As stated older cars were rated by gross horsepower #s which were often inflated even more than the lack of accessories would have accounted for. Only a handful of engines from that era had ratings that were close to reality. Early 340's being a good example. Late models from every manufacturer seem to be pretty close. Lastly the dyno #'s used by car mag's and most engine builders are much more like the old car #'s were, which is to say that after you pull it off the dyno and actually install it in your car the #'s will fall. Anyone wishing to have an engine dynoed would be wise to ask just which correction fact was used to determine the output. IMO dynos are a very useful tool for determining the effect of changes, but the actual number isn't all that important. In the end the only numbers that actually count are time slips. Vehicle weight and trap speed tells more about actual power output than dyno #'s that vary according to individual dynos and their operators.