Oil viscosity, bearing clearance, oil pressure question big block chevy

[GRTfast]

Been doing a lot of reading. Need some experience based advice. Engine is a 2 bolt main big block Chevy street motor. Forged crank and rods. Main clearances are .003, rods are .0025

High volume oil pump. With 5w30 it will maintain 22ish psi hot oil pressure. At 2000 rpm it’s 50. At 6000 rpm it’s 75. (Florida summer).

I’ve always heard that larger clearances like I have require thicker oil. If I use 20/50, hot oil pressure at idle us 35 ish. 2000 rpm is 65, it’s 75-80 by 3500 rpm and stays there to red line. presumably I’m in bypass at this point.

With 10/40 the pressures are about half way between what they are for 5/30 and 20/50.

The old school knowledge my dad put in my head says to run the 20/50, but the research I’ve done is saying thinner is better as long as I can maintain adequate pressure.

Also, at cold startup, idle oil pressure is 45 with 5/30, 55 with 10/40 and 65 with 20/50.

Thoughts?

[engineguyBill]

The oil pressure readings that you are achieving with 5W30 are fine, use it. 20W50 is too much viscosity for any high performance engine.

[Fireonthemountain]

Use to be 20/50 was the right answer, but things change. Today many many engines with tight clearances will not tolerate thicker oils

The thicker philosophy was because it was thought to protect better and made oil pressures rise, and high oil pressures were thought to be a sign of a healthy engine. And losing pressure or low pressure were bad.

Then as motors got hot the oil thinned and pressure dropped. Originally there were no multi viscosity oils and only straight weights, and 30 weight was about normal, with 40 or 50 advocated by some racers back then. The thicker oils were thought to cling better to parts, so when you started the engine the oil would already be there. There were also other reasons for all this, such as getting gasoline in the oil, at times, thinning the oil.

Today there might be very healthy engines showing no oil pressure at idle, but as long as the wedge of oil is there and just from large clearances built i,n then no problem. The minute you hit the gas the pressure shoots up.

Its ingrained in some still, to keep the pressure up at idle. Thick oil eats power and does not protect better, and we know this from testing.

[Newold1]

You should not try to over engineer your oil usage.
With the numbers you are giving your current oil will work just as it should.

[GRTfast]

You should not try to over engineer your oil usage.
With the numbers you are giving your current oil will work just as it should.

Understood.

[Fireonthemountain]

Use to be 20/50 was the right answer, but things change. Today many many engines with tight clearances will not tolerate thicker oils

The thicker philosophy was because it was thought to protect better and made oil pressures rise, and high oil pressures were thought to be a sign of a healthy engine. And losing pressure or low pressure were bad.

Then as motors got hot the oil thinned and pressure dropped. Originally there were no multi viscosity oils and only straight weights, and 30 weight was about normal, with 40 or 50 advocated by some racers back then. The thicker oils were thought to cling better to parts, so when you started the engine the oil would already be there. There were also other reasons for all this, such as getting gasoline in the oil, at times, thinning the oil.

Today there might be very healthy engines showing no oil pressure at idle, but as long as the wedge of oil is there and just from large clearances built i,n then no problem. The minute you hit the gas the pressure shoots up.

Its ingrained in some still, to keep the pressure up at idle. Thick oil eats power and does not protect better, and we know this from testing.

Thick oil is not so thick at operating temperature. Load capacity is a function of viscosity.

Thick oil is not as thick it is true as it warms up, but heavier also may not protect as well, from from wear, as a lighter weight oil, at higher temperatures. The weight of the oil does not determine the shear force needed from going metal to metal, at any normal or high oil temperatures. I once believed differently.

[digger]

Without knowing the operating temperature it's Meaningless

[digger]

Without knowing the operating temperature it's Meaningless

Pretty much a given on a liquid cooled engine isn't it? 180 to 220 degrees F, somewhere in that area.
One other small detail, we have seen photo's of cavitation bearing damage on here before, and what viscosity's are most likely to cavitate?

Easily Anywhere from 160-260 if not more depending on the application

A lot depends how good the design is (stiffness of parts) and the quality of machining.

[DCal]

I have the same clearances in my BBC and with 10/30 wgt and fully warmed up I get about 3lbs more than yours. I have 7500 street miles on mine and water temp 180-190 so in my book you're good to go.

