HTHS influence on oil pressure?

[FCD]

So say you have a 15w40 HDEO, with a HTHS of around 4.5.
And you have a 20W-50 PCMO, again with an HTHS of 4.5.
As i understand the main bearings are the place most influenced by higher or lower HTHS viscosities.
When cold i assume the oil pressure would be lower with the 15w40, but when hot would both oils give the same or a similar oil pressure?

 

[Shannow]

The mains, at high RPM are working in the shear rates that high shear viscometry is meaningful...took the industry a while to work out why multigrades gave inferior results to the same "traditional" grade (i.e. monograde where viscosity doesn't change with shear rate).

CATERHAM did some great work mapping oil pressure versus HTHS in his lotus replica, and bascially proved a relationship, and that HTHS versus oil pressure at RPM were related, not specifically the J300 "grade".

SAE papers did the reverse, measuring the flow rate at a constant supply pressure.

Your last sentence rings true as well.

CCS is a high shear rate viscosity, like a "LTHS" (Low Temperature High Shear)...CCS of a 20W is going to be higher than a 15W.

 

[zeng]

Pressure gauge is connected at low shear region of supply galleries before distribution to bearings, hence it's readings reflects LTLS or HTLS viscosities.
Hths viscosity has reduced influence on a pressure readings, I suppose.

 

[Shannow]

zeng,
it's ALWAYS high shear in the pressure/load zone of the bearing.

But on the feed side, and assuming that the gallery feed point is at the most open point...

A bearing with a c/D (clearance/Diameter) ratio of 0.001 (1 thousandths per inch of diemeter) has a shear magnitude of pi/0.001 per revolution (3,1415)...at around 350 RPM, this starts to get into the range of 10^6 shear rate of the HTHS.

Cams turn at half speed.

So at 4-5,000RPM, the entire bearing, and it's behaviours are governed by high shear rate viscosity.

So oil pressure at RPM (with the relief closed) correlates very well in a simple engine.

Pistons squirters and the like have their flow rate governed by oil pressure, and this tend to distort the reltionship a little bit.

but CATERHAM's relatively simple Astra based engine lacks a lot of the confusing elements, and his results were virtually linear.

 

[SonofJoe]

I'm guessing this is a hypothetical exercise because it would be nigh on impossible to have a 15W40 & 20W50 with equal HTHS's.

Thinking about this, you would need a 15W40 with a KV100 of say 16.25 cst and a 20W50 with a KV100 of 16.35 cst (16.3 being the 40/50 breakpoint). But doing this would mean the 15W40 would contain far more VII than a standard 15W40 and the 20W50 far less. I'm guessing this would mean that the HTHS of the 15W40 would now be far higher than that of the 20W50. One way to address this would be to drop the CCS-15 of the 20W50 to a level far lower than strictly needed to 'force' more VII into the oil. If you dropped the CCS-15 down to say 3505 cP (from the spec value of 9200 cP max) you would force in a lot of VII and it would still qualify as a 20W50 because the CCS-20, at 7010 cP, would be offgrade as a 15WXX oil. You now have a 15W40 oil and a 20W50 with almost equal HTHS's but you've only managed this by effectively making the oils equal!!

However, if I answer your hypothetical question, I'd say the 'normal' 20W50 would give a higher oil pressure than a 'normal' 15W40 even if they had equal HTHS's. This is because whilst differential pressure loss across the bearings is significant (and is HTHS dependant) there is also pressure loss through the rest of the system (galleries & top-end) which probably isn't so shear dependent. Also whilst it's true to say the oil is at its hottest as it exits the main bearings (so is at its thinnest), heat will get extracted from the oil by the water jacket downstream of the bearings thickening it up again.

Interesting question but I suspect no-one know the correct answer..

 

[FCD]

Ok, thanks for educating me on that.
Now my Capri consumes almost no oil so i'm considering lower viscosities than 20w50.

 

[slybunda]

so does that mean higher hths is needed for badly designed engines that have poor manufacturing resulting in loose bearing clearances and need the higher hths thicker oil to take up the slack?

 

[FCD]

My Capri specs 20W-50 as default for hot climates, this is because the oil galleries in the engine are huge and really with anythinh thinner than a 40 it doesn't have enough oil pressure on hot idle, the engine is not poorly designed they just made them like this, the main journals are also quite big for a V6, they are similar in size to a small block chevy.

 

[slybunda]

how big are oil galleries? like hose pipes?

 

[FCD]

Who knows, they might be

.

 

[LoneRanger]

And then there's the 540 RAT oil tests that contradict many traditional beliefs on viscosity.

 

[Shannow]

Originally Posted By: LoneRanger
And then there's the 540 RAT oil tests that contradict many traditional beliefs on viscosity.


He discounts viscosity because his testing machine is an additive test more akin to a differntial or bearing, not a test of "lubrication". I'm sure that his tests discount colour too, but that doesn't make the test universally relevent.

