Dyno tests

ir3333 said
When an engine is tested on a dyno, quite often there
is no alternator,fan,fuel pump water pump or exhaust
pipe.
... how much hp might this be?

Mine doesn't have a fan or a fuel pump run by the engine in the car.

Every engine dyno I've seen has exhaust pipe with mufflers.

A 100 amp alternator would be about 1400 watts output , or about 2hp, figure 50% efficiency that would suggest you need 4hp at maximum load.