Why HTHS Spec?

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6
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South Florida, USA
My car is an Audi 1.8T and the appropriate oil to satisfy the lubrication needs of the turbo must meet VW502. A key requirement within VW502 is ACEA-A3. Hope you will find the following useful and comments are welcome: HTHS specification ensures sufficient lubricant film strength for engine parts where the lubricant (oil) may experience a shear rate of approximately 1 million reciprocal seconds (106 s-1). The HTHS specification value will vary with an engine design and its permissible safe operating envelope. The shear rate of an oil film between two surfaces is the relative velocity between the two surfaces divided by the distance between them, hence the unit “reciprocal second”. Some authorities on the impact of shear rate (velocity gradient) on oil viscosity believe that if there is a reduction in viscosity it will not be felt until the shear rate is approximately 106 s-1. An understanding the meaning of 106 s-1 may be garnered by realizing that a bearing 2 in. in diameter with a radial clearance of 0.001 in. and spinning at 9550 RPM will reach a velocity gradient (shear rate) of 106 s-1. ACEA-A3 spec stipulates HTHS viscosity > 3.5 at 150 deg. F. and 106 s-1. This will ensure adequate oil film strength for turbocharger bearings that in general spin at very high RPM. Oils that physically shear apart (due to say VI) will negatively impact their HTHS spec because of the reduction in base viscosity. In conclusion, the HTHS viscosity if called out in an OEM oil specification may be regarded as a function of the engine design and the selected oil should at a minimum satisfy that requirement.
 
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324
Location
California
Hi, I just was going to ask the question related to HTHS and relationship to viscosity. If nobody answers it here I'll post it in a separate thread. What affects HTHS number? We can see that M1 5W-30 has 3.08 and 10W-30 has 3.17 although viscosity for both oils is the same at 100C. On the other hand GC has 3.58 having a viscosity of 12.2 cSt @ 100C. Assuming that HTHS grows linearly relative to viscosity (I think this is not totally correct, but if you round up I guess it would almost be true) HTHS number for M1 10W-30 with viscosity of 12.2 cSt @ 100C would be 3.867 which is considerably higher compared to GC. Another example is Redline 5W-40 HTHS number is 4.6 viscosity at 100C is 15.1 cSt, 10W-40 - 4.7 with viscosity at 100C of 14.6cSt. Compare it to Mobil 1 0W-40: viscosity at 100C is 14.3, but HTHS number is only 3.6 – closer to GC rather than Redline. That seems kind of confusing. I understand that base oil, additives, and VI's play role, but I still wanted to learn more about HTHS relationship to viscosity and oil formulation in general. Another observation: would it really make any difference if person uses M1 10W-30 or GC in terms of HTHS and protection. The difference in HTHS number is only 0.41 I personally doubt this will make any difference in a daily driver, or even occasional race run. I don't think that if GC has many approvals for whole bunch cars, M1 10W-30 would provide less protection for the same recommendations (again in terms of HTHS number). What do you think? Regards,
 
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764
Location
Fairfield County, CT
M1 10W-30 does not carry the same approvals as GC. If the manufacturer calls for ACEA A3 spec, that's what I'd use. Ted made an interesting post here, it predicts shear stability using the ratio of kinematic to HT/HS viscosity. For most oils, the ratio falls somewhere between 3 and 4, a lower number being better. I think Redline 5W-30 is actually less than 3! I also like Ted's OCI formula... good stuff.
 

Jay

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1,607
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Idaho Falls, ID
Titaniam_Alloy, HTHS is viscosity. It's a different way of specifying viscosity. Kinematic viscosity is measured under the flow of gravity (a very low shear rate). It's expressed in Stokes. HTHS viscosity, CCS viscosity, and pumping viscosity are all dynamic viscosities expressed in Poise. HTHS is a very high shear rate test, CCS high shear rate, and pumping viscosity is a low shear rate dynamic viscosity test. You can convert kinematic viscosity to dynamic viscosity by multiplying it by density in grams/cc. The polymer molecules used to increase VI are kind of springy and compress slightly under pressure--hence the lower dynamic viscosity for the big VI oils.
 

