Originally Posted by 1989GTA
I actually did this on my 1989 Trans Am. I built up a 369" SBC motor and put it on an engine dyno and it made around 530hp. Installed the motor in the car and put it on a chasis dyno and it made 420rwhp. Now you have to keep in mind these are different dyno's and the calibration's are most likely not the same but in this instance it tends to support the around 20% loss. This was with a 4L60E transmission and a Yank SS3600 converter.
That's the kind of data that I was hoping some people would share. With enough data points like this, one could come to a reasonable conclusion.