ProComp/Speedmaster aluminum heads
I'm not at all claiming that there's enough flex in an OHV head to account for 1.5 pts of compression.
That number is likely as old as replacement heads for flathead motors, which I could see flexing enough to account for 1.5 pts.
The thing is, most engines could get away with far more than +1.5 points without ever thinking about head material and so alum head makers can make that blanket suggestion without risking much. The +1.5 myth probably also helps sell alum heads because any performance gained would be from the compression, not the head material and no one wants to spend money for nothing..
However, head flex is 100% a thing. All heads 'lift' to an extent when under load, and all metal moves and deforms when under load. How much is highly variable, but alum tends to move twice as far as steel/iron for the same load, so in my mind the extra compression to account for deformation is the only thing which is based on science - but how much adjustment is needed? Likely not much if the engine is worth two ***** to begin with.
The last paragraph nails it. There isn't a cylinder head on any engine ever that doesn't lift and flex. There has never been one.
If there was, you could machine the head and block to flat and smooth and not use a gasket.
That's why you need a gasket. The head lifts and flexes and moves all over hell. Without a gasket you'd never get it sealed.
As one more proof I'm not nuts...if you use dead soft copper gaskets you know what a beeeeeeeeotch to get sealed. Not compression sealing, but coolant and oil sealing. That's because that dead soft copper gasket can not follow the head as it moves away from the deck and flexes with every single firing cycle.
My buddy had a blown iron 392 hemi we built and I begged him to let me Oring the the block and heads but some gasket maker promised him his dead soft copper gaskets would seal 10 pounds of roots boost without Orings. So I did it without the Orings.
It was in a roadster. When we fired it up and got it warm I walked around the engine and stuck my hand down by the head/block interface at all 4 corners. He said WTH are you doing...and I say I'm feeling the combustion leaking out past the gaskets!!!
So he sticks his hand down there and he feels it. He was pissed. That very day the engine came out, I took it back to work, pulled it all the way down and did the Orings.
You couldn't feel the combustion leaking at idle any more, but I promised him at full boost and full song it was leaking!
It's just the way that it is.