Tapdancer

Thread starter
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6
Location
South Florida, USA
Titanium: As I have tried to explain in my post, HTHS viscosity rating reflects the oil viscosity at a shear rate of 1 million reciprocal seconds. So, if the shear rate of the oil film between two surfaces being lubricated is greater than 1 million reciprocal seconds, there will be further reduction in viscosity. The shear rate experienced by the oil film is a characteristic of the design, whether we are dealing with turbo bearings, cams, cylinder walls... any two surfaces that move relative to one another and have a lubricant between them. If their relative velocity divided by the distance between them is much less than 1 million reciprocal seconds, shear rate will not have a significant impact on the "instantaneous" viscosity. I do not think that the viscosity change due to this operating phenomenon is permanent. The viscosity change that may be permanent is the physical change in the oil when VI breaks down or when chemical actions such as oxidation takes place. Note: In my initial post "106 s-1" did not copy over correctly. It represents 10 to the 6 power, times s to the -1 power, i.e 1 million reciprocal seconds.
 
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9,448
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USA
I think the most important number to me is HTHS number. I want to know what viscosity I can expect at the bearing and cam lobes. The HTHS number can also give insight into how many VII's are being used. The viscosity printed on the front lable is very misleading!
 
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324
Location
California
Hi, So as I understand that if an oil company would make let's say 5-20 with HTHS number of 3 or above, it would be totally safe to use it in the engine calling for 5/10W-30? I see that you all guys say that HTHS number is more important than viscosity. Is this correct or I'm not getting something? Why then car makers like MB/BMW recommend oils A3/B3 from 0W-30 (HTHS usually around 3.5-3.8) to 10W-40 (HTHS usually around 4.5-4.7). This spread looks very big to me. Why then Mobil 1 can't make 5W-40 (with viscosity of around 14 and HTHS above 4) or 0W-30 (with viscosity around 12 and HTHS around 3.6, like GC) instead of their 0W-40. Seems like 0W-40 has 2 pitfalls: relatively high viscosity at 100C (i.e. higher fuel consumption), 14.4; and relatively low HTHS number for 40 weight (i.e. less protection), 3.6. Am I on the right track of thinking? Regards,
 
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23,591
I think there is direct link between viscosity and HT/HS. I doubt it's possible to make a very "thin" oil with a very high HT/HS. BMW may well have established that oil with an HT/HS of above 3.5 is safe for their engines. Sounds like an engine designed with the A3 spec in mind. I'm sure Mobil can make a 5W-40 with a higher HT/HS than their 0W-40 has. M1 D1 has a higher HT/HS than 3.6, and a new M1 5W-40 has been announced. We will see the specs on this particular oil soon, I hope. I don't think the 40 weight is a pitfall. I think this viscosity will offer better bearing and cam lobe protection. For all I can assume (A bad thing to do, I know!), a thicker oil may also prevent crank vibration (Is THAT what I hear in my engine with M1 0W-40???) and even piston slap. I'd also trade a couple percent in improved fuel economy (from 30mpg to 30.6mpg?) for less wear. If I had to choose a thin oil, I'd pick the one with the highest HT/HS in its category.
 
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204
Location
Cordova, TN
I have been a long time M1 5w10w-30 user but this is why I recently switched to Amsoil's ASL/ATM line that meets A3 and GF3 specs. It would seem Mobil could/should easily be able to up their M1 10w-30 HTHS rating of 3.2 to 3.5 with a little higher viscosity and/or ester content and have an A3 rated oil that still meets GF3 economy specs. I like M1's additive package better but like Amsoil's A3 rating and perhaps the use of a higher ester content more. Guess it is all about price points and margins and the average consumer not caring less.
 
