Viscosity ?

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Jun 30, 2003
Lafayette, La.
Im new to the forum so if this has been mentioned already forgive me. I am wanting to try thicker oil in my boat to increase oil pressure. I have a 455 olds that I built and the clearances are on the loose side. I have been told that thicker oil can help increase oil pressure. I run 20W-50 Castrol now. Im wondering how do you figure out how thick the blended oils are relative to straight weight oils. Example would be, whats thicker 20W-50 or 40W, and so on.


[ July 01, 2003, 01:09 PM: Message edited by: Kevin Vidrine ]
One thin to consider is volume verses pressure. They are inversely related, so going to an even thicker oil to increase pressure will decrease flow volume. Kind of like kids blowing water through 2 staws. A more narrow straw will increase pressure and let you shoot farther, but will decrease the amount of water that is shot out. Food for thought.
Sorry, I didn't answer your second question. The second set of numbers in a blend are your viscosity at operating temp, so any XXw40 oil will be a 40 weight when fully hot, same as a staight 40 oil, and so on. The W or 'winter' rating simply means it will flow easier when cold and is better for cool climates than a staight weight oil. A 15w40 won't be thicker when hot than a 10w40, just different flow characteristics at cold start.
BUILDING A LONG STROKE ENGINE ON THE LOOSE SIDE IS A GOOD IDEA. I like .0025" rods and mains on small blocks, and .0025"-.003" on big blocks.
I make up the oil pressure with a better oil pump instead of thicker oil wigh these re-builds.
For example several engines have their oil pumps at the back of the engine and feed oil towards the front. With a thick oil, even with loose clearances it is going to take a while to get oil to the front rods and mains.
If you install the best hi-volume pump you can afford, and use say a 15W40 instead of a 20W50 engine oil.
The thinner oil will reach the critical engine parts sooner, more oil will be pumped through the engine and the high volume oil pump will keep the pressure from dropping as the oil heats up.
I believe that more big-block engine failures can be attributed to guys starting their engines and showing off the power before the engine is warmed up and oil has reached critical parts than from any other cause.
You see this at the beach all the time. Some jack@$$ will start his hot boat and blast off all in the time span of a few seconds.
I like it the best when his crank goes through the bottom of the boat about 100 yards off-shore.
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