Thin oil as it relates to oil pressure

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700
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USA
You will not notice a difference between a thin 30w and a thick 30w. Example: was that i put Valvoline Racing 30w (11.0 cst)in my boat (Ford v-8) and Chevron Delo 30w (12.5 cst) and there is no difference on the pressure guage. I think the thicker oils run hotter because of more friction and work to pumpthe oil and thus it heats up and is the same as a thinner oil. I would rather use on the thin side of the scale and have faster-cooler flowing oil. I use M1 5w-30 in my Jeep and have 70 psi at highway speeds, and 35 at hot idle.
 
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9,427
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Pensacola & Vero Beach FL
Consider the overall lubrication system and how it all works together, with oil as but one part of it. The pump is designed to put out a greater flow at a higher pressure than the engine really needs, to allow for spikes in demand, and this is controlled by a pressure relief valve. One pressure factor that seems to get overlooked is the speed with which oil "escapes" from the pressurized part of the system. If there's an internal leak, or if the oil is so thin that it moves through and escapes too quickly, then you'd probably see a pressure drop. When it sets off warnings is a matter of the calibration of the sensor. Like any liquid, oil, thick or thin, is a non-compressible fluid (gasses being the compressible type of fluid). As such, it cannot be "crammed" into a smaller volume. So, as long as there's enough pressure to keep oil where it's supposed to be, I'd think that any oil should be able to keep parts separated. My big question is at which point does an oil become so thin that it can escape the pressure "circuit" so quickly that pressure drops and the ability to keep parts separated is compromised.
 
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9,427
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Pensacola & Vero Beach FL
Oh yeah, another thought from my flying days in the Marines: in my jet (the EA-6) excess HIGH oil pressure was considered just as serious an engine emergency as was low O/P. Much above 50 psi, and you were running a serious risk of blowing out oil seals. In fact, an engine shut-down was required if the pressure could not be controlled by throttle adjustments. Not a popular option if your were recovering aboard ship, but better than having hot pressurized flammable oil spewing all over the inside of your engine compartment.
 
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144
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98281
As a guide, I have always tried to make sure that at 2000 rpm I had oil pressure that related to the maximum RPM the motor would see. So if my day to day red line RPM was 5,000 then I made sure that at 2000 rpm , I had 50 psi.
 
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308
Location
Houston, TX
the oil pressure in the oil passages serves to display the flow relative to restriction, and to insure there's enough pressure to reach the whole oiling system. for instance, if you only had 5psi right after the oil filter in a sbc, it's questionable whether or not any oil is flowing to the front cam bearing, or front main. if you only had 20psi, you're probably getting oil flow throughout, but there's not enough flow to cool the bearings under high loads. the pressure within the bearing itself is basically a function of the oil properties (and viscosity). bearings are self-pressurizing. with that in mind, my guess is the pressure warning lights are calibrated keeping the severe service limits of heat and recommended viscosities in mind. it also matters where the switch is. if it's on the far side of the oiling system from the pump, after every passage is fed, 5psi wouldn't necessarily be dangerous, for instance. i guess i didn't answer your question, but it's something to think about. -michael
 
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9,427
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Pensacola & Vero Beach FL
quote:
Originally posted by Michael SR: the pressure within the bearing itself is basically a function of the oil properties (and viscosity). bearings are self-pressurizing. -michael
Right, but just like the primary pump, the bearings will be self-pressurizing whatever oil they are fed, and if that oil is so thin that it "escapes" or flows away too soon, then wouldn't you be looking at the dreaded metal-to-metal contact?
 
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12,385
Location
Northern CA
quote:
Originally posted by ekpolk: My big question is at which point does an oil become so thin that it can escape the pressure "circuit" so quickly that pressure drops and the ability to keep parts separated is compromised.
It's a complex relationship. Oil pressure does not directly control how well a journal bearing works. All pressure does is get an adequate supply of oil into the bearing. I think that if youran through the numbers or did some test work you would find that a thinner lubricant requires less pressure to get an adequate supply to the bearing. If you dig into some engineering references like Marks Mechanical Engineers handbooks, you will find some ugly sets of equations that will give you some insite into how viscosity and supply interacts with other characteristics of a journal bearing. In a given engine, you can have oil too viscous for the pump to deliver an adequate supply of oil to the bearings, or you can have oil so thin that the pump can't supply the volume required to make the bearing work. Aother side effect of thinner oil is that the running clearance in the bearing is reduced. So if you are running 0W-20, fine filtration is more important than if you are running 10W-30. Running clearnaces get down into the very low micron range, even though nominal bearing clearance may be 25 microns.
 
