4.3 Vortec intake port stalls above .430" lift

[GNNOVA]

I have a set of stock 2001 4.3 V6 Vortec heads that I am porting. In stock form when I flowed the intake port, it would lose about 4-5 cfm above .400" lift. When I did some light pocket porting, the stall point increased to .440". When the lift goes .010 above .440, the flow will hunt up and down with the test pressure hunting up and down. When test pressure goes up, flow goes down. When test pressure goes down, flow goes up. If it goes to .460 or above the flow will drop 4-5 cfm and stay there regardless of lift. I'm pretty sure this is a turbulence issue in the pocket area. I have not flowed any V6 or V8 heads so just looking for some info from someone that has work with the Vortec V6 or V8 SBC head (the ports are the same) before I blindly start grinding. Thanks in advance

Ken

[GNNOVA]

I found some info on Youtube about porting the Vortec V8 head so I used it to do some porting on my V6 Vortec head. I opened up the area under the valve to to 85% of the valve diameter and blended, reduced the valve guide boss size, smoothed the back cut on the valve and unshrouded the valve in the combustion area (this provided the best improvement for the work done). These mods stop causing the flow to go down above .400 and increased the flow though out the lift range as much as 20cfm. It still flattened out above .400 but at least it did not go down.