Can a high HTHS ever be detrimental to engine protection?

[ZeeOSix]

It could be the main bearings in these engines that are the limiting factor then. MOFT in the main bearings seems to be lowest at low rpm, which seems to be the opposite of rod bearings, which have lower MOFT at high rpm. Or it could be due to the other advantages discussed.

LS series engines never really had any journal bearing issues. Many go over 250K miles with no bearing problems as long as the oil pump stays in healthy condition.

The Subaru oil pump is driven at crankshaft speed. It's got a lower flow per revolution than the GM pumps, but it has a higher output rating since it's flow rating is based on the pressure being lower than the 102 psi pressure relief. It will only start going into pressure relief at high rpm when the oil is thicker than ~12 cST. If you compare the dimensions of the gerotor in the table with that of a GM pump, I'm sure the Subaru pump is smaller.

The only flow data I have for my turbo FA20 engine is at 600 rpm and 6700 rpm. It must be from a running engine since the pressures given correlate well to actual engine data in these conditions.

Flow per revolution is 12.3 L/min per 1,000 rpm at idle, dropping to 9.0 L/min per 1,000 rpm at high rpm, which is a 27% drop in flow per revolution by high rpm. Pressure per revolution is 8.5 psi per 1,000 rpm at idle, dropping to 7.0 L/min per 1,000 rpm at high rpm, which is only an 18% drop in pressure per revolution.

So that seems to indicate increasing restriction with rpm. The increasing restriction may have to do more with the restriction of oil passages than the bearings. Comparing your Z06 pressure curve with the Melling curves, there is the same relationship.

Here's the two RPM vs flow specs from the Subaru table above plotted for a visual. Yes, the swept volume of the Subaru pump is slightly smaller than the LS pump (0.75 in^3 vs 0.98 in^3 per rev). I'm assuming the pump slip is near zero at the low 600 RPM and 5.1 PSI output. If the RPM vs flow is plotted for the ideal pump (orange line) vs the actual flow output (gray line), it looks like typical pump slip increasing as the flow and pressure increases (the 27% decrease in flow from 600 to 6700 RPM). The pump is putting out 1.95 GPM at 600 RPM, vs the LS pump putting out a hair above 4.0 GPM at 600 RPM (per the Melling curve), but ~2.5 GPM at 600 RPM per the volume/rev curve (they don't agree at idle for some reason). So yes, at idle it looks like the Subaru pump wouldn't be in pressure relief at 1.95 GPM and 5.1 PSI. The Subaru oiling system looks to have some relatively low flow restriction paths in the system (like maybe oil cooler, flow paths to the top end, etc) since the OP spec is 46.6 PSI at 15.9 GPM. The hot oil temp/viscosity will help lower the OP vs RPM, but that's still pretty low oil pressure for that much flow.

As far as the RPM vs P curve on that engine, if it drops 18% below linear from 600 to 6700 RPM, then the pressure curve isn't even linear because it has to be arching downwards as RPM increases like seen on the Z06. If the system acted like a fixed flow resistance, the RPM vs P curve would arc upwards somewhat like shown in post 65. So the effective flow resistance of the system is decreasing as RPM increases, most likely due to the journal bearings self-pumping factor. That's the only thing going on in a running engine that can effectively act like a decrease in system flow resistance. And the more journal bearings there are in the system, the more their self-pumping effect will have on reducing the oil pressure curve as the RPM increases.

Increasing restriction at higher flow rates is what you would expect if the flow is turbulent enough. The higher the flow rate and lower the viscosity, the higher the Reynolds number and the more turbulent the flow will be. Oil filters and oil coolers have increasing restriction with flow rate, and it's the same for oil passages in general when the flow isn't laminar.

Yes, you can see the transition from laminar to turbulent flow in the example in post 65. Pretty much all flow components will have a flow vs P curve shaped similar to post 65. Once the flow goes turbulent, the pressure curve starts going exponential as flow increases. The only thing that can change the shape of that curve (ie, turn it down to more linear or even roll it over) is the flow resistance of the system becoming lower as the flow increases, like the self-pumping effect of the journal bearings have on the oil pressure between the pump and the bearings.

 

[burbguy82]

HTHS essentially is the rating of a lube's ability to retain vis in extreme heat.
How in the world would a lube with high HTHS value be "detrimental to engine protection"?
As opposed to what as an alternative; a lube with low HTHS which degrades quickly?

