Your system should "find its own level" (if it's like mine) in the expansion tank. My expansion tank runs about 1/2 full. I would not be surprised if that volume is more than 20 oz. That's less than a big glass of beer.
So if you overfill (expansion tank FULL), and then it vents into the overflow tank, and gets sucked back in when cooled, it's still overfilled.
My advice.... Let your system reach its happy level (after dumping some coolant on a couple local drives), and it won't do it anymore.
I might suggest you don't ONLY drive it to shows, where venting is a little embarrassing (but not harmful), and ensure your cooling system has stabilized the fluid level first. This will give you more first-hand experience with the behavior of the system.
Perhaps you have done all this.
I can't comment on the thermal observations you are seeing. Your comments make sense about the fans shutting off via the thermo switch while driving at speed, so that’s a good data point on the ACTUAL temperatures…
I'm sure you have a "calibration" on your gauges and are highly confident that 220* indicated on your gauge is 220* at the sender.
Anyway - let your system vent, find it's equilibrium, and then drive it for a bit.
Has it done anything wrong except push out "excessive" fluid (which isn't technically "wrong") yet?
- Boiling sounds at engine shutdown?
Rocky