i made up a spreadsheet with formulae derived from decades of (someone else's) empirical research. it's more for analysing engines, than for predicting power output. the above numbers are actually very decent starting points. fabbing an intake can be a wonderful learning experience - and minimal economy benefit. can you even get "eco" cams for the 250?
fwiw, 1D simulators like engine analyzer pro aren't accurate or useful. i just got a quote today for professional software that costs $18,000.00 for a 1 year license! (or buy 2 get 1 free
the amount of information you need to input is staggering (and what it can do is mind blowing!). not many professionals have this software, and nobody in Canada does. I would love to buy a license this year. It amazes me how much most OE and pretty much all aftermarket miss the mark.