160-degree thermostat too cold?

My 408 has a 10.5:1 C.R. with ported "J" heads and has been detonating pretty badly on 91 octane, so I thought I'd try a cooler thermostat. I bought a 160-degree t-stat, and when I removed the old one I noticed that the engine builder had installed a 195-degree unit. What will be the effect of installing the 160-degree unit? Is it too cold? If so, what is the minimum temperature t-state you would consider using?