Originally Posted By: CATERHAM
Originally Posted By: OVERKILL
If an engine is spec'd for a 40-weight and it generally runs the oil around 14cSt when hot, and this is the recommended grade by the OEM, this would be relatively close to "optimum" for the engine as defined by the OEM. Of course there's going to be a bit of leeway in either direction but generally, let's say that in the range of 12-16cSt is "optimum" for this particular engine.
No, a 40 grade oil (12-16cSt) is way heavier than optimum @ 100C
oil temp' for any engine. In fact most engine oil pumps will still be in bypass at that temp' with a 15-16cSt oil.
An oil's optimum viscosity in HTHSV terms will be somewhere in the 3.5-4.5cP range at all oil temp's if such a lubricant could be invented. (I'll be posting on recent developments in that regard when I find the time).
Can you expand on this please? The "optimum" visc for any engine is going to depend on bearing width, diameter and load, it isn't a static value. If an engine is designed for an oil >=3.5cP HTHS; KV100 somewhere around a heavy 30, light 40, then that's optimal for that design; that design, at standard operating temperatures will be protected adequately with that viscosity.
I know my oil pump certainly isn't in relief when my oil is much above 75 degrees for example (based on data logs from guys on M5 board with oil pressure gauges). It takes reasonably high RPM to activate the relief and that point of course shifts up as we approach 100C sump temps. Similar to my experience that I've posted about with the SBF on here before as well, where, at standard operating temperature and "normal" summer oil temps, relief actuation is close to redline. Oil pressure was in the mid 30's at idle, relief was 65psi.
I'm not 100% sure on what you are saying, and that's why I'm asking for you expand on this. As it stands, oils with the HTHS value you've cited are all in the KV100 range that I mentioned; the same range at the opening of your post you are saying are too heavy but then go on to say that those same oils, but using their HTHS visc values, are "optimum"
We know bulk sump temp and bearing temp are different, that's why HTHS is measured at 150C instead of 100C like KV100 is, as they are designed to reflect "normal" operating temperatures for sump and bearings.
As per your last point, yes, an oil that doesn't change in visc due to heat or cold and could maintain a constant viscosity would be ideal, but of course that doesn't exist. All we can do is try and minimize the amount of change between our coldest starting temperature and highest operating temperature. I know that's why you chase VI, and that's the same reason I like to look at CCS and MRV when choosing an oil that is within the viscosity range specified by the OEM for my vehicle. I'm more concerned about the coldest temps I'm going to encounter than I am about visc change between a hot start in the summer and operating temperature. I have several decades of experience with boats and in the last 15 years, car engines, to show me that the latter isn't an issue. I've never worn out an engine with this approach; I've never had an engine fail, and my tear-downs and tear downs of guys I know in the Mustang scene show that running a high quality lubricant changed at a sane interval works to prevent wear. We've never looked at VI as a contributor to oil choice, always the hot viscosity rating relative to power output and ensuring adequate oil pressure was maintained.
Originally Posted By: CATERHAM
A 2.6cP HTHS rated 20 grade oil has a HTHSV of around 6cP at 100C so it's still thicker than optimum at that temperature for all engines.
It's the reason more and more manufacturers are specifying that grade and lighter. So aside from availability issues with 20 grade oils, 30 and 40 grades are spec'd to deal with viscosity loss associated with very high oil temp's, fuel dilution and one oil shear.
Except bearing temp and sump temp aren't the same. I believe you and Shannow have discussed heat rise in bearings vs bulk oil temp before, no? We can't just look at bulk sump temp, extrapolate HTHSV from that temp and assume it is too heavy in the bearings if the bearings want an HTHSV of 4cP for example. At the current bearing temp, that oil may BE 4cP. That's why bearing design, width, diameter, clearance...etc are important factors.
Engines built for 20 grade oils "from the ground up" are:
1. More rigid in terms of bottom end construction. A great example of this is the Ford Modular. Ford put a lot of effort in ensuring that, compared to its Windsor predecessor, the Modular would keep the bottom end in one place and this was done to accommodate the push to thinner oils.
2. Have wider bearings.
3. Have higher volume (and often higher pressure) oil pumps. See the Modular again for this.
4. Are often fitted with oil coolers to keep oil temperature under control to maintain a minimum viscosity. See the HD towing package and police package on Modular equipped trucks, Police Cars and taxi cabs for examples of this
I believe it was Honda we discussed recently that has gone to wider bearings on their new engines in anticipation of the adoption of the 16 grade oils, no?
I know that a lot of this stuff, we like to think it is static to make it work with our point of view, but a lot of it is incredibly dynamic and changes based on many of the factors we've already discussed like the intended operating environment of the engine, various design parameters, power density of the engine, which will heat the oil faster and also put more pressure on the bearings, the number of cylinders; a high power 4-banger may have greater cylinder load and subsequently pressure on the rod bearings and a forced induction app guarantees this.
And on top of that, every lubricant is a compromise in some way, shape or form. There's no "perfect" oil. An oil that will protect a high strung engine on the track will be too heavy for optimal efficiency tooling around town in a short tripper.
This is what I find fascinating about what Honda's doing right now in Japan with their new engines specifically designed for these ultra-light lubricants; lubricants so light they don't even have an SAE grade associated with them, LOL! These oils also have INCREDIBLY high VI's, using, admittedly, a great deal of VII, and high quality, but extremely light base stocks. The big issue with them, as mentioned in the one paper somebody linked, is that volatility (NOACK) becomes an issue and will be a big issue with approvals. And there is part of our compromise. Shannow has also mentioned increased levels of boundary lubrication as well as drag caused by these wider bearings in order to allow these oils to be used, but obviously the advantage offered by the thinner lubricant must be more than enough to offset the increased friction in these areas or Honda wouldn't be doing it.
I also found it noteworthy that Honda SPECIFICALLY states NOT to use these lubricants in engines other than those that were designed for it. Obviously they feel it is too thin to provide adequate protection in conventional applications.
It can be a factor on 15k mile changes.
