Not sure what you mean by "loose" or "lose".
You certainly lose HP through your drive train and a chassis dyno will tell you what you're making at the rear wheels. Engine dyno would tell you what you've got at the flywheel. Not even sure how engine dynos work...wouldn't the motor have to be out of the car for that? ::
Yes, the engine must be out of the car. I had my engine on the engine dyno before it went in the car. It was fun watching the process. The engine is hooked up in a little room. It looks like it is in intensive care with all the wires and stuff around it. From another room, the engine is ran and monitored. I think more things can be monitored on an engine dyno. Mine was tuned on the engine dyno, although once in the car, it was set too lean. I had to bump up a couple of jet sizes.
Oh yeah, forgot to mention...word is that they'll be raffling off some sort of dyno session with Blood at my club's show this weekend. I probably won't make the show...but if you're in the area, drop in and buy a ticket...you might get lucky. ::
If you have the money, do the engine dyno. If you want street numbers, numbers you can say "meet the ground", do the chassis dyno. It would be nice to have both though. If you had both you would see how efficient/inefficient your drivetrain/chassis really is. I know the shop that I did my chassis dyno at told me to "leave my ego at the door". A lot of people leave a chassis dyno with their jaw on the ground expecting much higher numbers cause they "think" their motor makes this much power. ::
I went in hoping for 300RWHP, I left the shop with 270 and some tuning to do.