It is weird though. I used several cars that engine dynoed about 15% higher than what this calculator shows.
I used a 500" Mopar that we dynoed at 512 HP. The calc shows 425.xx hp using the trap speed indicator.
I also used a blown SBC stroker that dynoed at 526 HP. The calc showed 432.xx hp using the trap speed indicator.
Last but not least, I used the Beast. The engine dynoed at 813 HP. The calc showed 681.xx using the trap speed indicator.
Who knows what the truth is?? ... lol
Also, there are several folks using this calculator, (or ones similar), that get almost the exact same HP numbers with the calculator and the chassis dyno, which measures RWHP.
Either way, these calculators are pretty neat if you like fiddling with numbers....