does anyone know what the motor's operating temperature is for r33 rb25det??

The normalised engine operating temperature is between 85 and 93 degrees celcius (185 and 200 degrees fahrenheit) But that is a figure for all manufactures. It differs from one brands to another, but it won't be higher then 200 fahrenheit.

Derby

I have seen some as low as 160 and as high as 210, but Derby is correct, in that ballpark. Not the RB25, just engines in general.

My RB runs a lottle on the hot side, I would say 190, if I had to guess, comparing it to were other skylines I have been in.

I know Mercedes tries to keep at 192.

And most of the manufactures set the max at 230.
That should help.
@SkylineUSA. A higher water tempurature ruins efficiency. I have seen test results for a engine that was kept at 1290 F and the efficiency was horrible because all the heat generated by the burning fuel was flowing back into the material around the flame (piston, cilwall)

Derby

1290, That is a bit hot.

I can see there is a point were it will ruin efficiency. If I remember correcetly, the Older EFI Mustangs ran a little over 200f.