I am wondering at what speed the auxiliary fan on a 123 car starts windmilling.

RPM has nothing directly to do with actuation of the aux fan. It is turned on by a high pressure switch located at the filter drier. It's purpose is not general engine cooling, rather it is turned on to bring down the high side pressure to keep it from getting dangerously high.

I think he maybe meant at what vehicle speed the forward motion of the car will force the fan blade to turn without having any power applied to it. And I don't know the answer to that. I would think that it does freewheel though.

Good to see a post by you, Larry.

Tom W

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

The 2nd reply is the correct question I am asking. Sorry about the confusion. (At what forward speed of the car does the aux fan start to windmill - turn freely)?

if you really want a project, disconnect the power leads and connect a volt meter,go for a drive and when you start generating voltage the fan is turning.
larry perkins

Not a bad idea. Thanks!