Viscosity requirements vs. operating rpm

[Curious Kid]

I was wondering just how much weight should be given towards heavier oils when selecting a grade when the engine tends to run at low rpm (1600-2200rpm). A big weight selection factor for me is the long time frame it take to rack-up miles and the temperature extremes throughout the year (-20's to +90's deg. f). The engine redlines above 6K.

I understand that hydrodynamic film strength increases with rpm, and so when I'm knowing of the road ahead, I manually shift the auto transmission down a gear which brings the rpm's up another 600-800. I however prefer not to have to change gears with increasing rpm spans as this put more strain on the transmission/wears clutch material).

Perhaps I'm fretting over nothing other than to gain information. I love to learn, but when the costs can quickly add up, that's where things start to out-weight interest.

Vehicle: '95 Volvo 850 wagon w/94K, 2.4l 5-cyl, non turbo w/automatic. Used for transport, leisure, and occassional towing.

 

[Jimbo]

The loads also tend increase with rpm, so the hydrodynamic oil barrier and bearing loads should hopefully balance out as the rpms rise. The problem is that most of us do not know the calculated values the engineers used in sizing the bearings in our engines and oil viscosity recommendations are influenced by factors other than just the needs of journal bearings. CAFE, hydraulic lifters, cam chain tensioners and others may dictate oils on the thin side.

You are basically saying that lugging the engine is bad, and that is certainly so. At the high rpm end is the BMW M3 fiasco, where they apparently undersized the rod bearings and use 10W-60 oil as a bandaid fix.

 

[Curious Kid]

Thanks very much for the info guys.

I have ordered two more books from Noria, and am waiting patiently for their arrival.

 

[1911]

quote:

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Originally posted by Jimbo:
The loads also tend increase with rpm, so the hydrodynamic oil barrier and bearing loads should hopefully balance out as the rpms rise. The problem is that most of us do not know the calculated values the engineers used in sizing the bearings in our engines and oil viscosity recommendations are influenced by factors other than just the needs of journal bearings. CAFE, hydraulic lifters, cam chain tensioners and others may dictate oils on the thin side.

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YEs yes......all true but I will add a bit here. Film thicknesses are much more sensitive to RPM than they are to load even though they are indeed influenced the load in the manner said above. The exponent in the film thickness equation is higher for the RPM. However, what is not clear to me is exactly how peak main and rod journal loads vary with RPM, this is an excellent question.

1911

 

[bbcmat]

I don't have the theoretical info your looking for.

I do have recommendations for that engine type - as the earlier version of my own 5 cylinder non-turbo in the 2000 S70 SE

1) Any continuous rpm below 4500 rpm will not result in abnormal wear, and is not considered hard or stressful service at the engineering level at Volvo's technical training facility.

2) ANY spark knock / detonation caused by lugging the engine is bad - but this condition is generally not allowed with the controls for the auto transmission / engine management system

3) A North American issue with the valvetrain in these motors is exhaust valve and stem deposits - the motors do not see extended higher-rpm loads and do not cook the deposits from the valve seat and valve face. The exhaust valves are equipped to rotate to help clear the deposits, but this only consistantly occurs over 4500 rpm. A good practice with these engines is the routine "clean out" - 10-12 minutes on the highway on a fully warmed up motor - sweeping the the rpm 4500-5500 including a number of "throttle off" deaccelerations (back down to 4500 rpm) build heat and rotate the valves.

I generally do this in 3rd gear of my 5 speed auto in the cool of the morning - doing it 4rth puts the speeds up where I'm driving instead of listening. I try to do it once a month - but time goes by so fast I usually do it every 6-8 weeks


MAT

 

[mechtech]

Low RPM motors [in general], like heavier oils, and high revers like the thinner stuff.

 

[Curious Kid]

I sense that mechtech, but how much of that fact is by design?

I understand that there's going to be a compromise, but If I were to use a heavier oil to account for my generally low rpm operating conditions, what may come of it if I happen to wind it up to 5 or 6K?

Thanks again.