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Thanks everyone!

There will always be a big surge as it first starts spinning since the BEMF will be zero at that point. Once it gets going, it will stabilize somewhat.

If you compare the amps at 11v vs. 13v for example, assuming the same load, the motor will draw more at 13v and it will spin faster. Power will be a function of voltage, not constant.

Right. Take any motor, attach an ammeter to monitor amps, and try changing the voltage.

The load created by cranking an engine will increase with rpm also.

Well, the power input is about as constant as the load on it's output.

If the voltage is lower and rpm is lower and load is lower, then the power is also lower.

I agree it's not constant power (that would be a hell of coincidence) but I think there is a bit more to it, not least because the starting battery is not a fixed or unlimited voltage or current source.

For example, if the load on the starter is higher due to cold temperatures, the starter speed will be slower, but the current will be greater and the voltage will be lower. Well, until the point I=V/R kicks in as the battery voltage has sagged.