Man oh man, did rhamm1320 hit the nail squarely on the head!!!!!! Excellent reply to the question.

I use a DynoSim Version 4.20 with Pro Tools and haven't had so much fun in years. The results obtained with my software against actual dyno pulls have shown to be within 2%, so I'm happy with that. But like rhamm said, it's the DIFFERENCE in hp and torque that you see from changing parts around that makes the DynoSim so much fun to use. You can build 5 different combinations of the same motor within an hour and all it costs is your time. It's not like buying the actual parts and doing an expensive dyno pull with them. Here's a link to four sims I did for a fellow on another forum. Note my disclaimer at the bottom.....
http://www.hotrodders.com/forum/whats-deal-155307.html