Also, isn't ZDDP something that gets consumed by cold starts and metal to metal contact? That is, as long as there's a high enough concentration which may not have to be very high at all, the extra ZDDP is just going increase the required oil change interval? That's a question, not a statement: I don't really understand all these issues as well as I'd like to.

Of course, the higher heat and additional load from making more power add to the oil requirements but not directly in terms of turbocharger lubrication. Common sense tells me that the oil will likely run hotter if the rod bearing loadings under compression are three times higher than stock and the engine makes three times as much power. That's why I'm kind of thinking that SAE 60 hot viscosity might be the right pick.

Re: Symptoms of too high or too low motor oil (hot) viscosity
Postby mekilljoydammit » Thu Jan 12, 2017 9:55 am

I'm going to basically keep this at a high school level - IE, this is what affects what and why, but leaving out the math. I hate doing appeals to authority, especially when I'm setting myself up as the authority, but when I was doing the R&D stuff we had 3-4 people with doctorates and years of experience in the field doing the simple version of the computer code to figure out this stuff. The code they came up with got increasingly less accurate the closer to the limit things were loaded to, and that's much simpler scenarios (more mechanically complicated designs, sure, but basically static loads) than we're seeing in engine bearings. I may have picked their brains as much as I could, but I don't know nearly as much as they do, so I'm not going to pretend I'm up to coming up with mathematical solutions, you know?

Knock does interesting things actually - I think I've seen examples where the top (piston side) rod bearing was the one damaged! If the bearing has to shift relative to the shaft all of a sudden, there's going to be a sudden pressure drop on the unloaded side where air will come out of suspension and the oil may drop below its vapor pressure - cavitation in other words. Then when things catch up, the oil collapses all those bubbles and the water hammer effect erodes material away... in addition to the possibility of breaking through the film on the other side, of course.

Air entrainment is a huge issue too, but I do really agree that oil temperature is something that needs to be controlled. Too far in either direction on hot viscosity (that is to say, viscosity of what's actually getting to the bearings) can lead to runaway situations - too thick and there's too much heat generation, not enough flow to pull heat away, and localized hot spots kill your bearings, too thin and the film gets too thin, there's excess heat generation and hot spots kill your bearings. Fortunately, in the real world, viscosity of oils that you would put in your engine don't vary enough to produce really disastrous effects - for applications where you're not on the ragged edge, going a bit too thick will just mean that it gets a little hotter, the viscosity comes down and it self corrects, albeit at a little more friction than optimal, too thin may mean that it doesn't heat up quite as much, etc. As a rule of thumb, I'd prefer to err on the side of too thick for stuff that's getting flogged (I come from an amateur roadrace background) because of the next issue.

See, the next issue is rod bearings. First, an ideal bearing looks like a crank setup - fed from the side, everything goes out. In this situation, engine speed has no bearing (ha!) on oil flow, and actually if you have thin oil (thinner than you'll see on any car) you can get away with incredibly small supply pressures if the flow is enough. The problem is rod bearings are fed from the crank. By the nature of the drillings, the oil has to fight centrifugal force to get to the center (most cranks) or a bit in from the journal (Cosworth style drillings) and that reduces the effective pressure. Worse, if the effective pressure at the part of the drilling closest to the center of the crank gets below the vapor pressure of the air/oil mixture you're actually feeding everything, you just starved your rod bearings completely. Intuitively you might think that centrifugal force flinging out to the rod pins would help, but it's kind of a "you can't push a rope" thing - or more scientifically, once you get below the vapor pressure, it's not liquid anymore. This is why, btw, Formula 1 does things like feeds the crank from the nose, and why they were experimenting with hollow cranks with big oil reservoirs in each main.

If I was Roush or Cosworth or someone and was trying to do this, what I'd do is have a bunch of thermocouples embedded in any bearing I could get to, plus in the path of the oil coming out of the bearings, then do endurance testing at high loads - race track simulation on an engine dyno at realistic oil temps ideally. Make sure nothing is getting too hot during the test and then do a test program with teardowns between each rebuild distance and stepping oil viscosity to the minimum that will avoid wear. What I do for race stuff in the real world where I have to pay for it is run somethingW50 and go bigger on the oil cooler.

The exact way it gets consumed is a bit of a mystery to me, but I think that ZDDP decomposes ... If this is true, then isn't it the case that more ZDDP would be providing longer protection but not better protection.

Furthermore, if my speculation is right, elemental analysis of oil such as mass spectrometer "burns" the ZDDP, zinc, and phosphorus at something like 10,000 F using plasma and by my logic one can't tell the difference between available ZDDP and already consumed ZDDP and the resulting zinc and phosphorus.

Thus, I don't know how to measure the depletion of ZDDP.

By the way, my current opinion is that the oil test data on the page maintained by "540 Rat" is very relevant for mixed and worse lubrication regimes, such as camshafts-lifter interfaces. However, for bearings that are intended to be mostly in hydrodynamic lubrication regimes, such as rod bearings, my current thinking is that the correct (hot) oil viscosity is more important than any test results from tests that mimic the mixed lubrication regime.

The reason why I'm wobbling between lower viscosity and higher viscosity oil is load-bearing ability vs. cooling. Viscosity*bearing speed / bearing pressure is a design value for bearings such as rod bearings. If I am increasing the peak load on the bearing by say 50% (the average load is going up by a lot more), then to keep the bearing as far from mixed lubrication regime as stock I'd have to also increase viscosity by 50%.

One probably will have to take some more risk, but clearly the higher peak loads on the engine point towards higher that stock oil viscosity.

So cooling is an important problem: I'll have to be able to cool the oil in order to run high enough viscosity that can protect the engine.

And there will be a big-*** oil cooler in the stock GT cooler location that will be correctly ducted to the airflow.

The feature of adding a thicker aluminum radiator is protected in the fan shroud design, but I haven't done anything on that yet as I believe that air flow thru the radiator is a bigger problem for 928 that the radiator size per se.

Finally, I'm just going to "improve" cooling by simply running the engine somewhat hotter than stock.

There's one more viscosity related question. Higher viscosity oil drains slower to the sump. The engine oil pump also ingests oil from the pickup at a lower rate if oil has higher viscosity. It's probably a wash in normal situations. However, when the car is under severe g-load, perhaps the oil can't drain to the sump. In this case, I am thinking that higher viscosity oil has a longer lifeline under those high g-forces because the oil pump sucks the sump dry slower.

Here's a post by a person on speedtalk.com who makes sense to me on the topic of oil viscosity: