Oil Flow vs Pressure

[CyFi6]

I have a pretty simple question.... Why is flow so important rather than pressure? As long as there is pressure, doesn't that mean the bearings are coated? Why must you have so much flow, its not like the oil needs to carry away particulates or something like that, so as long as the bearings are coated with oil what is the issue?

 

[jdub]

LOL...that is not a simple question

Without going into fluid dynamics, you have to understand how an open bearing (like rod and main bearings) work. In a simple sense, the bearing "water ski's" on a wedge of oil as the journal rotates. The oil is pumped in and exits out the bearing sides...it has to be constantly replenished to provide the volume of oil required for film thickness in the bearing. A secondary, but very important, function is to keep the bearings cool. This pic might help visualize it:



What's important is the volume of oil supplied, not the pressure...flow is going to give you the greater volume and is dependent on the size of the pathways. Pressure (you can look at it as flow rate) will too, but only to the point where the oil channels in the block, crank, etc can handle that volume. Once that point is reached, you have very little additional flow for a much larger rise in pressure. This is why you will hear "pressure is resistance to flow". Higher viscosity oils have an intrinsic resistance to flow...thicker fluids are harder to move through a given size channel.

Keep in mind this is the simple version explanation...it's really a lot more complicated than this.

BTW - There is a fair amount of particulates floating around in your oil...soot from combustion, particles your air filter misses, etc. You oil carries this crap to your filter so it can be removed.

 

[CyFi6]

Ok, thanks, that helps a little bit. So now i ask why is it so important that there be about 10psi of pressure for ever 1k rpm? If you have 10 psi of pressure at any rpm, doesn't that basically mean you have enough flow? As you stated, once you reach the point where there is basically max volume output, a rise in pressure will really not increase flow very much at all, so why do you need more pressure at higher rpms?

 

[jdub]

10 psi per 1000 RPM is an old school rule-of-thumb...it's not hard and fast. It does insure that you will have the flow you need to the bearings to accomplish what I posted above. As RPM increases, so does the amount of oil required to keep the bearings "floating" from the increased force form the piston/rods and to remove the heat produced. Higher pressure at lower RPM is not going to hurt a thing.

The pressure has to get up the 90-100 psi range on a 7M before you start to reach the volume flow capacity of the oil channels in the block...above this more pressure will give you more flow, but the incremental gains in flow is greatly reduced vs what you get below 90 or so. You also start running the risk of popping oil lines or the filter itself.

One more time, the focus on pressure is misplaced. You can run a thick oil and get high pressure, but the flow is less than what you see with a thinner oil at lower pressure. Or the much higher flow with a thinner oil at the same pressure you see with the thick stuff. Pressure, flow and viscosity all run hand-in-hand and are dependant on the capacity of the pump

Another thing to remember about the 7M-GTE is the oil cooler circuit functions due to a relief valve in the filter head...it operates at ~40 psi. This effectively bleeds pressure from the overall system and dumps the oil straight to the pan...you don't usually see pressures above 40-50 psi as a result. One of the best things you and do to improve flow is to switch the cooler system over to a thermostat controlled full flow circuit.

I created a new thread for this...the subject was kind off topic for the original.

 

[Nick M]

Pressure goes up by trying to move more into a fixed area. That confuses people into wanting more pressure.

 

[CyFi6]

Decided to post here rather than in a different thread. I understand what all you guys are saying about pressure, but now comes in a problem. According to my Prosport Oil pressure gauge, at idle, i have about 4-5PSI. This is the stock pressure minimum. I have this pressure with Mobil 1 0w40 synthetic oil. According to the charts this has a much thicker operating temperature viscosity than say the Mobil 1 0w30 oil. Does this mean i should stick with my 0w40 instead of going to a 0w30? Common sense would tell me that with the 0w30, my idle pressure would drop below the manufacturer minimum at operating temperature.

All pressures off idle are at LEAST 10psi per 1k rpm, and most cases more. 3k RPM driving usually has a pressure of 40-50 PSI. Switching to the 0w30 oil would not put my off idle pressures out of the safe range, but im afraid it will put my idle pressures below the safe range. What is your input on this?

I will add just in case...
7mge N/A
88k original miles

These pressure readings i gave in this post are with Mobil 1 0w40 after approximately 3,000 miles.

 

[IJ.]

10 Psi per 1000 is for maximum pressure only NOT across the range.

 

[Nick M]

Decided to post here rather than in a different thread. I understand what all you guys are saying about pressure, but now comes in a problem. According to my Prosport Oil pressure gauge, at idle, i have about 4-5PSI. This is the stock pressure minimum. I have this pressure with Mobil 1 0w40 synthetic oil. According to the charts this has a much thicker operating temperature viscosity than say the Mobil 1 0w30 oil. Does this mean i should stick with my 0w40 instead of going to a 0w30? Common sense would tell me that with the 0w30, my idle pressure would drop below the manufacturer minimum at operating temperature.

All pressures off idle are at LEAST 10psi per 1k rpm, and most cases more. 3k RPM driving usually has a pressure of 40-50 PSI. Switching to the 0w30 oil would not put my off idle pressures out of the safe range, but im afraid it will put my idle pressures below the safe range. What is your input on this?

