oil pressure vs. heat

[jkcrewsn]

I have been contemplating this for a while, and my curiosity has overcome my fear of feeling like an idiot so I thought I would ask. What I am wondering about is this: when driving in traffic, the cooling effect from airflow is minimal at best. While puttering around in second or third gear at 30 - 40 mph, I notice the oil pressure is quite low. Dropping down a gear makes the pressure rise, but wouldn't that also make the temp. of the engine rise due to the greater rpm's? Where is the point of diminishing returns? Is a thousand rpm's or so worth the extra oil pressure? I guess part of the solution to this question is whether or not MORE oil gets pushed through with a higher pressure or not. I would assume more oil would go through the cooler though. I rarely get over 210 now that I have an external oil cooler and a replaced oil line that was picnched, so it's not a desperate issue, I was just curious if anyone has thought of this, or if I need a hobby.
Also, does anyone have problems with water spots even with a towel dry? It seems to be from hard water or something, anyone have a solution for this? Thanks for reading.

[Roland Kunz]

Hello

OK very simple.

The engine temperature is relatet to the load and not to the rev level.
OK higher rev level under same load generates more temperature but your fan also works more air trough for cooling.

However the oilpressurelevel is relatet to the rev level and to the oiltemperature.

If your engine gets overheatet then you will see the pressure droping and if you then select a higher gear at the same speed the engine cools better

Grüsse

[CamB]

The specification is about 60psi (4 bar) at 4000 rpm, if I remember right. If you have this and you don't see much over 210 deg you should be ok.

I have a different question. How come different engines run at different oil pressures. I have a Jag XJ6 which (barely) makes its spec oil pressure of 45 psi when hot.

I have a 911 which does 60psi at 4k hot, a 911 which does AT LEAST that (the '75) and the BMW.

The BMW never really drops BELOW 40psi. It charges up to nearly 80psi under load, even when being raced and getting pretty warm.

So how come the variance? Why isn't it one size fits all?

[juanbenae]

i had the same question but in more laymens terms. so if i read this right (my spelling has been questioned on this board, but i read at a 6th grade level so i understand most things i read here) it would be better in hot conditions to use the lower gear at a higher rpm than to use higher gear at lower rpm? i figured that was the case,, more R's greater rate of oil circulation, more fan action moving more air, cooler motor.

please advise, & thank you

[Carrera Charlie]

Regarding water spots ...

You're most likely right that they're from hard water deposits. I've had great success removing them with a very mild paint cleaning clay like Griot's (http://www.griotsgarage.com/catalog.jsp?&SKU=11153). You have to make sure you apply a good wax afterwards, or you'll get spots again quickly.

Regards,

[Roland Kunz]

Hello

I have a different question. How come different engines run at different oil pressures. I have a Jag XJ6 which (barely) makes its spec oil pressure of 45 psi when hot.

XK engines have a complex oil system and many places to lose pressure.

Maybe Jaglovers is a place to find more.

Also oil pressures arn´t absolute numbers and not comperable between different engines.

Each manufactorer has specbooks showing the pressure ranges and most engines will run with even half the pressure.

@k911sc

OK there isn´t a direct line but on most engines they work best in a medium field.
If you rev to high you have the problem from the additional from the revlevel generatet heat. This is normaly a lower generatet heat then the benefit from the better cooling by faster cool medium cycling.

If you optimize a engine for best efficient cooling on low rev then those engines will break in at high revs or need additional aids.

Most heat is generatet under full WOT load at low revs and low speeds.
Those conditions apply mostly on towing or on very step hill driving.

Grüsse