Oil pump mystery...

[Chris Coker]

Adding shims behind the relief spring only increases the max oil pressure the pump will deliver before popping the relief valve. It doesn't affect flow at all. The relief valve basically allows oil to recirculate inside the pump once it opens. That being said, once the valve opens, your flow becomes a constant. Up until that point, flow is basically a function of pump rpm. Pressure is basically controlled by the restrictions in the oiling system (bearing tolerances, diameter of the oil galleries). This is a simplification, but gets the basic idea across.

This isn't a big deal in a street car, but the general rule of thumb is to have 10 psi of oil pressure for every 1000 rpm. So, at 8000 rpm one would want 80 psi. Some guys do get by with less pressure, and the less pressure you run, the less engine hp you use to drive the pump.

[itsa68]

If the gears in the U20 are taller, I would try a gear swap to the R16 body and use a metal spacer gasket with a figure 8 cutout to make up the difference.
Going from memory, on the high mileage R16 pumps that I have taken apart, most of the wear was on the cover from the gear faces ,causing pressure lost over the top/bottom of the gear faces.
Secondly, I noticed that the stationary shaft of the slave gear was worn, but the bore of the gear was not opened up drastically. This probably increased the gear backlash and did not maintan a good positive seal between tthe gears teeth.
In summary I would also be tempted to clean up the U20 cover face (by milling flat), shim with a metal figure 8 cutout shim/gasket, press in a new shaft for the slave gear, and possibly change out the drive gear shaft or re-bush it at the lower end of the body.
Of course most people might not have access to the equipment required to perform what I mentioned above.

I will have to look further into the possiblity of producing some metal shim/gasket spacers if it would be a way to swap to a R16 pump body.

Hope I stirred some creative juices out there and some more ideas are presented.
RAy B.

[spl310]

Has anyone seen a rating on the U20 with regards to flow capacity? The reason that I ask is that although the rotors may be taller, the discharge port is the same size. That could limit the flow.

I will take the pumps that I have apart and check them out in detail one day. I think that if the G/R/H pumps are shimmed for more pressure, then you may have sufficient flow to support the needs of a U20...