bearing thoughts

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9
Location
bellevue WA
hello all, I have a question about the merits of oil viscosity choices for a high performance engine. With new engines being built much tighter than before I wonder if lighter weight oils wouldn't serve better than thicker? On another forum I see hobby racers using 40 or 50 weight and saying its for bearing protection. Would't a thinner oil fit in the tight spaces better? I would also think a thinner oil would cool quicker, (and heat quicker) which in a long work session would be helpful.
 
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537
Location
California
I have no idea why, but JPL did a study on this very topic a looong time ago, like in the late 60's. They found that narrowing the engine clearances required lower viscosity oil. I have no idea how they tested this, I just remember the result.
 
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2,387
Location
Chicago area
Smokey Yunick used straight 30 in real race cars many years ago. Nascar cars often use 5W in qualifying, and the same engine has to be used later for the race[s].
 

brucer

Thread starter
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9
Location
bellevue WA
SO a 10 weight oil would be a good choice? If I understand it correctly a 0/30 is like a 30 weight oil when hot? MarkC what are you going to strike Seattle with?
 
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783
Location
Austin Texas
It is not the thickness of the oil that matters, it is the viscosity (e.g. resistance to flow) with is completely different than SAE weight (which is a target band of viscosities at a given oil temperature.) Your engine is designed for a particular viscosity range. Say 20cSt down to 2.5cSt. There is a minimum viscosity where wear accelerates and it become impossible for the engine to survive the 120K miles government mandate for emission controls. Any viscosity above this minimum will allow your engine to lead a nice long life. Most modern engines can live a nice long life as long as the actual viscosity remains above 2.5cSt. Since few engines pull heavy loads all the time, the manufactures target about 10cSt of oil viscosity for the normal operating condition. This is a thick 20W or a thin 30W oil. At the other end of the viscosity spectrum, there is a maximum viscosity above which the oil pump cannot push enough oil to keep the various bearings lubricated. Here is a regimine where overly thick oils will actually hurt your engine. Viscosities above 1000cSt provide marginal lubrication. No oil is thin enough at startup, even in the 100dF heat of Texas summers! And until your oil gets up to 100dC (212dF) it is too thick. But there is another factor, until the oil gets hot, the antiwear package is not working all that well. So, manufactures design the oil system to reach operating temperature fairly rapidly (5 to 20 minutes). So at normal operating conditions (10cSt at 100dC) the oiling system has plenty of margin (above the 2.5cSt minimum). Now tow a load, or take the car to the race track, and the oil temperature rises. When a 30W oil reaches about 265dF it also reaches about 3cSt and the margin in the system is removed. Is rapid wear happening, no, but it is not far away from happening. In similar situations a 40W oil will reach that 3cSt at about 280dF and a 50W at 300dF. So, based on the operating temperature of the oil you choose the oil that remains above the design viscosity of the engine, and you will be happy. On the other side of the coin, thicker oils allow startup wear to continue far longer until the oil heats up and thins out. So for an application where many short trips are 'de rigor', the thinnest oil possible prolongs the life of the engine. So, how does one achieve a thin oil under operating conditions and a thick oil under stressful conditions? One uses an oil cooler with a thermostat! Only high dollar sports cars have these kinds of systems *Ferrari, Porsche, Lamborghini}. Even the venerable Corvette only has the oil cooler on the Z51 option package, and here it has no thermostat to avoid overcooling the oil. Racing cars (especially endurance racing cars) use thick oils. We have one of these critters in the shop. With a 750 HP SBC (Lola T70 CanAm car), the startup instructions read something like this. 45 minutes becore cranking the engine over, turn on the oil heater. When the oil gets up to 100dC, run the electric oil pump until you have 60 PSI of pressure through the engine for 1 minute. Turn off fuel pumps, turn off both sets of the ignition, crank the enigne over for 20 seconds. Turn on the fuel pumps, prime the velocity stacks with a tablespoon of 110 octane gasoline for each stack. Turn on startup ignition. Attempt to start the engine, but do not crank for more than 5 seconds. If engine catches, spin it up to 2.5K RPMs until it will run cleanly, then reduce speed gradually, until it will hold idle at 1,200 RPMs. Then turn off the oil heater! Simply attempting to start an engine like this without the oil being hot will cause it to self destruct--instantly--most likely the crankshaft will break. So, if you have an application where you stress the oil heavily, then is is better for you and for your car to add an oil cooler with thermostat and keep the xW-30 or xW-20 oils in the crankcase.
 
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9,461
Location
Not Seattle, but close.
quote:
Originally posted by brucer: SO a 10 weight oil would be a good choice? If I understand it correctly a 0/30 is like a 30 weight oil when hot? MarkC what are you going to strike Seattle with?
For now, just large amounts of contempt. Next time I'm at the lakeshore or actually over there, I'll launch a few "birds".
 

brucer

Thread starter
Messages
9
Location
bellevue WA
Mitch, most informative! and thank you for the description of the race motor start-up. do you mind if I copy and paste parts of your post? in its own humble way my mitsu evo 8 has a oil cooler & thermostat
 
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39,805
Location
Pottstown, PA
[Welcome!] I don't think it's that one dimensional a question. I don't think you can boil it down to just thick vs. thin. It depends on pressure/flow with a broad margin that can be indexed either way without "bumping the walls". If you've got ample pressure with a light weight oil ..then you don't have bearing worries. The main "fear factor" for those in the know (I'm not one of them) is valve train wear on non-roller type engines.
 
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12,385
Location
Northern CA
quote:
Originally posted by carock: I have no idea why, but JPL did a study on this very topic a looong time ago, like in the late 60's. They found that narrowing the engine clearances required lower viscosity oil. I have no idea how they tested this, I just remember the result.
A journal bearing design book from 80 years ago would tell you the same thing. Thinner oil is required for optimum performance with smaller clearance, you can go a lot thicker than optimum and get by with lower bearing performance. Runs hotter and higher drag, which are two sides of the same coin.
 
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1,910
Location
Vista, CA
Basically, that's how you start an engine with a higher viscosity oil. The end result is that when the engine is up to temp the oil viscosity is just right. The oil can now take abuse a 'thinner' oil could never stand. In high performance street cars you don't hold the throttle down very long, and in a race car you hold it down as long as possible.
 
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2,387
Location
Chicago area
I wonder if that Lola would have a different starting sequence if the newest/best oils were considered. It's not something that I would like to do every morning in a daily driver!
 
Messages
783
Location
Austin Texas
Brucer: Sure, cut and paste to your hearts content. Mechtech: This kind of engine has a maximum life of 40 hours anyway (with a $25K rebuild, or almost $1K/hour of use). Getting 750 HP from a (naturally asperated) 350 SBC is not easy trick, and keeping the engine alive for 40 hours is not a mainanence free ordeal. BTW we do use the best oils we can get our little hands on. This engine lives on RedLine. But nothing on a 36 year old real race car is maintance free.
 
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