I just wasted the morning reading posts on different websites about engine vs chassis numbers. There are too many things to nail it down the but some guys have done back to back testing and have come up with a 16-23% range of hp numbers. I guess the best you can do is go right in the middle without doing it your self. Of course higher hp engines have a different range so I tried to pick out the ones in the 3-400hp range where most of us sit. They say a man car will generally be a couple of percentage points less that an auto. They say a Mustang will give lower numbers than a DynoJet machine as well. In the end it's just a best guess unless you want to pull your motor and take it somewhere for an engine dyno pull.
Dyno Day