This is my first post on this site I just joined today. I have always wonderedaboutsomething. I know the 4.3has alwayshad Vortec heads on it, but in the ThrottleBody daysnone of the engines were ever advertised as Vortecs until 1996 and the power ratings looked like this (in 1995 full size trucks) 4.3=160 HP 237 TQ, 5.0= 175 HP 265 TQ, and the 5.7= 210 HP 300 TQ. The 1996 and up was rated at 4.3 200 HP 260 TQ, 5.0 230 HP 290 TQ, and the 5.7 260 HP, 330 TQ. The 4.3 went up 40 HP and 30 TQ and the 350 went up the exact same numbers, the 305 went up 55 HP and 25 TQ. So did the 5.7 already haveVortec or some othergood heads on it or what? The 4.3 has been a Vortec since 85, and the 350 only increased in power the same as the 4.3. The 305 got a big 55 HP gain showing it is the capable engine I belive it to be(don't hate mefor loving the 305 please). I'm not sure of the numbers(I think 240HP380 TQ to 290 HP 410 TQ)but I think the 454 gained 55 or so HP and 30 TQwhen they started calling them Vortec. So are the 305 and 454 the onlyV8's to really get new heads or were they just choked that bad?

 

There were a lot of changes to the 96+ engines that resulted in the net gains you show. The 96+ 4.3L engine was rated at 190HP/250lbft. Without looking up the rest, I cannot comment on the gains you show.

I believe that the CPI vortec motors (said vortec in large letters on the upper plenum) were rated slightly higher than the newer ones, but had very different HP & TQ curves than the current motor.

Things change, designs improve overall, not just in intake, chamber, and exhaust designs, but also in the bottom end, cam, and intake designs.