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324
Location
California
quote:
Originally posted by Tapdancer: The shear rate of an oil film between two surfaces is the relative velocity between the two surfaces divided by the distance between them, hence the unit “reciprocal second”. Some authorities on the impact of shear rate (velocity gradient) on oil viscosity believe that if there is a reduction in viscosity it will not be felt until the shear rate is approximately 106 s-1. An understanding the meaning of 106 s-1 may be garnered by realizing that a bearing 2 in. in diameter with a radial clearance of 0.001 in. and spinning at 9550 RPM will reach a velocity gradient (shear rate) of 106 s-1.
Thank you guys for answering my questions. Sorry for being pain in the ~~~ for asking so many questions. But here are two more: Since bearing clearance has significant effect on shear rate, engine with a larger clearance would exhibit smaller shear rate, right? Can we trace this to an engine design and differences between North American and European manufacturers and their oil recommendations (i.e. are cars sold in US by domestic and Japanese makers have larger bearing clearances compared to European car makers?) If higher HTHS viscosity would provide better protection why a growing number of manufacturers, including European like Audi/ VW/Opel/Saab/Volvo, recommend more and more low viscosity oils with lowered HTHS number? Regards, [ January 17, 2004, 08:34 PM: Message edited by: Titanium_Alloy ]
 

Jay

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1,607
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Idaho Falls, ID
quote:
Originally posted by Titanium_Alloy: Hi, ... I see that you all guys say that HTHS number is more important than viscosity. Is this correct or I'm not getting something? ...Am I on the right track of thinking?
Again HTHS is viscosity. Two oils with a HTHS of 3.1 cP will have about the same friction in a running engine, and develop about the same oil film thickness regardless of SAE grade. HTHS viscosity is a more accurate way of specifying viscosity than kinematic viscosity for an engine. Kinematic viscosity is much more convenient to measure, though. A higher HTHS isn't "better" than a lower HTHS viscosity any more than 50-weight is "better" than 30-weight. It depends on what the engine requires. Mobil 1 formulates their oils with lower HTHS viscosity than most other oil companies because they want their customers to see better gas mileage with their oils.
 
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23,591
quote:
If higher HTHS viscosity would provide better protection why a growing number of manufacturers, including European like Audi/ VW/Opel/Saab/Volvo, recommend more and more low viscosity oils with lowered HTHS number?
I'll speak only for VW and Audi, because I am quite familiar with them (I used to have an Opel, too, once, actually): The latest Euro spec engines from Audi, VW (BMW and Merceds, too), are equipped for LongLife Service, which requires the vehicle to be equipped with a flexible service interval display. There are also engine modifictions like different oil passages, different oil pump, modified cams, etc. LongLife Service usually mandates the use of lower HTHS oil (not always the case!). I know that European manuals clearly state that oil with the higher HTHS can be used, as long as conventional service intervals are maintained. I am also aware that quite a few people refuse to use LongLife service approved oil, and they stick with higher HTHS oil (in VW/Audi's case VW 502 00 and VW 503.01). I guess lower HTHS oils are easier to tailor for extended drain? Also, not all new VW/Audi cars require low HTHS oil. On the contrary, S4 and RS4 require VW 503 01, which just like VW 502.00, requires a minimum HTHS of 3.5.
 
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324
Location
California
Jay Sorry for the confusion I meant kinematic viscosity at 100C. I should be more careful and clear next time [Embarrassed] [Off Topic!] I read on some website that Porsche requires KV for all of their cars to be at least 11 cSt @ 100C. Website also mentioned that they refuse to test any oils of XW-30 weight, only XW-40 and above. They only made one exception - GC. Really makes you think about quality of 0W-30 Castrol. I don't have any info that Porsche approved any more 30 weights. Regards,
 
