10psi/1000rpm rule and running lower viscosity oil

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RangerGress

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The AEHaas Motor Oil articles on this site emphasise using 10psi/1000rpm as the general rule for selecting oil viscosity. How good of a rule is this?

BMW oil has a cST of 12.2. It is 10W30 or 10W40. One of the BMW recommended oils is Mobil1 10W40 which has a cST of 11.5 or thereabouts if I recall correctly.

I did an experiment the other day with oil viscosities and this 10psi/1000rpm rule. Mobil1 10W30 gave me ~25psi/1000rpm. Mobil1 0W20 gave me ~18psi/1000rpm. This indicates I should be running an even lower viscosity oil.

But Mobil1 0W20 has a cST of only 8.8. So I don't understand how to get out of this contradition. The 10psi/1000rpm rule says that 0W20 is better then 10W40, even if the higher viscosity oil is preferred by BMW. But 0W20's Shear value is far lower then BMW's preferred number.

What am I missing here?

The car is an '87 BMW 325 that races in a class called "SpecE30". Very few allowed mods. Motor is a high mileage stock motor with a new bottom end. The objective is to get a couple additional horsepower via low viscosity oil. When everyone's car is pretty much identical, a couple extra hp becomes significant.
 
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Quote:

What am I missing here?


What you are missing is that the 10 psi/1000 rpm is an old rule of thumb that doesn't apply to all engines.

You are also misapplying the rule. It is most useful (but not always correct) at higher rpms. i.e 60 psi at 6000 rpm.
 
Steelhead, so you are telling me....

1) 10psi/1000rpm is a reasonable and safe standard.

2) I shouldn't worry about the fact that the low viscosity oils have a cST lower then what BMW recommends. My engine life should suffer dramatically.

It's not a DD. It gets ~60 track days a year tho, so it gets run pretty hard. I don't want to swap the motor every year, but I could probably live with putting in a new (junkyard) motor every 3 yrs.

Am I understanding your thoughts on this correctly?
 
Originally Posted By: XS650
Quote:

What am I missing here?


What you are missing is that the 10 psi/1000 rpm is an old rule of thumb that doesn't apply to all engines.

You are also misapplying the rule. It is most useful (but not always correct) at higher rpms. i.e 60 psi at 6000 rpm.


Ok, but if I get 60psi at 3000rpms, I should switch to a lower viscosity oil, correct?

Anything unique about 80's BMW engines that would make the rule invalid?
 
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RangerGress,

I also think you are oversimplifying AEHaas' guidelines. The key thing that you are not considering is expected oil temperatures.

Based on your experiment, the 0W20 provided you with more than satisfactory oil pressure at idle. But how hot was the oil? And how hot will the oil be when you are flogging it down the track?

At highly elevated oil temperatures that are expected on a race track, you will most certainly find that your oil pressure is under specification due to high oil temperatures and low oil viscosity at those temperatures.

If you were driving very gently around town so as to not heat up the oil excessively, you might find that the 0W20 would give you adequate protection and pressure. However, for those track days, you should stick with the recommended viscosity and use a synthetic 40 weight to keep the oil in the correct viscosity range at higher temperature ranges.
 
Originally Posted By: il_signore97
RangerGress,

I also think you are oversimplifying AEHaas' guidelines. The key thing that you are not considering is expected oil temperatures.

Based on your experiment, the 0W20 provided you with more than satisfactory oil pressure at idle. But how hot was the oil? And how hot will the oil be when you are flogging it down the track?

At highly elevated oil temperatures that are expected on a race track, you will most certainly find that your oil pressure is under specification due to high oil temperatures and low oil viscosity at those temperatures.

If you were driving very gently around town so as to not heat up the oil excessively, you might find that the 0W20 would give you adequate protection and pressure. However, for those track days, you should stick with the recommended viscosity and use a synthetic 40 weight to keep the oil in the correct viscosity range at higher temperature ranges.


Ya, I've been thinking along those lines. My test of 0W20, conducted in the garage, did lack a certain realism. I ran it at 3k RPM for ~10min. I'll be at the track Thur and I'll give it a better test.
 
