Understanding HTHS

[Danh]

High Temperature High Shear viscosity is a (the most?) important specification for each oil viscosity. But I’m not sure how it relates to everyday engine protection.

Using 20 weight oil as an example, my understanding is:

* 100C viscosity should be in the range of 6.9-9.2 cSt
* HTHS must be at least 2.6 cP @ 150C
* cP and cSt are related, but cST is not density-adjusted and is about 15% higher than cP, making 2.6 cP roughly 3.0cSt

I realize oils in passenger cars are unlikely to reach 150C except for very short period around piston rings, but this spec is there for a reason. Does this mean that an engine spec’d for 20 weigh oil would happily survive with a sump full of 3.0 cSt oil? Or is it only a spec relevant to very brief exposures to very high temperature conditions in very limited areas where lubrication needs are different?

For those of us that panic when a UOA where our 20-weight oil’s viscosity has been sheared or diluted out-of-grade to a 6.0 cSt, this seems an important spec to understand.

Thanks for comments.

 

[Artem]

I mean... even the very best, Fomula 1 racing spec oil will stumble in a poorly designed engine that's cursed with fuel dilution problems since it's conception. THAT unfortunately is the result of either poor engineering OR, even a sheered and fuel diluted oil can still keep that engine running for 200k and the auto manufacturers simply don't care any further then that.

So basically us stressing over oil MIGHT just be wasted effort but I've yet to confirm this for a fact.

I say we blend one ULTRA Synthetic formula and have all cars on it. Just a blank bottle with nothing but Engine OIL on it. Hahaha

 

[Dallas69]

If you are worried about shear on a 20 wt,just go up to a 30 and sleep well.

 

[SonofJoe]

I'm sure Shannow will chip in on this. He knows far more about this than me. However, the way I see it is this...

Under sensible driving conditions, your maximum bulk oil temperature should be around 100°C-ish because thermodynamically, your oil temperature 'follows' your coolant temperature. This is quite a way away from 150°C. However translation of frictional energy into heat causes the temperature of the oil to rise as it traverses the main bearings. So momentarily, your oil conceivably hits 100°C+, causing it's viscocity to drop and DIRECTIONALLY (but not necessarily ABSOLUTELY) increase bearing wear.

What '+' translates to in °C probably depends on your engine's RPM, with + being a few degrees when you're tootling along at 2500 rpm and a lot when you're redlining it. The HTHS spec temperature is probably set to reflect a worst case scenario.

I sort of recall reading some Toyota paper which basically says you really don't hit really serious bearing wear issues until HTHS drops below 1.6 cP. As such, all modern oils, including the thin ones are 'safe'.

Yes fuel dilution is bad but it's important to not necessarily see it as the Kiss Of Death for your engine. Fuel dilution tends to be associated with very cold weather conditions when your oil will generally be slow to warm up. If a heavily fuel diluted oil never gets above 80°C, then even at 80°C+, the oil will be relatively thick and not prone to excessive wear.

 

[OVERKILL]

Originally Posted By: SonofJoe

What '+' translates to in °C probably depends on your engine's RPM, with + being a few degrees when you're tootling along at 2500 rpm and a lot when you're redlining it. The HTHS spec temperature is probably set to reflect a worst case scenario.


A big factor here is whether the oil temperature is controlled via a coolant/oil heat exchanger like is the case on many modern vehicles or not. Oil/air exchangers are also out there, though not as common. These are usually thermostatically controlled.

Other factors are sump volume, oil pan material and exposure to air.

Shannow did an experiment with a thermocouple down the dipstick tube measuring the impact of RPM on oil temperature. There was a significant difference at the same road speed between being in top gear tooling along at low RPM and being in a lower gear with the engine revving much higher, despite overall load being basically identical.

 

[BrocLuno]

So what is the actual question?

Much of the requirement for any given engine is based on the bearings clearances at the time. Rods and mains at 0.0025" are happy with one viscosity. Rods and mains at 0.0045" are somewhat different and that's where you are at around 100,000 miles or so, give or take.

Rods and mains at 0.006" are at end of useful life and they need a much more robust film. The engine will have a few more mile left in, but you need to dose accordingly ...

 

[Danh]

Originally Posted By: BrocLuno
So what is the actual question?

Much of the requirement for any given engine is based on the bearings clearances at the time. Rods and mains at 0.0025" are happy with one viscosity. Rods and mains at 0.0045" are somewhat different and that's where you are at around 100,000 miles or so, give or take.

Rods and mains at 0.006" are at end of useful life and they need a much more robust film. The engine will have a few more mile left in, but you need to dose accordingly ...


The question is what is the significance of HTHS? And if, in an engine specified for 20-weight oil, does the corresponding HTHS of 2.6 cP (or ~ 3.0 cSt) mean the engine will safely operate at that viscosity?

 

[Gokhan]

HTHSV is the more relevant viscosity as far as the bearings are concerned. It not only corrects for temperature but also for the temporary shear of VII in tight spaces such as bearings.

For wear in the valvetrain, timing chain, and piston rings, base-oil viscosity is most important. For that usually higher the x in xW-y is better.

