Engine analyzer (and pro) was off by ~4HP for me when compared to the actual dyno. You have to have all the flow data entered properly as well as the little stuff like rings, water pump, etc...
My DesktopDyno was way off since it guessed at port flow.
I could not get DD2000 (demo copy) to match either.
You'll pay a lot more for Engine Analyzer however.
1970 Cougar, 3660 lbs
418W, C6, 3.50 posi, slicks & street trim
Best 1/4 mi: 11.24 @ 119 MPH Best 1/8 mi: 7.13 @ 96.32MPH