Brake HP, Net HP what's the truth?

[Unit 5302]

For a while now I've been thinking about the differences between brake hp and net hp.

From what I understand, net hp is the engine's rating with all the accessories hooked up, running like it would on the street.

I've also gathered that brake hp is the engine's rating when only the necessary accessories like the charging system and waterpump/fan are connected. So it'll be less accurate than net.

How much will net hp differ from brake? I've heard anything from a few ponies to 30%.

I've decided a commonly quoted 30% is unreasonable. That's like saying the 87 5.0HO actually has 322bhp, and a modded 5.0 with 300rwhp is making 500bhp naturally aspirated. How many people really believe it cost 100hp to run the accessories? You'll only pick up a couple tenths by bypassing them, which would indicate maybe 20-30hp. The air pump uses about 5, the power steering pump maybe 10, and the A/C pump maybe 5 when not in use. That makes 20hp difference vs brake hp. Not 100hp.

I've begun to become annoyed when people re-rate the older hi-po motors in net hp by taking 30% off their power ratings. Say a 427 dual quad, rated at 425bhp would now have 298hp, and a sick 250rwhp. Now we all know the old motors were conservative, but come on. Even if we say 500bhp, that's still 350 net. Knowing the 427 is pretty wigged out from the factory, could a 300hp or 350hp car move a 5500lb '63 convert to a low 13 stock? (my dad ran it, I've seen the timeslips) I say, no way! Plus it would seem to me, since the 427 had no A/C pump, or air pump, it would not suffer as much as a modern engine in rating.

Anybody want to chime in?