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Airlie Beach Australia
Hi, Titanium_alloy - Porsche commenced endorsing 10w-30 oils ( SF/CD ) from at least 1988 and as stated in a later Technical Bulletin as follows In Technical Bulletin 9213 ( November 2/1992 )they endorsed mineral 10w-30 oils for from -25C to 20C Porsche then endorsed synthetic 5w-30 oils for from -30C to 40C+ ( and a 5w-40 synthetic for the same conditions ) They also endorsed synthetic 10w-30 oils for from -25C to 40C In MY1993 ( from August 1992 ) they commenced factory filling with 5w-40 synthetic oils - firstly with Shell and then and currently with M1 I cannot be sure but I believe they endorse or use a M1 0w-40 as well now Regards Sorry - see later post! Or Moderator please delete this - thanks! [ January 18, 2004, 01:21 AM: Message edited by: Doug Hillary ]
 
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204
Location
Cordova, TN
[/qb][/QUOTE] Mobil 1 formulates their oils with lower HTHS viscosity than most other oil companies because they want their customers to see better gas mileage with their oils. [/QB][/QUOTE] I have no doubt M1 could achieve a HTHS 3.5 target and meet GF3 fuel standards if they wanted to (i.e., thought consumers would pay the extra 15 cents per quart for the A3 rating and/or inherent higher quality oil). I also think GC could possibly meet GF3 if it's additive package contained moly and/or boron. While M1 10w-30 is a good synthetic oil for the money, it would be a killer if it met A3 and GF3 specs and, at that point, qualified to meet most European specs.
 
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5,112
Location
Airlie Beach Australia
Hi, Titanium_alloy - Porsche commenced endorsing 10w-30 oils ( SF/CD ) from at least 1988 and as stated in a later Technical Bulletin as follows In Technical Bulletin 9213 ( issued November 3/1992 )they endorsed mineral 10w-30 oils for from -25C to 20C Porsche then also endorsed synthetic 5w-30 oils for from -30C to 40C+ ( and a 5w-40 synthetic for the same conditions ) They also endorsed synthetic 10w-30 oils for from -25C to 40C in the same Bulletin In MY1993 ( from August 1992 ) they commenced factory filling with 5w-40 synthetic oils - firstly with Shell and then and currently with M1 I cannot be sure but I believe they endorse or use a M1 0w-40 as well now Of course their "Approved Oils" are required to meet ACEA A3/B3 specifications Regards Doug Hillary MY02 Subaru Outback 2.5 AWD ( Delvac 1 ) MY98 BMW Z3 2.8 ( Delvac 1 ) MY89 Porsche 928 S4 ( Delvac 1 )
 
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526
Location
Manitoba Canada
To further complicate the HTHS issue, the SAE J300 test clearly differentiates HTHS for a "light duty" (eg, Mobil 1 0W-40 rated ACEA B4-02) and a "heavy duty (eg, Mobil Delvac 1 5W-40 rated ACEA E3, E4, E5, CI, etc) oil. LD x-40: minimum 2.9 mPa s @ 150 C HD x-40: minimum 3.7 mPa s @ 150 C This is quite a difference in apparent viscosity. Otherwise, remember that the grade on the bottle is fairly meaningless, as it usually has little to do with the oil's true performance. This is why commercial engine makers have their own test approval method, such as Cummins CES 20078, and why in Europe VW has the endlessly confusing VW specs of 500.x, 502.x, 503.x, 505.x, and now 506.x We in North America have to place our faith in that worthless "starburst" certification from SAE. This is the worst possible application of "lowest common denominator." It should be common sense that it's all but impossible to develop one "universal" motor oil to satisfy all climates and all engines. For example, I do worry about HTHS, especially for things like turbochargers where one expects very high shearing forces. I also worry about MRV BPT at -42.
 
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9,448
Location
USA
heyjay, I agree with you that to date no universal oil yet!!! I do have high hope though for my current test of synthetic 5W40. I am preety sure that we are going to see a good showing with these. Currently I have Redline in. I am also going to try Delvac-1. In Michigan we get temps from -25F to 90F so we will see. I am hopeing that it proves to be a year round viscosity for me.
 
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