If at 5,000 feet, with 10.5:1 compression, is an engine less prone to detonate on 91 octane than it would at let's say seal level or 1,000 feet?

 

Yes

 

For each location density altitude changes every day.

 

An engine is more prone to detonation at lower altitudes. Higher altitudes can get by with less octane and more timing advance.

 

Think of higher altitudes like running a restrictor plate motor. The reduced air that reaches the cylinder reduces the cylinder pressure. Reduced cylinder pressure allows for less octane and more timing as previously stated.

 

Would one be "safe" at 5,000 feet with 10.5:1 compression with iron heads, 91 octane and 30 degrees timing?

 

Was the compression ratio from measured data or from published head and piston specs? What is the seat timing of your camshaft? What is the lobe separation and advance? Did you degree the cam in? If so, what is the actual advance or retard?

A small block Oldsmobile set up with true flat top pistons and an as cast iron head will be around 10.25:1. With a camshaft of around 310 degrees of seat timing on a 104 intake centerline, it’ll run on 93 octane without problems at sea level. If you run the factory W-31 cam with it’s long closing ramps and 113+ intake centerline, 91 octane is possible.

But, if you haven’t measured anything and are seeing if it will run on your current setup, go out and test it. Start out low on the timing and see if it knocks with a load on the motor.

 

YES, if the engine is solid.

 

That should be no problem.
I’ve done a bunch of iron headed 455’s for guys in CA. They were all 10.0:1 and run fine on 91. The tune and combination are key.