[Kevin Johnson]

... And then I found this. https://www.sciencedirect.com/science/a ... 5815604942

China is very proud of a researcher; Li-li WANG (王丽丽)
College of Mechanical and Electronic Engineering, Shandong University of Science and Technology, Qingdao 266590, China:

Full text; no paywall:

http://or.nsfc.gov.cn/bitstream/0000190 ... 598568.pdf

[GRTfast]

... And then I found this. https://www.sciencedirect.com/science/a ... 5815604942

China is very proud of a researcher; Li-li WANG (王丽丽)
College of Mechanical and Electronic Engineering, Shandong University of Science and Technology, Qingdao 266590, China:

Full text; no paywall:

http://or.nsfc.gov.cn/bitstream/0000190 ... 598568.pdf

So it looks like pressure increase of supply pressure reduced cavitation in the bearing, as does lower viscosity. Seems like the ideal scenario (as reducing cavitation is concerned) is low viscosity oil with high oil pressure, which is the scenario that yields the most flow.

[Newold1]

While some here will discuss the long term affects of morning sunlight versus early evening sunlight, I will just refer to my old simple explanation of the OP's question.

"IF IT AIN'T BROKE DON'T FOOL WITH IT OR FIX IT!"

[GRTfast]

While some here will discuss the long term affects of morning sunlight versus early evening sunlight, I will just refer to my old simple explanation of the OP's question.

"IF IT AIN'T BROKE DON'T FOOL WITH IT OR FIX IT!"

[David Redszus]

Determining the correct oil viscosity for a given engine and operating condition is no simple matter.
Trial and error is often employed and often results in trial and error, error, error, error, etc.

Oil viscosity is a function of the oil itself and operating temperature; the units of actual viscosity are CentiStokes (cSt), which can be predicted at any temperature using cSt values at 40 and 100C.

A very useful equation used to predict required oil viscosity is given by the Sommerfeld Number. There are a number of
variables to be considered:

Journal diameter (in)
Bearing clearance (in)
Bearing length (in)
Bearing load (lbs)
Bearing speed (rpm)
Oil Temp (F)
Oil cSt @ 40C
Oil cSt @ 100C
Oil Specific Gravity
Oil pressure (psi)

Using the above inputs, a spreadsheet can be built that will give:
Absolute oil viscosity (cSt)
Dynamic viscosity (cP)
Bearing area (in2)
Bearing unit pressure (lbs/in2)
Bearing flow area (in2)

Now we can determine the Sommerfeld number inflection point; an oil viscosity that will yield the minimum friction under any set of operating conditions.

Oil that is too thin or too thick will both produce excessive bearing wear due to increased friction.
Oil that is too thin has the additional benefit of producing engine destruction.

Oil that approaches a too thin condition must rely on the oil anti-wear additive package to survive.

OE mfgs work closely with oil companies to design and specify the correct oil for intended operating conditions.
Then we over-rule them and race the engine beyond its intended limits.

[GRTfast]

Determining the correct oil viscosity for a given engine and operating condition is no simple matter.
Trial and error is often employed and often results in trial and error, error, error, error, etc.

Oil viscosity is a function of the oil itself and operating temperature; the units of actual viscosity are CentiStokes (cSt), which can be predicted at any temperature using cSt values at 40 and 100C.

A very useful equation used to predict required oil viscosity is given by the Sommerfeld Number. There are a number of
variables to be considered:

Journal diameter (in)
Bearing clearance (in)
Bearing length (in)
Bearing load (lbs)
Bearing speed (rpm)
Oil Temp (F)
Oil cSt @ 40C
Oil cSt @ 100C
Oil Specific Gravity
Oil pressure (psi)

Using the above inputs, a spreadsheet can be built that will give:
Absolute oil viscosity (cSt)
Dynamic viscosity (cP)
Bearing area (in2)
Bearing unit pressure (lbs/in2)
Bearing flow area (in2)

Now we can determine the Sommerfeld number inflection point; an oil viscosity that will yield the minimum friction under any set of operating conditions.

Oil that is too thin or too thick will both produce excessive bearing wear due to increased friction.
Oil that is too thin has the additional benefit of producing engine destruction.

Oil that approaches a too thin condition must rely on the oil anti-wear additive package to survive.

OE mfgs work closely with oil companies to design and specify the correct oil for intended operating conditions.
Then we over-rule them and race the engine beyond its intended limits.

Thanks for the clues, time to fire up MathCad.