Boost Timing Retard Vs. Altitude

[Street_Kings]

I have noticed here in colorado, the cars that I have tuned can get away with less timing retard in boost without preignition as compared to cars i've done in lower altitudes. Has anyone else noticed this? or am I just seeing things wrong? At first I had thought it was due to lower atmospheric pressure, but then again when logging, MAP trace shows the same cells (which reads the absolute pressure vs. RPM,) so that would have no relevance. Any ideas?

CLIFF NOTES:
Q: Is preignition effected by altitude? if so, how/why?

[john]

Less oxygen per measured volume == less octane needed for the same tune at sea level.

You won't need to retard timing as much at altitude as there isn't as much O2 to burn, so the burn will seem/be richer.

Make sense? Are you carburated or FI?

[Street_Kings]

All were FI'd Honda's.

I didn't even think about that. Even though the Absolute Pressure in the intake stream is the same, there will be less oxygen molecules in the charge, which means less oxygen per amount of fuel, and more fuel means less chance of detonation.

BUT, if it were less molecues of oxygen to the same amount of fuel, wouldn't that throw the AFR's off also? I always shoot for a 12.5:1 AFR, regardless of where I am, yet the lower elevation engine can have timing less retarded to ward off detonation than the higher altitude engine, even with the same AFR of 12.5:1?

[HONDA GHANDI]

What john said. Plus you can get away with higher AF ratios here. Case in point, my civic is running at a near 13:1 ratio and .65 degrees of timing retard for every 1psi of boost. At sealevel the car pings but not here. I have a completly different tune for lower altitudes as I have for here.

[STIBungy]

Hence why the 18:1 Cavalier lives to run another day. :D