High Temp/High Shear Viscosity and Fuel Efficiency Under Steady State Conditions

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All things being equal, the best predictor of engine oil fuel efficiency is HT/HS viscosity. This is the viscosity of the oil in the main/rod bearings and in the valvetrain; under high shear rate conditions. For every 10% reduction in HT/HS viscosity, you can expect a fuel savings of approx 1.0%, based on the testing I've done and a number of SAE technical papers I've reviewed. For example, if you run a 5w-20 oil with a HT/HS of 2.8 Cp, you can expect a fuel savings of about 1.0%; compared to a 3.1 Cp, 5w-30 oil and 1.5%; compared to a 3.1 Cp, 10w-30 oil. The improved fuel efficiency of the 5w-30 over the 10w-30 is from reduced viscous drag during the engine warmup phase....I have run 2.9-3.0 Cp, 5w-20 and 0w-20 synthetics and seen measurable fuel savings with both, compared to the 3.5 Cp, Amsoil 5w-30 synthetic. The reason why fuel efficiency with low quality oils actually INCREASES with use is that they suffer significant viscosity losses due to polymer shear ....This trick is also used to help these formulations pass the Sequence IV, fuel efficiency test that is part of the API, "SL" specification. It's actually very hard to pass this test with a shear stable, ACEA "A3/B4" quality synthetic .... Of course the potential drawback of the low vis approach is that bearing wear will increase dramatically, if the HT/HS viscosity falls below the threshold value needed to maintain hydrodynamic lubrication in the bearings. So you pays your money and you takes your chances .... Tooslick Dixie Synthetics
 
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I would just like to know what happens when you have all this 'bearing wear' I keep hearing. Does consumption increase, power decrease or what? Is it different from piston wear or is this also occuring? And what is the remedy?
 
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quote:
Of course the potential drawback of the low vis approach is that bearing wear will increase dramatically, if the HT/HS viscosity falls below the threshold value needed to maintain hydrodynamic lubrication in the bearings.
Anybody got a feel for what that minimum HT/HS number is? Are there any labs out there that do this test? EDIT: We sure are full of questions on this one, aren't we. [ February 18, 2004, 11:29 AM: Message edited by: doyall ]
 
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quote:
Originally posted by doyall: Anybody got a feel for what that minimum HT/HS number is? Are there any labs out there that do this test?
The results of such a test were posted here awhile back. If I remember correctly, bearing wear increased significantly once the HT/HS dropped below 2.6.
 
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quote:
If I remember correctly, bearing wear increased significantly once the HT/HS dropped below 2.6.
I suppose that means that Pennzoil 5W-20 with a HT/HS of 2.65 (per the Typical Physical and Chemical Properties sheet dated May 2002) wouldn't be a good choice if I were to use a 5W-20.
 
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quote:
Originally posted by doyall:
quote:
If I remember correctly, bearing wear increased significantly once the HT/HS dropped below 2.6.
I suppose that means that Pennzoil 5W-20 with a HT/HS of 2.65 (per the Typical Physical and Chemical Properties sheet dated May 2002) wouldn't be a good choice if I were to use a 5W-20.

Not necessarily. SAE J-300 specifies that a Xw20 oil must have a HT/HS of >2.6. The test I was referring to showed that oils that had an inherent HT/HS of less than 2.6 showed abnormal bearing wear. To paraphrase Molakule's comments about this test, he said the results showed that any modern engine will show acceptable wear levels under normal conditions running any 5w20, 5w30, 10w30, or 10w40 oil.
 
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I think any oil labeled "Energy Conserving" will have a lower HT/HS; likewise with the starburst symbol. Most high mileage oils are not energy conserving and don't have the starburst; likewise for most 10w40 and higher weights. Valvoline told me the 10w40 Maxlife I am using has a minimum HT/HS of 3.5 Cp. I notice that Exxon Superflow 10w40 is even better at 3.7 Cp. [ February 18, 2004, 01:15 PM: Message edited by: TallPaul ]
 
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Tn
HT/HS, I think is very important. My Syn doesn't rate all that well, but it is very shear stable, so it's a meaningful number. Some of the 10-40 dinos go out of grade so quickly that it's not a meaningful number.
 
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So, would a lower HTHS with a shear stabile oil still be considered a way to go? Or would a higher HTHS be better if your using ONLY dino or a shear unstabile oil?
 
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quote:
Originally posted by haley10: HT/HS, I think is very important. My Syn doesn't rate all that well, but it is very shear stable, so it's a meaningful number. Some of the 10-40 dinos go out of grade so quickly that it's not a meaningful number.
Would be interesting to compare HTHS of 10w40s and 15w40s. I believe 10w40s generally have a thicker base oil than 10w30s, perhaps close to 15w's in some cases, and with the better polymer technology, I am not sure the 10w40s are all that bad. If the viscosity goes out of grade, I should see it on my oil pressure gauge. I get 45 with 10w40, but only 36 with 5w30. If pressure is sustained throughout the OCI, then I am probably OK. I am 1800 miles into my Maxlife 10w40 and no drop whatsoever, but that is not many miles. We'll see.
 
