My SM heads replaced stone-stock 360 heads that were in good shape.
So, depends on how it's looked at. I could say "not much" because it dropped about 2 solid tenths... maybe 2.5.
But then I can say my cam is so small it didn't take advantage of the heads. My cam is 204@050 .427 lift.
I also used the factory stock rocker assembly/pushrods. So, with geometry loss, I can imagine my lift was probably an actual .410 or less.
At least a half tenth would be attributed to the weight loss, not flow.
So then I can say the 2 - 2.5 tenths isn't too bad... when all is considered. For 700 bucks fully loaded delivered to you door, I say they are a great deal. Just a valve job with new springs and guides on the cast heads can come close to that, especially if you have to buy new valves.
