0w30 vs 5w30
- Thread starter mco30mco
- Start date
Excellent choice
Yeah, the estimates in that spreadsheet are more of a "standardized" VII content assuming the same VII type, but what matters in terms of the oil's performance is not the actual percentage of VII, but the difference in high shear viscosity between grades.
I saw your cold weather oil pressure test that you linked in another thread recently, but I interpret the results differently than you do. An oil pump bypass may start bypassing at a lower pressure than the nominal bypass pressure rating, and pressure can also rise much higher than the bypass pressure rating at higher rpm.
On the initial cold start at -18 C, the oil pressure peaks at the bypass rating of 65 psi, then settles to 62 psi, but even at 62 psi, the oil pump is still bypassing. It may even be bypassing when the oil pressure is 59 psi at 30 C oil temperature. You'll be able to tell when the pump stops bypassing when the pressure starts dropping more sharply as the oil temperature rises.
If we assume that the oil pump was not bypassing at 30 C and 59 psi, with ~100 cST oil, then at -18 C with ~1000 cST oil, the oil pressure would be 10 times higher, or around 590 psi, if the flow rate was the same. Restriction and pressure rise roughly linearly with viscosity at a given flow rate (Poiseuille's law). This all assumes that the engine doesn't have a variable discharge oil pump.
The chart here below an example of the non-ideal behavior of an oil pump bypass. It shows that even when the pump is in bypass, the pressure changes a bit with changes to oil viscosity. It also shows the linear relationship between viscosity and oil pressure when the pump is not in bypass (80, 100, and 120 C data points at 1000 rpm).
On this engine, the oil pump is bypassing at low rpm whenever the oil is colder than 80 C. This isn't the most typical example, but the oil pump on my Subaru for example is still bypassing at high idle with the oil at 40-50 C. I doubt there are very many automotive engines that don't bypass oil at 10 C, unless they have variable oil pumps.
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HTHS, which, we unfortunately don't see very often anymore. The point being, I'd be careful basing anything off those calculations, since blending doesn't typically follow the methodology necessary to validate that generalization. The Mobil examples illustrate this quite well.
Yes, you can overdrive the bypass, which I've seen many times with SBC's with HV pumps in stock clearanced engines. My buddy's dad made that mistake, pump had a 55psi relief and it would regularly produce 75psi on a cold start, as the relief couldn't pass the necessary volume of oil to keep system pressure at the relief pressure.
The engine doesn't have a variable discharge pump, it's a standard crank driven PD pump. The Pentastar engines have a vane pump, obviously, that's a different animal. The action of the pump shearing the oil is going to immediately heat it and reduce the viscosity, as is the act of compressing it. So, even if we assume your 1,000cSt figure for the oil in the pan is accurate (visc calcs aren't accurate below ~0C), as soon as it enters the pump, that's no longer the case. This is also why the bulk oil temperature increases so quickly, as the cold oil is acted upon by the rotation of the camshaft, crankshaft, rods, rockers, pistons...etc, then goes back into the pan, rapidly increasing the bulk temp.
I'll see if I can get you some more data from my Jeep. I'd consider my 6.4L pretty typical of an engine with a crank driven PD pump and its pump is higher volume than many other examples. I know the Subaru's have higher volume pumps, what's the relief pressure on them?
My old Windsor (I had an Autometer mechanical gauge on it) also had a 65psi relief, and it had to be reasonably cold for it to engage the relief on a cold start. As long as you were gentle with the throttle, you could keep it off the relief driving it as well.
Yeah, 1,000 cST was just a very rough estimate. A VI calculator puts a 5W-30 at 2,600 cST, but when oil is this thick will be fully shear-thinned which will lower the viscosity. I don't think the oil pump contributes that much to oil heating. On my car it can take 30 seconds for the oil temp sensor located at the pump outlet to go up more than 1 degree, after which the temperature starts to rise more rapidly (though I haven't tested this in extreme cold). Most of the oil heating is probably in the journal bearings, pistons, and head.
The pressure relief setting on my WRX is 102 psi, though Subaru engines more typically have a spec of 75-90 psi , with high flow rates of usually around 60 L/min. The cold idle speed is high as well at 1,800 rpm (due to modern emissions standards), which also contributes to the bypassing on a cold start.
Here's a video of a Subaru engine in bypass at cold idle with 40 C oil. I think the bypass setting is around 85 psi on this engine. The oil pressure hits 140 psi when the engine is reved up. I'd be interested to know how much higher it can get when the oil is thicker. Needless to say, I've started to pay more attention to burst pressure specs when choosing an oil filter.
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You can work CCS visc backwards, it doesn't give you KV, but you do end up with a CCS viscosity figure. You halve it for every 5C, so the Ravenol was 4,560cP at -35C, so it would be ~570cP at -20C, using this method. If we wanted to compare this to a PAO base oil, KV -40C for SpectraSyn8 is 19,000cSt, while CCS is 4,800cP at -30C (Brookfield is 17,590cP at -40C).
I believe the pump contributes to localized heating, through the shearing it imposes on the oil pumping it, which results in localized thinning as the oil is pumped. The cold start I'm sharing below, the oil temp went up by 6C within about a minute just idling, this is a 7 quart sump.
Wowzers, that's like Cleveland SBF relief stuff right there, lol!
So, I went out with my son (recording) and did a 10C start and drive. I was surprised by the results of the start, but not the drive. I'll preface this with the fact that the HPL 0W-40 is heavier than the Ravenol by a bit. CCS is 6,118cP vs 4,560cP, KV40 is 83.79cSt vs 74.4cSt, HTHS is 3.9cP vs 3.7. That appears to make a difference. I don't believe I was on the relief, but I was close:
After backing out of the driveway, here is driving oil pressure, you can see that I was able to hit the relief at one point when I was accelerating a bit more briskly, otherwise, it stays off the relief:
I will do the same with my wife's 1500 once I have a chance, I expect we'll see considerably lower numbers, since it has 0W-20 in it.