Dyno tests

A good clue to the answer is in the HP derating that occurred when the HP ratings of US engines went down, from gross HP to net HP. Similar changes in test methodology took place; gross HP, prior to the change, was without the pumps and fans, etc. (Dunno about the exhaust for gross tests, but one might resonably suspect that gross HP tests were done without normal manifolds, etc.) 10-15% HP drops were the norm when that change in HP measurement methodology was adopted.