Viscosity, Pressure and Flow

[Jim Allen]

How about some analysis of this, please.

1) The ideal is to have the lowest viscosity oil needed to maintain the hydrodynamic barrier at the temperature range specified.

2) When you view the engine manufacturer's oil pressure specifications, which usually indicate a specific rpm and specify a fully wamed up condition, can you assume that in those conditions the engine has adequate oil flow to maintain the hyrodynamic barrier?

3) If "2" is true, if you were to reduce the oil viscosity while monitoring that oil pressure, and it stays within the recommendations, are you copacetic with regards to getting adequate lubrication?

3A) How do load conditions enter into this? If "2" and "3" are true, and you reduce viscosity, but maintain the recommended pressures, how much effect will the drop in viscosity have on the hydrodynamic layer at low rpm, high load conditions? My thinking is that a one or two grade drop (say from a 40 to a 30, or a 30 to a 20), assuming the oil pressure recommmendations remain in the green, there should be little or no difference if the pressure is correct.

My underlying question here is to determine if monitoring oil pressure is a good way to determine if you engine can safely operate on a lower viscosity oil. I would think oil temp would enter into this too, but if you drive 20 miles, you can be pretty sure the engine oil is as warmed as as it has to be. Also, if you monitor pressure in various climactic conditions, you can test how much that effects the engine.

 

[TallPaul]

Without detailed analysis I would say not to go thinner than the manufacturer allows, which on many Fords is 5w20 (back specified for many 90s engines). Still that assumes a relatively tight engine.

Chances are you will not stay within manufacturer's recommended pressure if you go thinner than they specify.

I question point 1. In what way is it ideal to have the lowest viscosity needed ...."? Certainly is from a CAFE standpoint, but I prefer to go to the higher end of the allowable viscosity range for a given temperature range.

Higher rpm probably can go thinner with enough pressure/flow. Lower rpms need thicker oil. Lugging needs very thick oil, but best avoided.

Certainly you can run 5w20 in an engine requiring 20w50 if you never drive more than 2 or 3 miles, i.e. you never warm the oil enough to get it very thin.

 

[Merkava_4]

I can't copy/paste this for some strange, so here's a link:

Hydrodynamic Fluid Film

Imagine if that speed boat had to try and form a hydrodynamic wave on lacquer thinner rather than water, it would have to travel much faster to achieve the same results.

That's what's happening when you put a 5W-20 oil in an engine that's not designed for it; it's like a speed boat trying to become suspended on a lake full of lacquer thinner.

 

[Rickey]

Assuming that there is a range of oil viscosity's that an engine can operate comfortably and reliably within:

And that the thicker end of usable oil viscosities is limited by the minimum temperature at which the engine will be started, and furthermore that the thinner end of usable oil viscosities will be determined by the maximum temperature that the engine will be operated at while under severe loading at max throttle.

Why would anyone in their right mind want to test either extreme outside of a laboratory?


Rickey.

 

[Curious Kid]

I have yet to read the link FWIW.

As it is, it is likely that even within the manufacturer's "ideal" viscosity range, there is still various states of lubrication amongst the many parts in the engine, like hydrodynamic for mains, mixed for tappet valves, and the whole range amid rings and cylinder walls ...give or take. Throw in the effects of base oil viscosity without consideration of running viscosity as made possible by additions of VII's as the straight weight thread got me to thinking, and that hints of importance HTHS could impart for less tolerant machines as stresses/output demands increase, and well, it all starts to seem like a jumble of pending factors.

For better or worse, spec's and recommendations via manufacturer just might be the savior unless one is watchful and following-up with testing, unless throwing caution to the wind of course. Going from a 30wt to a 20wt however has less of a points change than from a 40wt to a 30wt, as far as the averages for PCMO's goes from what I've seen...BUT consider shearing as energy conserving oils might be said to have an increased tendency to do, especially when foreseeing high output demands of any sustained sort.

Take care.