They basically used math, knowns, etc, they also went from gross to net @ 71, but they also lied especially on the hit motors like the HO's and RA's because GM had a corporate rule that said nothing over 1HP per 10LBs of vehicle weight shall sail, period, so if you check your weight you'll find the hot motors suspicioulsy top out at @ 360, 366, 370 HP WHICH IS NO WAY, NO WAY, A 400 with good heads and 10.75-1 comp and the 068 or 041 cam is going to be in the 400-425HP range, has to be, I laugh when I read about the factory comparing a 69 RAIII at 366 HP and the RAIV at 370, right it only jumped 4HP with the 041 cam and 1.65 rockers and round port heads, yeah right.