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Old 09-17-2011, 03:46 PM
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15w50 vs 10w30 oil pressure

I just did my first oil change after break-in... went from Joe Gibbs BR 15w50 to Joe Gibbs HR 10w30. My hot idle (950rpm) oil pressure with the 15w50 was 57psi. With the 10w30 it's virtually identical! I warmed the engine up to ~160* oil temp and the idle pressure was still 56psi. The only explanation I could think of was that the oil still wasn't hot enough to get into the 30 weight viscosity band? To be honest, I was expecting the hot idle pressure to drop by around 10psi, so I'm a bit confused.

Any ideas?

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Old 09-17-2011, 04:07 PM
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Quote:
Originally Posted by v8hed
I just did my first oil change after break-in... went from Joe Gibbs BR 15w50 to Joe Gibbs HR 10w30. My hot idle (950rpm) oil pressure with the 15w50 was 57psi. With the 10w30 it's virtually identical! I warmed the engine up to ~160* oil temp and the idle pressure was still 56psi. The only explanation I could think of was that the oil still wasn't hot enough to get into the 30 weight viscosity band? To be honest, I was expecting the hot idle pressure to drop by around 10psi, so I'm a bit confused.

Any ideas?
All this data like 10W30 or 15W50 means is at some low temp like zero F, the oil flows like a 10 or 15 weight receptively. At 210 degrees F it flows like a 30 or 50 weight respectively. Between a 10W30 and a 15W50 if you graphed the flow characteristics there's a cross over point where at some temperature they have identical flow characteristics.

Also, at 57 PSI you might be looking at a case where either weight oil at 160 degrees F has the bypass open in which case the system pressure would be the same regardless of oil viscosity.

Bogie
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Old 09-17-2011, 04:12 PM
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Quote:
Originally Posted by oldbogie
Also, at 57 PSI you might be looking at a case where either weight oil at 160 degrees F has the bypass open in which case the system pressure would be the same regardless of oil viscosity.

Bogie
Don't think that's the case... oil pressure rises way above 57psi with engine RPM... 5,000rpm is around 90psi @ 160f oil temp.
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Old 09-17-2011, 06:59 PM
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Why don't you ask your "builder"?
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Old 09-17-2011, 07:51 PM
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Looks like you got ya a new oil pump along with the bearings.
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Old 09-18-2011, 02:29 AM
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Originally Posted by sqzbox
Looks like you got ya a new oil pump along with the bearings.
Yep, it's a tight ship... fresh build. Guess I shouldn't complain about good oil pressure lol
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Old 09-18-2011, 02:15 PM
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I've been suspecting for some time there's more to oil pressure than the general theories tell us.

Here's my oil pressure all measured at 200degF and at 2000 rpm. Even though they all should be acting pretty much like 50W oil does when at 200degF, I find the differences between them not only interesting, but backwards to what I expected:

Castrol Gtx 20W50 45 psi
Mobil 1 Syn 5W50 42 psi
Mobil 1 Syn 15W50 38 psi

They may be all 50 weight, but supposedly the synthetic is more stable at temperature, but I'm not the only person who has experienced lower oil pressure with the synthetic 15W50 than a dinosaur equivalent.

Googling for information doesn't always give you understandable results.

For example, there are a large number of discussions on oil where it is stated that "for the same viscosity, the synthetic flows better". Now how can that be? Isn't the viscosity supposed to reflect the flowability?

I have a sneaking suspicion that the heavier the viscosity, the more the oil becomes heated due to friction, and thus tends to thin down more, thus making it behave closer to a lighter weight oil. This might be due to tight bearing clearances, and I suspect it's what's happening in the OP's engine.

I still havn't found out whether it's higher or lower viscosity oil than transfers heat better. But if heavier viscosity oil doesn't get rid of heat as easily (from the oil pan surface), then it may merely be hotter, and thus thinner, giving close to the same oil pressure as a lighter weight, lower viscosity oil.

Sidenote:various oil viscosties do become closer the hotter the oil becomes:

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Old 09-19-2011, 11:15 AM
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Quote:
Originally Posted by v8hed
Don't think that's the case... oil pressure rises way above 57psi with engine RPM... 5,000rpm is around 90psi @ 160f oil temp.
Why are you running 90 psi of pressure? Unless this is a racing at Daytona at wide open throttle turning 9000 RPM for 2.5 to 3 hours without let up except for a couple pit stops, there is absolutely no need for this kind of pressure.

Bogie
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Old 09-19-2011, 11:26 AM
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Quote:
Originally Posted by oldbogie
Why are you running 90 psi of pressure? Unless this is a racing at Daytona at wide open throttle turning 9000 RPM for 2.5 to 3 hours without let up except for a couple pit stops, there is absolutely no need for this kind of pressure.

Bogie
The higher pressure is possibly due to the oil pump (Melling 10550). I went for the 10550 because it has steel pinned drive gears and a decent housing. That and my bearing clearances are obviously 'healthy'.
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Old 09-19-2011, 11:44 AM
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Old 09-19-2011, 12:01 PM
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Quote:
Originally Posted by v8hed
The higher pressure is possibly due to the oil pump (Melling 10550). I went for the 10550 because it has steel pinned drive gears and a decent housing. That and my bearing clearances are obviously 'healthy'.
High volume and pressure pumps are for engines with wide bearing clearances and really good oil control devices in the pan, usually where a racing situation is rules limited to a wet sump.

Even for me and I tend to hang on to old proven methods for a long time after better ones are developed, this is a method that has largely been replaced. Better fuels and oils, and machining have allowed tighter clearances to be used thus reducing the need for so much extra oil to float the bearing journals against high leakage rates caused by wide clearances.

But before making any further recommendations, I'd need to know exactly how this engine is being used and what oil control devices are in the pan.

This really opens up a can of worms on opinions it won't be long before this discussion in 10 pages or more in length.

Bogie
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Old 09-19-2011, 02:04 PM
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Hot street motor... 7qt baffled pan with trap door filled with 6qts incl. filter.

I'm not concerned with the pressure... just curious as to why it didn't drop when I switched to 10w30 from 15w50.

Understand re. the 'no need for a HV pump' debate... like you said, it's a can of worms which I have no wish to open. I chose the pump I have more because of its build quality than its HV capability.
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Old 09-19-2011, 02:49 PM
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If you would need less pressure, you can adjust the bypass spring pressure, so that it will open sooner.
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