His conclusions that viscosity, and HTHS are irrelevent are particular to his test set-up only...marginally relevent to cam/lifter (which is where he aims), but misses 90+% of other engine components...bearings, rings, skirts, chains which his test has ABSOLUTELY nothing relevent to say...but he says it, and the schmucks believe him.

It's junk "science", not geometrically equivalent to the only part of an engine where it MIGHT be relevent. And his use of statistics (significant figures) demonstrates he simply doesn't understand that part of it either.

 

[Shannow]

Originally Posted By: slybunda
so does that mean higher hths is needed for badly designed engines that have poor manufacturing resulting in loose bearing clearances and need the higher hths thicker oil to take up the slack?


No, not really.

Design is an iterative process...you pick a design point (temperture and viscosity, having known the load), then design the bearing around a bunch of knowns...analyse (mathematically), and find out that the heat rise is more or less than you calculated, then iterate down to a design...then test.

Diameter, Clearance and Length, are your mechanical variables that you can change.

So to say that a particular design is "bad" when you don't know what the designer was designing for/around is incorrect.

Engines are total packages, and piston friction is a big part of overall power losses...thus the strange skirt shapes that we see. Honda in some of their papers appear to be focussing on viscosity to reduce piston skirt friction, and are having to make blocks stiffer, and bearings bigger and tighter to provide sufficient MOFT on these thinner oils (and more bearing friction/higher oil exit temperatures)...they are scraping the last few hundreths of an MPG out of engine friction. Now they are on the teeter totter of trading piston and bearing friction, they will have to look more at additives, and start to abandon hydrodynamics as the basic protection mechanism.

 

[FCD]

Some people do seem to think that 540 RAT's tests are the word of god himself.

 

[Shannow]

There's a little cadre who have it in their minds that the ILSAC oils, and those that followed are there for the benefit of engine longevity ("massive flow improvement"), and will grasp at anything that shows that viscosity and NOACK (one of their cult leaders is anti-NOACK) is irrelevent...RAT helps satiate their confirmational bias, even if neither them, nor RAT understand hydrodynamics...nor what he's ACTUALLY testing.

 

[Olas]

Originally Posted By: FordCapriDriver
Ok, thanks for educating me on that.
Now my Capri consumes almost no oil so i'm considering lower viscosities than 20w50.


Would a higher volume pump give you acceptable pressure at hot idle with a 40?

 

[FCD]

I imagine a 15w40 HDEO would give me good hot oil pressure with my current oil pump, i'm not touching the internals of the engine any time soon.

 

[MolaKule]

Originally Posted By: Shannow
There's a little cadre who have it in their minds that the ILSAC oils, and those that followed are there for the benefit of engine longevity ("massive flow improvement"), and will grasp at anything that shows that viscosity and NOACK (one of their cult leaders is anti-NOACK) is irrelevent...RAT helps satiate their confirmational bias, even if neither them, nor RAT understand hydrodynamics...nor what he's ACTUALLY testing.

 

[Shannow]

Bump to fix pics in light of current discussion

 

[Shannow]

Here's CATERHAM's data

[URed Line]http://www.itinerant-air-cooled.com/viewtopic.php?t=8721[/URed Line]

Quote:
All oil pressure readings were made at 6,500 rpm and oil temp' of 95C:
Mobil 1 5W-50 ..... KV100 17.5 cSt...HTHS 4.21 cP...OP 92 ...psi
Mobil 1 0W-40 ..... KV100 14.0 cSt...HTHS 3.7 .cP...OP 86 ...psi
Red Line 10w30 .... KV100 11.0 cSt...HTHS 3.8 .cP...OP 87 ...psi
Red Line 5w30 ..... KV100 10.6 cSt...HTHS 3.8 .cP...OP 87 ...psi
GC 0w30 ..... KV100 12.2 cSt...HTHS 3.5 .cP...OP 83-84 psi
Red Line 5W-20 ..... KV100 9.1 cSt...HTHS 3.3 .cP...OP 80 ...psi
Mobil 1 5w30 ..... KV100 11.3 cSt...HTHS 3.09 cP...OP 78 ...psi
Pennzoil Platinum 5w30 ..... KV100 10.3 cSt...HTHS 3.1 .cP...OP 78 ...psi
Red Line blend, 3qts 5W-20 and 1qt 0W-10 race oil *
...............KV100 8.2cSt est HTHS 2.85 cP...OP 74 ...psi
Toyota (Nippon Oil) 0W-20 virgin, less than 30 miles on oil
...............KV100 8.8 cSt...HTHS 2.6 .cP...OP 71 ...psi
Toyota 0W-20 used with 150 miles on oil **
...........est KV100 8.0cSt est HTHS 2.4 .cP...OP 65 ...psi


And here are some graphs I made of the above indicating a 99.4% accuracy in oil pressure predictabilty using HTHS viscosity, vs ~73% for kv alone.
There is some relationship in the latter, but only as far as HTHS tends to go up in oils of thicker weights.


(those charts are photobucket broke)...