Lou,
your observation on the relationship compression - propellers/rpm is right, but my guess is that you confound the cause with the effect.
Hcci charge starts firing basically at a given temperatore for a given stoichometric ratio. Now two are the reasons for which the temperature of the charge rises up to the ignition temperature. The first and most obvious is that the charge is subject to an adiabatic compression, the second, absolutely not to forget , is that the charge is heated from the hot liner, piston and head and this heating depends from walls temperature and time. As far as this heating time increases due to low rpm, this heating plays a bigger role in determining the temperature and following ignition of the charge. On the other side if you run speedier, for avoiding the ignition delay due to the reduced heath contribute from walls, you have to compensate with an enhanced temperature contribution from the adiabatic compression. This means you have to rise the compression setting.
Ugo