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Two years ago we undertook an exhaustive study of the Vortec head in numerous modified states with different valve sizes, throat cuts, valve jobs, port mods, guide mods, etc. We used up about 10 heads, numerous valves, and about $50,000 worth of labor. The study generated 100+ pages of flow and swirl data.
Here is the short version. Out of the box, .480 valve lift, 350-400 HP dependant on the CR, cam, ring seal, oil control, blah, blah, blah.
Larger valves increase flow, chamber mods not needed, trade off between shrouded vs unshrouded valves not worth the decrease in laminar flow and swirl.Throat cutting behind larger valves compliments the larger valves. Open the throat to the seat, remove the edge left by the cutter in the port.Blend the seats into the chamber, you don't want an edge here to disrupt flow and create turbulence.
"Bowl blend" and shortened guide in port also improves flow. Taper and blend the iron boss.
Minimal porting increases flow, too much increase in port size or loss of the shape of the stock port will decrease efficiency.Vortec heads like straight stemmed valves. Undercut valves create unwanted turbulence and a decrease in intake charge velocity (they add volume (slowing the gases)to the overall "port" volume just behind the valve where max velocity is required.)
Generally speaking, Vortec's stall at between .500 and .550 valve lift. This is where flow actually begins to decrease. But their true strength is low lift flow which gives more area under the total flow curve. And if you think about it how long are your valves at peak lift?
They spend much more time at .400 and below, where the Vortecs outperform most other heads. This combined with high velocity, lack of turbulence and superior combustion chamber design are where the Vortecs stand out.
Unported, with all the other tricks in place, the Vortecs will flow about 235-240 CFM at .500 I and 165-170 cfm at .500 E, on a 4" bore at 28" . With some careful porting there is another 5-10 CFM or so to be had. But again the low lift numbers are unsurpassed at .100, .200, .300, etc. lift.
For example the Vortecs flow as much air at .400 as .500 and no 23 degree head that I'm aware of can match them at .200-.300 lift for the combination of flow and swirl. Even the Fast Burn head can't touch them at low lift, it's ports are too big (flow is similar, swirl is less), it does of course out perform them at lift over .500.
Unported Vortecs with the "tricks" can produce 500 HP on well built, high CR, drag race short block. 425-450 HP is more realistic for a killer street engine running on pump gas.
Your are wasting time and money runing a vortec head engine with an extreme high rpm cam,and intake, especially on a larger 383-427 displacement engine, the heads are designed to make massive torque in the 2000rpm-5500rpm range on a 350 displacement engine, they will not efficiently feed a 7000rpm combo on a larger engine so build your engine with that in mind
