] [I dont know] [Confused] [I dont know]
 
Messages
23,591
quote:
It really seems like an oil thickness should be tailored to the bearing clearance.
Well, yeah, but tailored to fit which bearings best? Proper lubrication in crank bearings is surely important. [Wink] But what about lubricating the cam bearings and cam lobes? It's not really any less important, is it? Rod bearings and rings, valves? Not all bearings operate at the same rotational speed or under the same load. Even if clearances vary between bearings in an engine, compromises seem unavoidable.
 
Messages
24
Location
Loma Linda, CA
And nobody has even mentioned PLAIN BEARINGS! These aren't even really bearings in the classical sense. Here you are floating on a cushion of oil that is under pressure (no balls in these things!). Nice little explanation of how plain bearings work: http://mototuneusa.com/circular_logic.htm This is where you are going to get the worst wear at startup, before the oil pressure comes up! Here, an oil that is very "pumpable" when cold is absolutely essential. You want an oil that is thin enough to come up to pressure VERY quickly, yet will still not break down after the temperatures rise (nice high HT/HS). Just one more advantage of the Ester Synthetics is that the charged polar esters act like magnets. The oil sticks to metal surfaces after the engine is shut off, and doesn't "drain down" as badly as other oils. This gives you at least a little more protection for your bearings at startup. It's a different world now. In the old days, if you needed an oil that didn't break down so readily, all you did was just up the bulk viscosity (just make it thicker to start with). But in tighter tolerance engines, that solution doesn't work any more. [ February 04, 2006, 10:36 AM: Message edited by: Wayne Kelln ]
 
Messages
13,132
Location
By Detroit
Very interesting article Wayne Kelln. Noteworthy is the statement under "How Does A Plain Bearing Reduce Friction?":
quote:
Bearings are fed pressurized oil. This pressure is critical, because although the crank can push the oil aside, it's the pressure which keeps the crak from breaking through the oil film to contact the bearing.
So I wonder which is better for an engine that performs well, but is experiencing lower oil pressure after many miles, sticking with the manufacturer's recommended oil weight or stepping the weight up a notch to stay within the manufacturer's specified oil pressure?
 
Messages
3,478
Location
Millbrae, CA
quote: -------------------------------------------------------------------------------- Bearings are fed pressurized oil. This pressure is critical, because although the crank can push the oil aside, it's the pressure which keeps the crak from breaking through the oil film to contact the bearing. -------------------------------------------------------------------------------- This is dependant upon crank speed and load since under load and speed the crank will pull oil into a wedge which is what floats the crank NOT oil presure from pump. Pump oil flow is needed after full hydrodynamic lubrication starts to feed oil leaking out the sides at that time oil vis, pressure and flow have to be adiquate or bearing will starve. Large industrial units have high presure oil pumps to "float" the crank from full stop untill operating speed starts the hydro wedge to occur from then on less oil pressure can work as long as it provides enough flow to feed the wedge. bruce
 
Messages
13,132
Location
By Detroit
The time we need pressure most we have none. Should have electric oil pumps that can be started briefly before cranking over the engine. Since full hydrodynamic lubrication should be in effect promptly after starting the engine, this Motoman is apparently all wet in this area (but he sure makes it sound like he knows what he is talking about). And so maybe the 10 psi per 1000 rpm rule is not even all that good. Maybe a minimum pressure should be achieved across the board. RPM may not have that much to do with it. Perhaps an engine should have say 25 psi as all speeds. Another argument for electric oil pumps, which we don't have presumably because mechanical pumps are cheaper to manufacture.
 
Messages
24
Location
Loma Linda, CA
There is a device that helps keep the oil in your bearings for quicker pressure rize at start-up. I think it is just some kind of check valve in the line, or something. I will try to find it and post a link. In a worn engine that is experienceing low oil pressure, would it be counter productive to go with a thicker oil??? Sure, you might get more "pressure" at the pressure meter, but will it just be "back pressure" from pumping something that is thicker, and not realy reflect the pressure in your plain bearings????? On the other hand, the bearing tolerances on a worn engine are getting slacker, so a thicker oil might be helpfull. In any case, just be sure you have an oil pump that will pump it! Another issue I have heard of with thinner vs thicker oils is COOLING. The theory is that a thinner oil pumps faster, and makes the circuit from the engine to the oil cooler/sump quicker, so it carries more heat away with it. A thicker oil pumps slower, so it doesn't get cooled as well, and doesn't take the heat away with is as efficiently. ???????????????????????????
 
Messages
2,338
Location
Charlotte Metro area
One thing often left out of these conversations is the obvious Pressure/Flow relationship. Correct FLOW is what we are after, because it's the flow of oil that is what is really important to cooling. You can have high pressure, but a clogged oilway, and zero flow to that area. There is probably a calculation that engineers can use that describes the ideal flow to a part at a given pressure, and this can be tweaked with oil pump capabilities, viscosity, oilway length, and oilway diameter. THIS is why the poor hand-wringers that fret about 5-20 oil in their modern engines, and run 20-50 instead, could be setting themselves up for premature wear by using too thick viscosity for their particular engine's requirements. Bruce, it makes sense that a thinner oil will remove heat from a bearing surface more quickly due to more flow, but, it doesn't mean the thinner oil will be hotter at a constant speed...you didn't say it would...I just didn't want people to assume that. Since the thinner oil also passes faster by the areas where oil cooling occurs, more heat can be removed from the oil, too. Thin oil is like a faster moving train, transporting the cargo of heat from one point to another more rapidly than what the slower, thicker oil can.
 
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