Hot Engine Oil Pressures

[scam75]

Hi All

After my recent engine rebuild I have been closely monitoring oil pressures. I just wanted to see how these hot engine oil pressures compared to others.

800 rpm - 2 bar (idle for my car)
1,100 rpm - 3 bar
1,800 rpm - 4 bar

I don't recall previously ever seeing under 3 bar at hot idle before my engine rebuild, however I've got a new sender and this one shows 0 on the gauge with ignition on/engine off. The old sender showed nearly 1 bar. I guess I'm maybe seeing actual pressure now rather than slightly inflated and I just need to get used to it!

Cheers

Stuart

 

[Hilux]

To be honest I wouldnt swear by OEM electrical gauges.

2bar at idle is 30 psi - plenty - the big ends dont need huge pressure - thats for the top end (cams etc) and most oil warning lights go on around 5-10psi.

What is important is that the pressure is engine related and varies with revs and importantly you get a good reading at full chat which it seems you will from those figures as oil is used not only to lubricate but its main/secondary role is to take heat away from rotating components (`swhy the engine cooling system water helps reduce the oil temperature via an exchanger) so it is actually volume that counts and the required pressure gives that throughput. Low pressure means low volume hence rotating parts then cook the oil and heat up and fail.

If you want a proper reading use a decent gauge such as Stack and a true reading is obtained using capillary gauges (in my view) but bearing in mind the above - if you had no gauge you wouldnt worry about it.

 

[zcacogp]

Oil pressures on a turbo will be higher than those on an S/S2 as you have half the number of valves to feed. You also need to specify how 'Hot' things are; I can see idle oil pressures of 4.5bar on my S2 when the temperature gauge is in the 'hot' position, but have equally seen pressures of 0.5bar after a very spirited drive on a stinking hot summers day and then sitting in miserable traffic. And don't forget that your choice of oil grade will have a bearing on it all as well ...

I recall someone on here saying that they were advised not to fit an oil pressure gauge to a car of theirs in their youth as it would just give them something else to worry about, and that the advice proved completely accurate! My outlook on an uncalibrated oil pressure gauge is to watch the trend carefully; if you know your car usually idles at those figures then only get concerned if they change (either up or down) for no explicable reason. Otherwise, don't worry about them.


Oli.

 

[scam75]

Thanks guys. Hot idle referred to just parking up after a reasonable blast, I was seeing about 4.5 bar at idle at operating temp before I stretched its legs. 2 bar seems to be the lowest it goes immediately after a good blast.

All would seem well then given the correlation to pressure/revs. As I said I'm sure my new sender is making the gauge read about a bar less than it used too so in my eyes and mind that is a change I can see!

I will stop worrying!

Stuart

 

[zcacogp]

ORIGINAL: scam75

I will stop worrying!

Always sound advice.


Oli.

 

[John Sims]

ORIGINAL: Hilux

...... if you had no gauge you wouldnt worry about it.

Indeed. The easiest way to cure low oil pressure is to put tape over the face of the gauge.

 

[zcacogp]

ORIGINAL: John Sims
The easiest way to cure low oil pressure is to put tape over the face of the gauge.

But DON'T do this with the low oil pressure warning light, as my sister did on a Golf she owned once. I got into it to move it, and noticed a bit of blu-tak in the middle of the dash, and when I removed it discovered what it was covering, and that the light was on! []

Seem to remember refusing to re-build the engine for her for some reason. I must have been in a strop with her. Can't remember why.


Oli.

 

[TTM]

ORIGINAL: zcacogp
Oil pressures on a turbo will be higher than those on an S/S2 as you have half the number of valves to feed.

As we are both members of the revered, praised and feared (and probably laughed at) Guild of S2 fellowship I will pray the S2 gods so that they don't cast a spell on you for making such an erroneous comment!

The pressure as read from the dash should be the same for all engines.
If pressure is too low, either main bearings are worn out or the oil pressure regulator in the cylinder head is stuck open and lets too much oil in the head.

 

[zcacogp]

ORIGINAL: TTM

ORIGINAL: zcacogp
Oil pressures on a turbo will be higher than those on an S/S2 as you have half the number of valves to feed.

As we are both members of the revered, praised and feared (and probably laughed at) Guild of S2 fellowship I will pray the S2 gods so that they don't cast a spell on you for making such an erroneous comment!

The pressure as read from the dash should be the same for all engines.

Really? I stand by the theory of what I said; the more devices that need to be fed, the lower the oil pressure. Is there some system that regulates the oil pressure regardless (i.e. bleeds excess oil pressure off?) If this is the case then surely the pressure will stay at whatever the 'limit' pressure is regardless of the temperature?

