SBM Edelbrock/Speedmaster Head Data
I think it could be chamber shrouding on the long side. The valves are sunk 0.080 to 0.090. This means there would be that much additional shrouding on the plug side. Look how quickly the flow rises at 0.700-0.950 lift. The valve is getting clear of the chamber. I bet a surface cut on the deck face would clear it right up.
I can say the best set of heads I did was a BBC casting. I forgot what castings they were but I had to use a bunch of filler in the ports bit at 1 inch of lift the flow curve was still going up.
And by that time I had sold my bench so a third party did the before and after testing.
With a tunnel ram and 555 inches it went 1100 NA and he dropped a 400 shot on it.
He rattled the tires so hard so many times it knocked some of the filler out.
The guy who fixed the heads took all the filler out and the engine never made the same power.
Those heads were only flowed twice. Before and after. I had done enough of them that I went for it.
That was with 50 degree seats. I probably should have been using a 52 degree seat on the exhaust.
My point is you are drinking straight from the fire hose!! Eventually you’ll get to the point with certain castings that you will know where the issues are and you’ll just flow the port where the issues are to a make sure you have them nailed.
All this data will pay off big time down the road. Any tests you can go and any test you can think of will give you information.
Keep pounding on it. BTW, a low port 18 degree head is a tough way to learn.
All that data will make it so you can charge enough to make porting worth it.
Once you get the flow numbers you want, you can go to the dyno. Then the track. When all three agree you’ll be making smoke. And you can charge extra for it.
Edit: the BBC castings were Brodie. I can’t remember the exact head but it wasn’t my first choice.