I use desktop dyno and find it to be a good "guide" and decent at showing you DIFFERENCES but not ALL that accurate. Others have had experiences where it's within a couple percent, which is amazing.
I know I ran my 408 through desktop dyno and came up with some goofy numbers. I tried to manually enter my head flow profile and that messed everything up!
I picked heads from the program that seemed like mine and it was pretty close. It was giving me 500/500 numbers pretty much. Then I put in my head flow profile manually and remembered that the cam specs that matter are at .050" and I switched that as well. New results: 410HP and 475TQ or something like that. WAY off.
I find it to be better for seeing the difference between a single plane and dual plane intake (more HP, more TQ respectively), or how much gain you might see by adding a quarter point compression or 57 cubic inches