Or am I being thick? (I look to fellow S2Guild members to put me straight if I am!)


Oli.

 

[edh]

I am amazed at the oil pressure in my 230k mile S2 - admittedly with 10w60 oil, but still

usually 4 bar on hot idle, dropping towards 3 after track use.

My turbo ran much lower than that.

I'm planning to go for 10w40 next time & see how that performs

 

[TTM]

ORIGINAL: zcacogp
Really? I stand by the theory of what I said; the more devices that need to be fed, the lower the oil pressure. Is there some system that regulates the oil pressure regardless (i.e. bleeds excess oil pressure off?) If this is the case then surely the pressure will stay at whatever the 'limit' pressure is regardless of the temperature?

There is an oil regulator in the cylinder head that lets the "right" amount of oil flow through the gallerys in the head (16V) or cambox (8V). This regulator is different between the 16V and the 8V, and though it may account for a difference of flow required between both heads, there is no difference in terms of oil pressure in the blocks - the same oil regulator is used and this + the one in the head make for the gauge reading.

16 lifters may draw more oil flow than 8, but the pressure will remain the same.

 

[Copperman05]

I don't see how the amount of valves would affect oil pressure and even if it did Porsche would most likely design around it and make it equal. My gauge shows 4 bar on hot idlle maybe dropping to 3.5 after track/hard use, I'm running Redline 5w 40.

Edd

 

[zcacogp]

ORIGINAL: TTM

There is an oil regulator in the cylinder head that lets the "right" amount of oil flow through the gallerys in the head (16V) or cambox (8V). This regulator is different between the 16V and the 8V, and though it may account for a difference of flow required between both heads, there is no difference in terms of oil pressure in the blocks - the same oil regulator is used and this + the one in the head make for the gauge reading.

16 lifters may draw more oil flow than 8, but the pressure will remain the same.

Sounds like a more complex system than those which I played with on 8v and 16v golfs. For a given pump delivery there will be a relationship between pressure and flow, and (by definition) 16 lifters will take twice as much flow as 8, so you'd expect the pressure to be lower - which explains why the idle pressure on an 8v 1.8 Golf GTi is about 3.9 bar and the idle pressure on a 16v is about 2.5 bar.

I guess the answer must lie in the oil regulators you mention. The golf engines had them, along with oil pressure relief valves in the oil filters (if you bought good quality ones) but there wasn't a regulator in the head. This may well explain the difference.

Edd, imagine you are standing under the shower when someone turns eight taps on in your kitchen. The pressure in the water system will drop, as will the flow. That's the effect of having an extra eight valve lifters in your cylinder head.


Oli.

 

[TTM]

Also don't forget lifters for the 16V heads are smaller than those for 8V heads. We can't just suppose that the oil pressure is halved.

 

[zcacogp]

No, and I suspect the oil pressure used by the lifters is related to the area of the lifters rather than the diameter or simply the number of them.

And the engine has a large number of other devices that use oil, which will be common to the 8v and 16v engines. (Same number of cylinders, big ends, little ends, main bearings and so on), so the pressure relationship will be more complex again. And the turbo engines have a turbo which will need feeding with oil too ... and so the list goes on.

I stand by my basic argument tho'.


Oli.

 

[TTM]

Where is oil pressure measured on a Golf?

 

[zcacogp]

TTM,

From memory, it was just downstream of the oil filter (which sat out on a small pylon at the front of the engine.)

Your description of the 944 arrangement with oil pressure relief valves in the block and head suggest that the measurement is quite a lot more complex. Once you consider flows to different areas, and pressure loss due to frictional drag in the different flows, any simple relationship has to break down. And fluid dynamics were never my thing (weaknesses with fluid dynamics and thermodynamics were what made me fail a Mech Eng degree about 15 years ago!)


Oli.

 

[Jonny944CS]

quote: edh
I am amazed at the oil pressure in my 230k mile S2 - admittedly with 10w60 oil, but still

usually 4 bar on hot idle, dropping towards 3 after track use.

My turbo ran much lower than that.
I'm planning to go for 10w40 next time & see how that performs

Just noted that a typical 10w40 oil has around a 60% lower viscosity than the Mobil1 10w60 between 40 and 100°C.

I wonder what the power penalty is?

 

[edh]

not just power loss, but it could also generate more heat.

I notice that you can buy "qualifying" oil - 0w20 or similar, so there may well be a measurable difference.

Dry sumping releases a fair amount of power as well doesn't it?

 

[scam75]

So what oil stays thickest at hotter temps? I have a noisy lifter that is quiet at start up and only gets noisy at hot idle after a decent run about.

Currently running 15/40 semi I think whilst running in engine. I am going to use a fully synth from next change.

Cheers

Stuart