[olddog]

Originally Posted by eschaider

BTW in the FWIW category the relationship between pressure and flow for fluids is a second order relationship where the pressure increase is equal to the square of the volume increase. If you wish to double the volume flow of a fluid thorugh a given orifice then you will need to increase the pressure by a factor of 4. The bearing clearances we run in our engines represents the orifice referred to above.

You should do things for your engine that you think are best for you (and your engine).

First Ed's post is one of the best posts to this site.

I wanted to clarify and expand a bit further on this paragraph. As a process control engineer (more on that later), I have used orifice plate flow transmitters. For accuracy you have a pipe with a plate clamped between two flanges. The whole in the plate is a smaller diameter than the pipe. The pipe must be straight (no elbows) for a length 10 pipe diameters upstream, and 5 pipe diameters down stream of the orifice plate. Now you measure the delta pressure across the orifice plate. (delta meaning the inlet pressure minus the exit pressure. You take the square root of the delta pressure and that is proportional to the flow.

Flow = K * (delta Pressure)^0.5

where K = a constant

K has to take into count the pipe diameter, the orifice diameter, and the viscosity of the fluid.

If you are pumping fluids like water the viscosity changes very little over its temperature range. The orifice plate flow transmitter is as easy as falling off of a log. It works perfectly.

If you are pumping a polymer like Polystyrene, the viscosity changes with temperature exponentially. That could be compensated for, but it gets worse. Polymers' viscosity also thins (lower viscosity), when it is put into a sheer plane. As polymer flows through a pipe the velocity of the molecules at the wall approach zero. Each layer of molecules further from the wall is traveling faster than the previous layer (closer to the wall), and the flow is greatest at the center of the pipe. This causes a shearing action between each layer of molecules. The velocity change between the layers is not all the same, therefore the viscosity change due to shear thinning is different at every layer and therefore the viscosity of the polymer is not constant across the pipe. The point to this paragraph is to explain that an orifice plate flow transmitter will not work, with a polymer flowing through a pipe. Although with a large finite element computer algorithm and a lot variables not mentioned, we can estimate pressures for design purposes, but there are so many variables that the errors in each measurement adds up to me using the words "estimate the pressure."

Ok Rick so why did you waste my time telling me about polymers when we are talking about oil? Well because the way they get oil to behave like a straight 5 wt oil at a low temp and also behave like a 50 wt oil at a high temp is that they attach molecules into chains that intertwine like a plate of sphegetty. "Attaching molecules into chains that intertwine like a plate of sphegetty" is the definition of how a polymer is made. Multi-viscosity oil is partially polymerized oil. So, it does not behave like water at all. It behaves similar to a polymer, but not nearly as extreme.

The further the spread of the viscosity numbers, on a can of oil, the more they had to polymerize the oil to accomplish that spread. A can of 5-20W oil is not as polymerized as 5-50W oil. They need longer chains of molecules linked together in the 5-50W oil, to get the 50W behavior at high temperature, because it is a 5W base oil.

So what does all this mean? Well it explains why oil pressure readings are often talked about, and there are as many opinions as there are people talking about it. You are looking at a reading that does not precisely tell you a damn thing (other than the pressure at one spot). Because there are so many other variables that you do not know, you could not possibly tell me how much oil is flowing based on that one pressure measurement.

So why does one person run 5-30W oil in his engine and another person run 5-50W in essentially the same engine and neither one of them has ever had a failure? Both emphatically states they are running the best oil. They cannot both be right. Right? Well maybe.

Because of the sheer thinning property of polymers, the oil kind of self adjusts or automatically compensates to some degree. If you put a high viscosity oil into your engine, the oil will flow slower, therefore it will stay in the bearing longer. The longer it says in the bearing, the longer is will get shear thinned, and the more heat that will put into the oil. The thinner the oil gets, the faster it will flow. Hence it somewhat finds it own temp, flow, and viscosity. So at 8000 rpm the behavior of a 5-30W oil in a bearing may not be all that different than a 5-50W oil. The temperature of the 5-30W will be less than the temperature of 5-50W oil when it exits the bearing. The shear thinning has a greater affect in the 5-50W because it is more polymerized. The viscosity of the two oils while in the bearing is much more close than you would think, given what they act like when you pour them out of a bottle.

One last point to make. Don't think, based on what I said, that you cannot put too thick of an oil into an engine, because it will just get heated and thinned more and self adjust to the right thickness in the bearing. I did not say that. The oil passages (pipes if you will) that the oil is pumped through to get to the bearings, has little shear thinning and heat affects on the oil. A high viscosity oil will only flow through the block to the bearing so fast. It is possible for a high viscosity oil to stay in the bearing so long, that the viscosity of the oil, while in the bearing, is much lower than what it would be, if a lower viscosity oil was used. Let me state that again. It is possible for a high viscosity oil to actually be thinner, and therefore less protection, while in the bearing, than a lower viscosity oil.

So from this there is a fairly large range of oils you can put into an engine, without issues. The higher viscosity oils require more hp to pump (think the shaft driving the pump getting twisted off), and they will run hotter. The oil only has to keep the metal parts from touching, beyond that you are wasting power and energy. It's a lot cheaper to waste some energy to be on the safe side than destroy an engine. However if you twist off your shaft to the oil pump and are not looking at the pressure gauge you can equally destroy an engine.

So for push-rod engines, I personally wouldn't go less than 5-30W oil nor more than 20-50W oil. I would take into count the typical oil pressure range for the engine in question and if that engine has issues with twisting off oil pump shafts or bearing failures. Generally speaking I stay away from heavy oils unless the engine runs low pressure or it is going to be pushed to the limits of holding together, such as racing. The heavier the oil the more care needed in letting the engine come up to operating temp. That said we are not talking an exact science and there is room for error and many opinions. In this range, it likely doesn't matter a whole bunch.

Not everything I have stated is 100% technically correct. I tried to put this into terms that most gear heads would grasp, so I left the crap that would put you to sleep out. I may have failed at that. I am by no means an expert on this subject, but I have some experience that applies. I just wanted to share in an attempt to reduce some of the misconceptions I read from time to time.

Disclosure: I worked for one of the largest Chemical Companies for 38 years. I started as a operator and worked 7 years as a control room operator. I went to college part time to get an electrical engineer degree. I was promoted into a job that was essentially an engineer position. I did not complete getting the EE degree. I have written process control programs that run a variety of easy to complex processes. Among many things, I worked on improving polymer pump designs, which uses the polymer that is being pumped as the lubricant that flows through the bearings. The last 8 years I have been full time in a process control engineer position. The company, I worked for, sold the plant that I work at to a new company, and I continue to work in that role, for the new company.