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TallPaul, As an example the HT/HS of Castrol 10w-40 is 3.9 , the Mobil Delvac 1300S is 4.4 . Use TooSlicks percentages and weigh out the cost of fuel difference between the Delvac 1300S and a nice 10.5 cSt oil over the course of a year/15k miles and thats at running temp . The cost will even be more when one factors in warm up time and or stop n go driving . Oil cost annualy is so very little compared to fuel .
 

TooSlick

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5,785
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The idea is not to go too thick in these situations .... A friction modified, 3.5 Cp gas engine oil will provide significantly better fuel efficiency than a 4.5 Cp, diesel oil that isn't friction modified. I'd say the difference would be about 2.5%-3.5% if I had to guess. Roughly 0.5% in going from a 5wt to a 10wt; 1% going from a 30wt to a 40wt and 1%-2% percent for the friction modifier. For a newer gas engine, a shear stable, friction modified oil in the 3.5-4.0 centipoise range is probably ideal ....in other words, just thick enough to meet the ACEA, A3/B4 specs. TS
 
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quote:
HT/HS, I think is very important. My Syn doesn't rate all that well, but it is very shear stable, so it's a meaningful number
I agree with this. It's obviously better to have a higher HT/HS, but it doesn't mean everything. As we have seen many oils show great numbers even at or below 3.0. If the oil is formulated well, a 2.6 M1 or Pennzoil 20wt does very well. So as usual, spec sheets alone don't tell all. Now for extended drains, the A1/A5/A3 ratings are very important. IMO. [Smile]
 

TooSlick

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Doyall, Under normal driving conditions in a domestic six or eight cylinder engine, you want to maintain 10-15 psi for every 1000 rpms of engine speed in the mid range. If you are running under high loads, like pulling a trailer or driving up steep mountain grades all the time, I think that 15-20 psi/1000 rpms would be a better figure of merit. You get more radial crankshaft deflection under high loads, so it might be a good idea to use a slightly thicker oil. Something like an ACEA "A3/B4" rated, xw-30, PAO/Ester based synthetic, or a 15w-40, commercial engine oil. Dr T, Engine performance and fuel efficiency will continue to increase, even as bearing and valvetrain wear increases. However, at some point catastrophic failure will occur if the oil is too thin. So you don't want to get too close to the edge. As a general rule, German engines run very high oil pressure and you'd probably want to maintain 15-20 psi/1000 rpms in a BMW engine; even for normal driving and 20-25 psi/1000 rpms for very hard or fast driving. For example, my 1990 Audi 100 still pegs the oil pressure gauge @ 5 bar/73.5 psi at anything over 2500 rpms, even though the engine has 214,000 mile on it ...I am running the 5.1 Centipoise, Series 2000 Racing Oil these days, since the bearing clearances have opened up a bit from normal wear. However, I ran the 3.5 Centipoise, Amsoil 10w-30 for years in this motor and it would peg the gauge @ 5 bar at anything over 3000 rpms, even if the ambient temps were 90F-100F in the summer. In your old BMW, a shear stable, 5w-40 or 10w-40, 4.7 Cp oil like Redline would give you excellent wear protection and better performance then that 10w-60 - especially in the winter. For hot weather use, a 20w-50 or even that 10w-60 will probably help you get a few more years out of the motor. For all practical purposes, the wear in my Audi motor has essentially stopped with the 20w-50 synthetic. The last oil analysis I did showed 20 ppm of iron after 12,000 miles of oil use, which is extremely low. Ted
 

TooSlick

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The best two ways I know to evaluate viscosity requirements are to use oil pressure as a guide - which does change as a function of engine wear - or to use oil analysis and test several times under varying ambient temp conditions. A 1.5L-2.5L displacement, four cylinder engine should be close to the 1.0-1.5 ppm/1000 mile range in terms of lead wear. A 3.0L-3.8L, V-6 should be 1.0-2.0 ppm/1000 miles, and a 4.0L-6.0L, straight six or V-8 will be up around 1.5-2.5 ppm/1000 miles of driving. If your rate of lead wear is 100% higher than these figures, the oil you are using is too thin, and/or you have coolant contamination and corrosive bearing wear.
 
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What if your not 100% higher, say 50%, would it still be wise to thicken it up a little? Like a 5W-30 to a 0W-40? Same brand??
 
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Hmm, 1 or 2% fuel savings with thinner oil, possibly at the cost of protection, doesn't sound like a good trade. What about saving fuel by going easy on the throttle, by not idling the motor while the wife ( [Razz] ) is getting donuts and coffee? What about running a slightly higher tire pressure than recommended (most car makers recommend less pressure than optimal for comfort reasons)?
 
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