oil cooler

[bobcowan]

Any good oil thermostat will allow about 10% of the oil to flow through the cooler when cold. That allows all of the oil in the system to warm up at the same rate. Otherwise, once the main body of oil allowed the thermostat to open, you'd get a sudden slug of cold thick oil into your engine. That's a bad thing. of course, on a cold morning, the oil temp may never get to operating temp.

When fully open, the thermostat bypasses about 10% back to the engine. The cooler and lines will create some restrictions; the bypass helps to maintain good flow in a hot engine. It also helps prevent high pressure waves through the cooler with a sudden rise in rpm's.