Originally Posted By: Steve S
IMO you are missing the most important write up, the factory recommended viscosity recommendations.


I didn't miss the OEM recommendations. It's just that they conflict with the AEHaas 10psi/1000rpm rule. This thread is an attempt to understand why the two ideas seem to conflict.

There's lots of reasons for an OEM recommendation about anything. Recommendations don't just come from engineers. They come from engineers, marketers, accountants and lawyers.

For example all OEM suspension alignment specs ensure understeer by requiring insufficient front negative camber. This results in a car biased towards understeer. That way if you take a turn too fast you are liable to go straight off, instead of spinning off. Car manufacturers have found that in a trial if the guy drove straight off of the turn, the jury often defeats the suit. But if a guy spins off of a turn, a jury often calls it a manufacturer's defect and awards $$.

So the lawyers over ruled the engineers and the result is that all of our cars understeer like pigs. OEM isn't always the best way, it's simply the OEM's way. It's up to us free-thinker types to determine what makes the most sense for a given application.
 
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I believe that if you were to put the 0W20 in your car once again, drive down the highway for an hour, and then let it idle, you will see lower pressures, probably below 10 psi at idle. This would be a lot more representative of your driving than idling in the garage after the oil change.
 
OK You put 20wt oil in and idled your engine then reved it up the oil perssure was higher than what you read it needed to be so you asked about needing a thinner oil? I guess that is free thinking.
 
Quote:

I didn't miss the OEM recommendations. It's just that they conflict with the AEHaas 10psi/1000rpm rule. This thread is an attempt to understand why the two ideas seem to conflict.


The 10psi/1000 rpm rule of thumb didn't originate with AEHass, it's been around since at least the 1950s. It is only a crude rule of thumb.

You might as will be asking why the 3,000 mile OCI is in conflict with OLMs and UOAs.
 
Engine speed and oil volume (as indicated by pressure) don't have to parallel each other on a slope of increase. You've also got some other physics that must come into play at a certain point ..at least as it's indicated by pressure. Your pressure will, more often than not, attenuate at some rpm and THAT hot peak will tend to be below your pressure limits.

We're talking "generically" here...(preemptive disclaimer - YMMV)
 
Gary, that was a good link, thx. Took a while to read it and all of it's links. I'm a mechanical engineer, but they went well beyond my grasp of lubrication theory and practice.

It's irksome to begin to understand that I can't accept the 10psi/1000rpm as a quick and dirty standard. I guess that was just too easy.

XS650: I didn't mean to imply that AEHass invented the idea, that was just the first place I saw it.

SteveS; Kindly not assume that I am an idiot. Just because the manuf recommends something doesn't mean that it's the best solution for how a person intends to employ their product. I let the car idle until it reached operating temperature and then I ran it at 3k rpm for 10min and watched the oil pressure. That's not a terrific test because it doesn't account for the peak temps that would occur under heavy load. But as a quick and dirty test on a non-street legal car, it's probably good enough to render uncharitable remarks, well, unkind.

That test showed 1) Oil pressure did indeed vary with viscosity, which I'd never played with before and 2) Maybe 20W was still too high. And that was enough to get me on this forum and ask questions. Which is what "free-thinkers" do. Question.

Thur the car goes to the track and if oil pressures seem low I'll swap out the oil with something heavier.

But back to the OP's issues. If I can't trust this 10psi rule, how do I determine the lowest viscosity oil that my motor can tolerate for hard use for a couple yrs?
 
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I reread the posts and found some confusion. I was testing the 10psi/1000rpm rule. I was not testing 10psi @ 1000rpm. I was testing the relationship, and doing so at 3k rpm.
 
UOA.

Don't take offense to SteveS. I've heard rumors that he works here composing captions. It's just his thing ..
21.gif
 
10 psi per 1000 rpm, but are we sure that the startin point is not higher, like maybe should have 20 psi at 1000 rpm, then increment 10 per 1000 from there? Check your manufacturer's spec, some give idle and cruising speed, others (Fords I have) only give cruising speed. At least make sure your 10 per 1000 hits spec, which may mean a higher starting point.
 
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