Oil again

So, I have changed the oil.

A week or so ago, I was getting 55 psi on cold start dropping to near zero when very hot, using cheapo 20W/50 for running in. 

I worked through the clearances I measured when I built the engine, to determine the flowing areas of each of the bearings, to understand where my engine was in relation to factory minimum conditions - When I rebuilt the engine I replaced all the bearings in the bottom end, but only changed one of the rocker shafts, because they are easy to change and Drags only had one in stock. What I found was that I can improve pressures by changing the other three, especially if I can return the rockers to minimum clearance as well.

However, obsessive tinkering is something I have always been afflicted with. I wanted to change the oil, since I knew the viscosity at 100 C for both 20W/50 and monograde 50, which are more or less the same; viscosity at 40 C is almost double for monograde 50 over 20W/50, and I know my oil sits in the tank at about 73 C, so I expected a significant rise from changing to monograde 50.

Today, I started out cold with 55 psi and the relief valve operating, so I know I will never see more than 55 psi with the plunger pump. After a 4 mile run around town, its dropped to 25 psi hot (idling), and the relief valve is not operating, which is as I would expect.

So, situation much improved, but more testing required.

More on Oil

So today, we have something like 6 degrees Celsius outside. I've just completed about 3 miles around town and I have measured the oil temperature using a meat thermometer (not the one that was stuck in the Christmas turkey last week), and we have 73 degrees Celsius.

Please don't stick it in the Turkey

If you use one of these though, you will need to stop the engine first. The RF interference driven by the HT cables is phenomenal and stops the thermometer working at all. I guess the thermocouple wires pick up the RF.

Now, looking at the oil data sheets, we can see that the kinematic viscosities are similar at 100 degrees Celsius. Plotted they look like this:

This gives us a clue, but we can't get too excited because there are only two data points per grade. The temperature/viscosity relationship is not linear. Look at this, plotted on a logarithmic scale:

This gives us a better picture. Notice that the viscosity of the SAE 50 is about 20 cst at 100 degrees C, which agrees with the data in  the first chart, but that at 75 degrees C the viscosity is about 40 cst. That should give us about a bit more oil pressure for the same temperature than the 20/50W we are using now.