If all numbers are not relative to each other, then how is it a marketing tool?
If I build an engine and it makes x horsepower here, y on a Dynojet in the car, and z on a Mustang in the car, then what is the standard, and who's to say that the engine dyno is just a marketing tool?
Turning the accessories actually takes a negligible amount of horsepower. Between the water pump and alternator, we have measured about 8 hp loss.
Of course we can't mount sidepipes on the dyno, and I send out a lot of engines where the customer doesn't have an idea on how much hood clearance he will have, so it's really pointless to mount an air cleaner. Do you lose hp to an air cleaner? Sometimes. Sometimes you don't. We actually have an air cleaner here that straightens the airflow out and will net a hp/torque increase.
The best I can do is get the A/F ratio as close as I can, get the water temp and
oil temp as close as I can and make a pull on it.