The dreaded prod pinch. This relates to a 440, but will be of interest to some.
I brazed up the prod hollow on the heads so that I could just about eliminate the prod pinch. Moved the rockers over etc. After about 3 mths of street use, noticed a plug oiling up. Sure enough, braze had cracked. I replaced the heads with another set that had the ports cleaned up, sharp edges removed etc. Car performed better everywhere in the rpm range with the new heads. Lesson learned. This was before I read any of D. Vizard's stuff. He divides the intake port into 3 sections, with the pinch being the least restrictive. He gives flow numbers for each section in his SB book. 300 cfm for the pinch area, 200 cfm at the short turn & most restrictive 140 cfm at the v/seat.
He says this: '...you should consider that this section [ pinch ] section of the port is already a straight shot into the cyl head. It is close to 100% efficient, & until the valve flow is substantially improved, the prod pinch point is is not a flow impediment. Sure, at full valve lift it may cause a little flow loss, but remember the valve is at full lift only once in the cycle. It's at half lift twice, so what happens at half lift is more important.'
I brazed up the prod hollow on the heads so that I could just about eliminate the prod pinch. Moved the rockers over etc. After about 3 mths of street use, noticed a plug oiling up. Sure enough, braze had cracked. I replaced the heads with another set that had the ports cleaned up, sharp edges removed etc. Car performed better everywhere in the rpm range with the new heads. Lesson learned. This was before I read any of D. Vizard's stuff. He divides the intake port into 3 sections, with the pinch being the least restrictive. He gives flow numbers for each section in his SB book. 300 cfm for the pinch area, 200 cfm at the short turn & most restrictive 140 cfm at the v/seat.
He says this: '...you should consider that this section [ pinch ] section of the port is already a straight shot into the cyl head. It is close to 100% efficient, & until the valve flow is substantially improved, the prod pinch point is is not a flow impediment. Sure, at full valve lift it may cause a little flow loss, but remember the valve is at full lift only once in the cycle. It's at half lift twice, so what happens at half lift is more important.'