KV100 -- y in xW-y -- is the least important viscosity. KV40 dictates your warm-up fuel economy.

 

[1JZ_E46]

The way I see it, KV100 dictates how well oil flows through oil galleys on a warmed up engine and HTHS dictates the actual load carrying ability in the bearings, piston rings/walls, and timing chain (holding engine design equal). Two oils can have the same KV100 but very different HTHS figures. The one with the higher HTHS would imply a thicker (higher quality) base oil requiring less VII (that will temperarilly shear under high stress).

 

[Shannow]

When oils started being graded, when VI was starting to be used, the only oils that were in use was oil that came out of the ground, and was refined.

These oils displayed "Newtonian behaviour", didn't matter how hard you worked them, they were the same viscosity all the way through.

By "worked", I mean shear...for example, two plates (or a bearing surface) travelling at 1m per second, and 1mm apart would have a shear rate of 1000/s.

Then we developed the technologies to change the behaviours of the oil with viscosity index improvers, and by observation, the new grades did not reflect their grade in their wear behaviour...so they started looking at why.

What was found that at very high shear rates, the viscosity modifier additives started to "flatten" out and the apparent viscosity if the oil dropped...in the above plate example, 1mm separation, the rate is like 1km/second. the shear rate in this case is 1,000,000/s

Given that the shear rate is the surface speed/the gap, there's places (like the loaded side of the bearing) where the oil film thickness (gap) is so small that the loaded part of the bearing is in this high shear regime.

That's the "HS" part of the equation.

Re the 150C, that's the sort of temperature that is seen in the big end bearings, even IF the sump is tooling along at 100C.

Most of the heat that the oil deals with is related to the work that the engine does on it through shearing (note them mechanical "force", not breaking it up)...bearings have a big temperature rise. Pistons and rings do too, but the oil there is on the cylinder walls and the heat goes straight to the coolant.

So High temperature - think big ends. High Shear - think big ends.

That's why it came into being.

 

[Shannow]

Now for the fingerpainting...my favourite bit.


One of the early studies where they tapped into the main bearing supply of an engine that they could run on a stand, and provide oil of different viscosities to it. They supplied the oil at constant pressure, and measured how much the bearing drew from that...see how the things changed at 2,000RPM ?

That's when the bearings entered the high shear regime and the polymers flattened out.


Here's measured temperatures on an engine at two different RPM points. Note that the heat flow is INTO the block from the mains.

 

[1JZ_E46]

Thanks Shannow. It’s too bad HTHS hasn’t been better adopted into the SAE grading system (or even better, replaced the KV100 metric all together). At the least, it should be a required disclosure.

 

[Shannow]

Originally Posted By: 1JZ_E46
Thanks Shannow. It’s too bad HTHS hasn’t been better adopted into the SAE grading system (or even better, replaced the KV100 metric all together). At the least, it should be a required disclosure.


Agreed...it's the KV100 with HTHS "minimums" that drive silly behaviours like TGMO and M1 Racing 0W50...

 

[Shannow]

Originally Posted By: OVERKILL
Shannow did an experiment with a thermocouple down the dipstick tube measuring the impact of RPM on oil temperature. There was a significant difference at the same road speed between being in top gear tooling along at low RPM and being in a lower gear with the engine revving much higher, despite overall load being basically identical.


100km/h on my commute in the L67 Caprice.

In "D" (~1,800RPM IITC) stopping the engine, and pulling over, dropping a type K thermocouple down the dipstick hole (right where the drain back is hitting), was 95C ish.

In "2", 3,800RPM, same would be 129-135C

So same road load and aero drag, just more RPM...lots more heat. No idea what the rods were in that event.

 

[robertcope]

Soooo... I'm probably doing the smart thing running Red Line 10W-30 (3.5 HTHS) in my S2K (9K RPM) and NSX (8500 RPM)? Technically they don't require anything special per Honda, ie 10W-30 API SG Energy Conserving.

Is there ever any harm in running a higher HTHS than required, other than fuel economy?

 

[1JZ_E46]

Originally Posted By: robertcope
Soooo... I'm probably doing the smart thing running Red Line 10W-30 (3.5 HTHS) in my S2K (9K RPM) and NSX (8500 RPM)? Technically they don't require anything special per Honda, ie 10W-30 API SG Energy Conserving.

Is there ever any harm in running a higher HTHS than required, other than fuel economy?


Perhaps more startup wear (oil is slower at getting to bearings, etc.) and in some engines (very few) it can affect variable cam timing solenoids. More heat as well due to higher friction.

 

[Patman]

Originally Posted By: 1JZ_E46
The way I see it, KV100 dictates how well oil flows through oil galleys on a warmed up engine and HTHS dictates the actual load carrying ability in the bearings, piston rings/walls, and timing chain (holding engine design equal). Two oils can have the same KV100 but very different HTHS figures. The one with the higher HTHS would imply a thicker (higher quality) base oil requiring less VII (that will temperarilly shear under high stress).


