[Ragtop Man]

It's a bit of alchemy. Scratch that. A lot.

Actual Pontiac CR's were listed high to give the NHRA racers as much advantage as possible, production CR's could be down by as much as a half-point versus published.

IMO, it is better to stay conservative on the CR, and bump the ignition advance, than to go hog wild on compression and retard the spark.

But, a lot of things affect 'spark knock'. The load on the engine (low numerical rear end) ignition system quality and tuning, carb mixture distribution and A/FR, cooling system quality and capacity, exhaust system restriction and tuning, high bleed lifters or standard, and most of all, quench in the chamber.

The last is the most important, because if the piston is too far down the hole, combustion will take place in the dead space between the head and top of the piston, but it can not tumble and swirl in the same way it does in the chamber. You get a lot of knock and detonation, overheating, and generally pissy behavior. It is significant, because most shops will just drop a rebuilder quality piston down the hole and call it good, then blame the crappy gas when it doesn't run right.

Not having built a 400, but having seen the board's typical advice, with an iron factory head of good valve sealing and port matching, 9.0-9.5 static, zero decked, is about right, depending on cam carb and headers. The E heads can take more, all other parts equal, 9.5-10.0:1 because aluminum rejects heat more than iron.

This assumes all the other factors are working right, too.