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308
Location
Houston, TX
quote:
Originally posted by ekpolk: Right, but just like the primary pump, the bearings will be self-pressurizing whatever oil they are fed, and if that oil is so thin that it "escapes" or flows away too soon, then wouldn't you be looking at the dreaded metal-to-metal contact?
true, but i would think there's something "wrong" in the engine to get that far. clearances of up to .0035" on rods and mains seem to work fine, just have slightly lower oil pressure than normal. if the clearances are small enough, residual oil still forms a wedge. i've seen race engines that ran a few minutes without the pump working (snapped shaft due to debris, or where the tech made an assembly mistake), with no bearing damage as a result. but if the clearances were huge to begin with, ya, i can see it becoming exponentially catastrophic. -michael
 
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308
Location
Houston, TX
sorry, i hope i'm not coming across as arguing. good points here, i'm just trying to say that slightly lower than normal idle pressure from thinner oil doesn't spell disaster, imo. -michael
 
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1,855
Location
Australia
Just my experience from over 10 years ago running 0w-5 oil in an engine. Without changing the bearing clearances (IIRC, 2.2 thou), we experienced cavitation in the mains. This is evidenced by minor pitting in the bearing surface. We also ran very low oil pressures (35psi @ 7000rpm, set on the pressure pump) Tightened up the clearances to suit the light weight oil, no more problems. Above 25*C I'd use 10w-30. Oil pressure didn't alter. Rick.
 
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12,385
Location
Northern CA
quote:
Originally posted by tdi-rick: Just my experience from over 10 years ago running 0w-5 oil in an engine. Without changing the bearing clearances (IIRC, 2.2 thou),
Interesting, what was the application? I hadn't heard of cavitation in journal bearings, but there's a lot of things I haven't heard of:lol: Was the cavitation occuring after the highly loaded are where the inner and outer bearing surfaces get further apart? Several years ago I designed and built a journal bearing about engine bearing size and speed that used fresh or salt water for lube. I did it using normal design standards and used some small clearances. It worked like a champ. No cooling problems [Smile]
 
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9,427
Location
Pensacola & Vero Beach FL
quote:
Originally posted by Michael SR: sorry, i hope i'm not coming across as arguing. good points here, i'm just trying to say that slightly lower than normal idle pressure from thinner oil doesn't spell disaster, imo. -michael
Not argumentative at all. BTW, the manual for my 2003.5 V-6 Camry says that a flickering o/p light at idle is not a cause for concern. I've never seen that, and don't want to, but at least I know it's OK if it does.
 
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2,480
How is the oil pressure measured anyway? Isn't there an "oil pressure sensor" or something? In that case, wouldn't it be calibrated to read 'low' once a certain bottom threshold is reached? As an aside...my BMW doesn't have a guage, but there is a sensor on the oil filter canister that once got wet from washing the engine and I had a 'low oil pressure' message on the computer...
 
Messages
12,385
Location
Northern CA
quote:
Originally posted by Dr. T: How is the oil pressure measured anyway? Isn't there an "oil pressure sensor" or something? In that case, wouldn't it be calibrated to read 'low' once a certain bottom threshold is reached?
The problem with oil pressure idiot lights is that they are usually set just below the lowest pressure expected at idle on a hot engine. Typically around 5 psi. Run your engine hard with pressure just above the idiot light pressure and you stand a real good chance of ruining your engine without the idiot light coming on.
 
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144
Location
98281
I have been following the thick/thin debate but have heard little talk about oil pressure. Now I can not speak for Honda's, but if you were to put 5w-20 in a VW , the oil pressure warning light will come on when the car is good and hot and idling. Also I think we have established that manufacturers do not adjust the mechanical clearances for each world region. Backlash on oil pump gears is the same in one model all over the world. Yet the same can not be said for oil viscosity requirements.
 
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