So that's why when we compare Mobil 1 5w30 ESP and Mobil 1 0w40 ESP, the 5w30 version actually has a higher HTHS (3.58 vs 3.53) even though it's viscosity at 100C is lower (12.1 vs 12.9) It's good to know it uses a better base oil and less VII, as it should retain it's viscosity better.

 

[Shannow]

Originally Posted By: Danh
Using 20 weight oil as an example, my understanding is:

* 100C viscosity should be in the range of 6.9-9.2 cSt
* HTHS must be at least 2.6 cP @ 150C
* cP and cSt are related, but cST is not density-adjusted and is about 15% higher than cP, making 2.6 cP roughly 3.0cSt



https://www.bobistheoilguy.com/forums/ubbthreads.php/topics/3742278/Cp_and_Cst_-_the_confusion_bet
Originally Posted By: Shannow
Has been some confusion on the board, particularly when discussing the "Harman Index" as to Centipoise viscosity and Cst Viscosity.

Look at a J300 table (old now, anyone got a link to the new one with the 12s and 8s in it ???)


Cold cranking and HTHS are in Cp, while KV40 and Kv100 ... the former are called the "dynamic" viscosity, and the latter "kinematic". The dynamic, is the KV times the density at the test point.

What's the difference ?

Because a lot of SI units are based around fundamental properties of water, for a fluid of SG=1, they are the same...e.g. Engineering Toolbox conversion table but only when density is 1.

So if you are wanting to have cst equivalence of the CP viscosities, you have to divide the Cp by the density of the fluid at that temperature...To convert the cst to Cp, multiply by the density.

So to explain what was going on This thread

Originally Posted By: Shannow
Basically, use the KV40 and KV100 into the widman operational viscosity calculator, and calculate for 150C...multiply by density, and 0.885.


The 0.885 is the density corrector for 15C to 150C.


http://www.bobistheoilguy.com/forums/ubbthreads.php/topics/3626023/Re:_High_VI_and_Base_Stocks#Post3626023

Originally Posted By: Shannow
I've been playing a lot with A_Harman's technique for comparing the high shear and theoretical newtonian behaviours of lubricants.

Basically, use the KV40 and KV100 into the widman operational viscosity calculator, and calculate for 150C...multiply by density, and 0.885.

That will be the "KV150". Divide the HTHS by teh KV150 to get the "stability" as a ratio.

In a Newtonian (non VIIed) fluid the KV150 and HTHS should be the same number, so the ratio should be 1.

Some examples
Edge 25W50, KV40 204, KV100 21, density 0.8883, HTHS 6.1, Harman Index 1.06.
Amsoil ACD, KV40 69.0, KV100 10.6, density 8.539, HTHS 3.4, Harman Index 1.02.

So both of those oils are probably "straight" grades, with little to no VII added...the latter must have no VII for Amsoil to be able to rate it as a mono.

TGMO, KV40 36.1, KV100 10.6, density 0.851, HTHS 2.6 (for a little while), Harman Index 0.844.

Indicates lots of propping up through VIIs.

Couple of others
M1 0W40, KV40 75, KV100 13.5, density 0.85, HTHS 3.8, Harman Index0.874.
Magnatec 5W30, KV40 66, KV100 10.9, density 0.8595, HTHS 3.2, Harman Index 0.915

My new and current favourite oil
Edge 5W30 A3/B4, KV40 71.8, KV100 12, Density 0.8533, HTHS 3.6, Harman Index 0.946

(Redline 5W30 calculates out at 0.917, but they only list one density for all grades, so bad numbers in makes it not worth the effort)

 

[Gokhan]

Originally Posted By: Patman
Originally Posted By: 1JZ_E46
The way I see it, KV100 dictates how well oil flows through oil galleys on a warmed up engine and HTHS dictates the actual load carrying ability in the bearings, piston rings/walls, and timing chain (holding engine design equal). Two oils can have the same KV100 but very different HTHS figures. The one with the higher HTHS would imply a thicker (higher quality) base oil requiring less VII (that will temperarilly shear under high stress).

So that's why when we compare Mobil 1 5w30 ESP and Mobil 1 0w40 ESP, the 5w30 version actually has a higher HTHS (3.58 vs 3.53) even though it's viscosity at 100C is lower (12.1 vs 12.9) It's good to know it uses a better base oil and less VII, as it should retain it's viscosity better.

Well... 3.53 vs. 3.58 cP is well within measurement and sample error.

There won't be any difference in bearing protection (HTHSV-dictated) but most other parts will be better protected by the thicker 5W base oil than the the thinner 0W base oil.

 

[Gokhan]

Originally Posted By: 1JZ_E46
The way I see it, KV100 dictates how well oil flows through oil galleys on a warmed up engine and HTHS dictates the actual load carrying ability in the bearings, piston rings/walls, and timing chain (holding engine design equal). Two oils can have the same KV100 but very different HTHS figures. The one with the higher HTHS would imply a thicker (higher quality) base oil requiring less VII (that will temperarilly shear under high stress).

As Shannow pointed out and CATERHAM empirically demonstrated, oil pressure (and flow) is also dictated by HTHSV, not KV100, as the flow through the bearing clearances is dictated by HTHSV, not KV100.