OIL Weight?

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David Redszus
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Re: OIL Weight?

Post by David Redszus »

A higher viscosity oil can capture and hold air bubbles. Since the air displaces the oil, the oil density decreases.

A lower density oil will not transfer heat adequately and will run at higher temperature.
As the oil temperature increases, the viscosity falls and releases the entrained (or dissolved) air.

It should be obvious that cold oil will behave differently than oil at proper operating temperature.
RDY4WAR
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Re: OIL Weight?

Post by RDY4WAR »

Yes, a higher viscosity is more susceptible to aeration. Higher oil pressure also increases aeration risk as it increases the capacity of entrained air. I don't like to run higher than 65 psi.

Note though that running an extra 0.5 qt of oil increases aeration far more than a 30 grade vs 50 grade oil.
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Re: OIL Weight?

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RDY4WAR wrote: Thu Mar 04, 2021 11:30 pm Yes, a higher viscosity is more susceptible to aeration. Higher oil pressure also increases aeration risk as it increases the capacity of entrained air. I don't like to run higher than 65 psi.

Note though that running an extra 0.5 qt of oil increases aeration far more than a 30 grade vs 50 grade oil.
I do not have an accurate measure of aeration as a function of viscosity. But operating temperature does
have a substantial effect on viscosity. For example:

.........10w.......30w........40w
100F....37.1......111.2......173.0 cSt
200F.....7.4........13.6.......24.3 cSt

Oil pressure generally increases with engine speed. If engine speed increases but oil pressure no
longer increases, the by-pass valve is open and high pressure oil is discharged back into the sump,
causing substantial oil aeration, especially when the oil is cooler.

A data logger plot of rpm vs oil pressure is often very interesting; not just for high oil pressure but low as well.
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Re: OIL Weight?

Post by RDY4WAR »

I know the oil is more viscous when it's colder. I'm not arguing that point, I agree with you. We're not really concerned with what it's doing at 100*F though unless you're an NHRA Pro Stock team (with a straight 0 grade oil). I'm concerned with what it's doing at >200*F.

Equally as important as preventing aeration is allowing sufficient dwell time in the pan for entrained air to escape. An open pump bypass doesn't contribute as much to aeration as the crankshaft does. I'd rather that extra oil be going back to the pan to dwell than swirling around the crank.

To physically see aeration, you need a sight hole of some sort. I got to observe it with a clear dry sump tank. You can see signs of it when logging oil pressure and temperature. The more aerated the oil becomes, the hotter it will get, and pressure will become erratic.
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Re: OIL Weight?

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RDY4WAR wrote: Thu Mar 04, 2021 11:30 pm Yes, a higher viscosity is more susceptible to aeration. Higher oil pressure also increases aeration risk as it increases the capacity of entrained air. I don't like to run higher than 65 psi.
What's your lower number cut off ?
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Re: OIL Weight?

Post by RDY4WAR »

JC565Ford wrote: Sat Mar 06, 2021 1:10 am
RDY4WAR wrote: Thu Mar 04, 2021 11:30 pm Yes, a higher viscosity is more susceptible to aeration. Higher oil pressure also increases aeration risk as it increases the capacity of entrained air. I don't like to run higher than 65 psi.
What's your lower number cut off ?
Realistically... about 20 psi at hot idle, increasing 10 psi per 1k rpm, is being safe. My own car only sees 7-8 psi at 200°F (though it rarely gets that hot) and just 30-35 psi at 6000-7000 rpm down track. No problems at all, but it's just a mild small block. Flow is more important than pressure.
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Re: OIL Weight?

Post by JC565Ford »

RDY4WAR wrote: Sat Mar 06, 2021 10:20 am
JC565Ford wrote: Sat Mar 06, 2021 1:10 am
RDY4WAR wrote: Thu Mar 04, 2021 11:30 pm Yes, a higher viscosity is more susceptible to aeration. Higher oil pressure also increases aeration risk as it increases the capacity of entrained air. I don't like to run higher than 65 psi.
What's your lower number cut off ?
Realistically... about 20 psi at hot idle, increasing 10 psi per 1k rpm, is being safe. My own car only sees 7-8 psi at 200°F (though it rarely gets that hot) and just 30-35 psi at 6000-7000 rpm down track. No problems at all, but it's just a mild small block. Flow is more important than pressure.
Thanks.

Curious if you have any thoughts on running the M1 4T (10-40) motorcycle oil in a performance engine as opposed to their 20/50 ?

Is there anything in the formula that wouldn't be good for a pushrod V8 with a roller cam ?
https://www.mobil.com/en/lubricants/for ... -4t-10w-40
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Re: OIL Weight?

Post by RDY4WAR »

It won't harm your engine, but note that the friction modifier content is different from PCMOs and HDMOs to make it compatible with wet clutch applications. It'll work, you just might give up a couple HP.

I would run the Mobil 1 FS 0w-40 before the 4t oil. That's if you want to stick with M1.
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Re: OIL Weight?

Post by JC565Ford »

Thanks for your input.
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