oil viscosity, weights etc-long/neat race car
Jul 26, 2006 | 06:27 PM
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From: Bellevue. WA
oil viscosity, weights etc-long/neat race car
I copied this off another forum where I asked if a thinner oil wouldn't be better for a modern performance car, even one used on the track. Since I found it so interesting here it is in total.
"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. "
"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. "
