This may stir up some controversy, but it wouldn't be nothing new to this board [img]/infopop/emoticons/icon_wink.gif[/img] I was in the midst of wanting to enhance my 455's performance in my 76 T/A without sacrificing factory appearance. With cost in mind, I was planning on spending some serious cash for some desirable Pontiac round port heads. I thought 197, 7F6 or 16 would be what I want. I figured who better to ask than Nunzi or Pete McCarthy.
On two separate occasions, I asked both which heads would be best suited for hard running street 455. I assumed that the SD head would be the most logical choice, but to my surprise, both said stay away. They both said that the SD ports are too large to keep a constant velocity in the port and that to take advantage of the port size, compression, camshaft and rpm capabilities must all be increased. Once all those factors have been met for optimum performance, the exhaust port must be extensively modified to match the flow of the intake port. Keep in mind, both said the heads are great performance heads, but for a low revving 455, flow is not that good.
Pete McCarthy further stated that the SD head was not a torque producing head. He said if it wasn't for the 455 having so much torque producing ability, the SD motor would have suffered greatly. He also stated that Pontiac engineers knew this and started factory installing 3 angle valve jobs, on the exhaust seat only, to increase off the seat performance. McCarthy and Nunzi both told me that the Edelbrock head was patterned more off the SD head rather than the RA IV that they claim. McCarthy considers a stock SD head a great disappointment.
When I asked both for their choice of what I should look for, both gave me the same answer, "455HO head milled .060" What was strange is McCarthy recommended the 7F6 where as Nunzi prefers the 197. When I asked for reasoning as to their choice, I got the following answers: McCarthy said the 7F6 is made to run on low compression and has a specially designed combustion chamber in relationship to spark plug area. He said that the "bump" can be ground out of the exhaust port and they will flow perfect for the stree 455. Nunzi prefers the 197 because "you can-a get more for less", meaning that you can get more flow with less work not having to grind the bump. Nunzi says he prefers the 197 over the 7F6 because it is basically a 614 with more cc.
As I discussed more and more with these two Pontiac builders, I realized how true there findings are. High flow numbers are not always the greatest factor. There is a optimum number, but its how much it flows in the low range. I had the oportunity to discuss this with Jim Hand and he feels the same way they do. When you get into high flow numbers, there is a price to pay and that is low lift flow. As I was told by some of the best, I forward this thought on to you; Any head may flow a high number at .550" lift,. but think of how long it is at that point. It reaches .50", .100", .200", .300", .400" and .500" twice a cam revolution. Higher flow numbers at those points are where the power is made.
I hope I have explained this so that I have not confused anyone. I am just relaying what I have been told by a few of who I consider some of the best in the Pontiac world. If anyone needs any clarification, let me know. Rocky
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