I will add just in case...
7mge N/A
88k original miles

These pressure readings i gave in this post are with Mobil 1 0w40 after approximately 3,000 miles.

What flows better? Pancake syrup that has just been nuked, or straight from the fridge?

Now, why do you want oil tha does not flow as well, giving you more pressure? You are still stuck on pressure. If your engine is aged, the bearings on the crank open up. This will lower the resistance to oil flow, which makes the pressure on the gauge lower.

 

[CyFi6]

Well according to the oil information write up from jdub's signature, it states you want the oil that will give you the proper pressure that the manufacturer recommends. In this case, at idle, that spec is 4.3 or so PSI. Yes "flow is more important than pressure" but if i run an oil that is going to give me 0psi of pressure at idle is that better because there's less resistance to flow=more flow? Doesn't sound right to me. If that was the case why doesnt the manufacturer call for 0psi at idle?

 

[IJ.]

Because you need x pressure to maintain the Oil wedge to keep the Crank off the bearings.

0 pressure the motor will last hours maybe even at idle speeds.

 

[CyFi6]

Thats why i don't agree with what Nick M stated, saying im "stuck on pressure". My initial question still stands, if i switch to a 0w30 in order to increase my flow of oil off idle, is it likely my idle operating temp oil pressure will drop dangerously below specifications?

 

[IJ.]

Only way to find out is to run it to operating temp and see with that Oil.

There are too many variables in a 7M that will affect volume/pressure such as how worn the Cam tunnels are and so on.

I prefer to find an Oil that gives me the required pressure at Maximum output and just accept whatever pressure I see at idle.

 

[Nick M]

Well Yes "flow is more important than pressure" but if i run an oil that is going to give me 0psi of pressure at idle is that better because there's less resistance to flow=more flow? Doesn't sound right to me. If that was the case why doesnt the manufacturer call for 0psi at idle?

What oil will give you zero pressure at idle? And who is suggesting zero psi at idle?

As long as there is pressure, doesn't that mean the bearings are coated?

As long as there is oil on the bearing, then yes, there is oil on the bearing. And yes, for your question, you don't want the oil too fast. Which is why there are pressure specs.

its not like the oil needs to carry away particulates or something like that

Then why do you think there is a filter? Rhetorical, just for you to think about for a second.

if i switch to a 0w30 in order to increase my flow of oil off idle, is it likely my idle operating temp oil pressure will drop dangerously below specifications?

If your engine is aged, the bearings on the crank open up. This will lower the resistance to oil flow, which makes the pressure on the gauge lower.

 

[CyFi6]

If your engine is aged, the bearings on the crank open up. This will lower the resistance to oil flow, which makes the pressure on the gauge lower.

I understand and agree with what you are saying here, but if my pressure on the gauge drops, do i not need to compensate for it? In example, if i switched to a 0w30, and my oil pressure at idle operating temp is 2psi because of the thinner oil used, isnt that a concern? Should i not compensate it back to the spec pressure using a different oil?

 

[suprarx7nut]

I understand and agree with what you are saying here, but if my pressure on the gauge drops, do i not need to compensate for it? In example, if i switched to a 0w30, and my oil pressure at idle operating temp is 2psi because of the thinner oil used, isnt that a concern? Should i not compensate it back to the spec pressure using a different oil?

Not with higher weight oil...


I'm no expert like the others, but here it goes:

You have a worn engine. Fact.

That engine will show lower pressure than a stock engine. Your bearings have more clearance(or space) between the rotating metal and the static metal (rod and bearing). This increased space will lower your oil pressure because there is less resistance to flow. Got it?

What will using a thicker oil do? It will flow more slowly/less freely and will raise your pressure. Will it coat the bearings better? That's a question for IJ or jdub, but I would guess no. :dunno:

 

[CyFi6]

That is sort of what im trying to figure out here. Its kind of interesting that my off idle pressure is slightly above normal but my idle pressure is below normal.

 

[Nick M]

I understand and agree with what you are saying here, but if my pressure on the gauge drops, do i not need to compensate for it? In example, if i switched to a 0w30, and my oil pressure at idle operating temp is 2psi because of the thinner oil used, isnt that a concern? Should i not compensate it back to the spec pressure using a different oil?

You aren't going to like this but... you should fix the engine. And not try and put a band aid where where surgery is needed.

A higher milage oil will have more big molecules than a regular mutlitgrade. That will do what you want. Slow down the oil a bit so the bearing stays coated.

It will still be a 10W or 5W, but have more of the larger strings.

 

[CyFi6]

well because of the fact that off idle pressure is perfectly in spec, i feel that the low idle pressure could be any number of things besides bearing wear.

 

[wiseco7mgt]

Im not understanding why you want to run a 0-30 oil in a aged engine.Is this what toyota recomended for the 7mgte?

 

[Doward]

CyFi - just stick with the 40 wt.

With a brand new, tightly built engine, yes, 30 wt would be the way to go. It's not like you're running 15w40 - I wouldn't worry in the least about your 0w40 oil