what would be a safe compression ratio for iron heads?

and aluminum heads?

 

Many factors to consider, first is induction system...carb or DPFI, with DPFI you can run one full point higher do to better fuel handling... aluminum gives better cooling to hot spots and allows around another point higher, so we got just around one full point more compression from the factory for those two additions in turn... another consideraton is the camshaft profile....long duration cams...(hotrod) tend to give much higher operating pressures on account of the way the duration, ramp shape and overlap is designed...static compression is probably secondary to cam design in determining factors for OP's....
now on top of that, roller cams come into play because much higher lifts for any given duration is possible due to using a roller on the cam surfact, which negates any weird wear patterms due to the cam grinding on the edges of the lifter, causing wear.....
I personally find in MY experiences NOT ONE instance of higher octane fuel having ONE even slight differance in audible pinging, especially with carb'd cars....not even once suspeced any improvement...not once...
also engine operating temps are important...need more cooling potential to cool a high compression engine....

so given all that BS above, in general so it seems the max allowable for MOST cams is about 9:1 for carbs and 10:1 for DPFI....this is somewhat overlapped between aluminum and iron heads....
aluminum is good for a bit higher due to cooling capacity...as stated above...

GENE :smash: :smash:

 

My recommendation: Refer to the manufacturer of your cam, tell him what carb, manifold and heads you have got, and he should tell you what maximum CR you should have.

 

What kind of engine are you talking about? I would say 9:1 for iron and 10:1 for aluminum as a general rule. You can go with more compression if you have more overlap on the cam. You may also have to have less compression if you have hardly any overlap. Higher octane fuel will help too but the first rule stated is good for pump gas.