I also disagree with your poorly defined statement that "a higher HTHS gives better wear protection". That's a very broad paint brush with which to paint. To be more specific, lubes with a high HTHS value likely will protect LONGER (extended OCIs), but in shorter OCIs they are not shown to do much different at all. Much of this would depend upon the specific application and use factors.

Would you agree that HTHS rating alone is not a proper way to determine detriment or superiority over another? Further would you agree it is more the concert of all the factors, such as chemistry that would play a larger role in this rating? In other words, just because an oil has an "high" HTHS, does not mean anything, without an analysis? Or would this be a good "blind" spec to look for in an oil?

 

[burbguy82]

LS series engines

That's like challenging Jordan to a game of 1 on 1.

 

[ZeeOSix]

Would you agree that HTHS rating alone is not a proper way to determine detriment or superiority over another? Further would you agree it is more the concert of all the factors, such as chemistry that would play a larger role in this rating? In other words, just because an oil has an "high" HTHS, does not mean anything, without an analysis? Or would this be a good "blind" spec to look for in an oil?

Without knowing the wear mitigation effect of every oil's AF/AW additive package with controlled testing, the HTHS viscosity is the best parameter to go by. A better AF/AW additive package may make a lower HTHS oil protect against wear as well or better than another oil with more HTHS and a weak AF/AW package, but in the end the viscosity will always be a primary strong parameter that keeps moving parts further apart, and therefore adds wear protection with increased MOFT. That puts less wear prevention responsibility on the AF/AW tribofilm (the "film strength"), until the HTHS viscosity and resulting MOFT isn't adequate enough to mitigate wear. I'd rather be relying on viscosity first to keep parts farther apart, instead of the tribofilm when the film thickness goes near or to zero.

 

[burbguy82]

Without knowing the wear mitigation effect of every oil's AF/AW additive package with controlled testing, the HTHS viscosity is the best parameter to go by. A better AF/AW additive package may make a lower HTHS oil protect against wear as well or better than another oil with more HTHS and a weak AF/AW package, but in the end the viscosity will always be a primary strong parameter that keeps moving parts further apart, and therefore adds wear protection with increased MOFT. That puts less wear prevention responsibility on the AF/AW tribofilm (the "film strength"), until the HTHS viscosity and resulting MOFT isn't adequate enough to mitigate wear. I'd rather be relying on viscosity first to keep parts farther apart, instead of the tribofilm when the film thickness goes near or to zero.

great explanation..........so in your opinion, blind, you would go buy HTHS?

 

[ZeeOSix]

Yes, because higher HTHS will give more parts separation (more MOFT) and therefore more wear protection headroom (insurance) before you have to rely on the AF/AW tribofilm to start protecting from where the MOFT left off. The AF/AW tribofilm is somewhat sacrificial, the viscosity isn't. More HTHS viscosity is more important if you drive hard or tow a lot ... ie, push the engine somewhat hard. More HP/engine load and RPM demand and the resulting heat on the engine parts will decrease the MOFT, and if you have higher HTHS viscosity to start with then that helps keep the MOFT up under more demanding engine operation. If someone is just crusing around at low power and low engine revs, then the thinner oil will be adequate. But if you want a little more wear protection headroom, then go up a grade (ie, from say 5W-20 to 5W-30).

 

[Mephy]

i stepped a honda K24 from a 5w-20 with an average HTHS value of ~2.7cP to HPL's 5w-40 with a HTHS value of 4.248. so far the engine is much quieter and hasn't taken a hit to fuel economy (already have 1600 miles on the oil)

How come you jumped 2 weights up?

 

[BrendanC]

How come you jumped 2 weights up?

wanted my truck on 5w-40 so the accord also got 40 weight

 

[burbguy82]

Yes, because higher HTHS will give more parts separation (more MOFT) and therefore more wear protection headroom (insurance) before you have to rely on the AF/AW tribofilm to start protecting from where the MOFT left off. The AF/AW tribofilm is somewhat sacrificial, the viscosity isn't. More HTHS viscosity is more important if you drive hard or tow a lot ... ie, push the engine somewhat hard. More HP/engine load and RPM demand and the resulting heat on the engine parts will decrease the MOFT, and if you have higher HTHS viscosity to start with then that helps keep the MOFT up under more demanding engine operation. If someone is just crusing around at low power and low engine revs, then the thinner oil will be adequate. But if you want a little more wear protection headroom, then go up a grade (ie, from say 5W-20 to 5W-30).

i agree