I'm not sure about everyone's apprehension with 70C+ temps. With 95F+ ambient temps and at stock speeds, my Z9 runs 73-76C with the fan set to auto. When I move the fan to 100%, the temp drops to 66-68C. When I move the unit up to 750Mhz and set the fan to auto, it runs close, but not completely at 100% with the same 73-76C temps. If I move the fan to 100% at 750Mhz, the temps stabilize around 71-73C.
The specified operating environment for the Z9 Mini is 0-40 ℃ (32-104F). So in an environment that falls within those specifications, the software is trying to keep my Z9 in the low to mid 70s. That tells me this is a fine temperature for the unit.
I ran mine for a couple days in my 102-104F mining room. Clocked at 750mhz the temps were in the low 80C's. I've since moved it to sitting on top of my water heater with a heat pump (see my post above) just to reuse the heat to heat the water. Temps are now in the low 60C's, plus I get about 2000 extra Sols/s with the cooler temps.
I don't think anyone really knows what the correct chip temps are. There is a lot of talk these chips being more like GPUs or FPGAs. Most people try to keep their GPUs below 75C, but temps into the high 80C's are not really an issue. In case anyone asks, my temps of my gpus are 55C - 65C.
I'll be interested to see how both of your are doing in the next couple of weeks. Most GPU's start throttling at 80C in terms of performance but who knows how these will hold up.
One person in this thread has reported theirs died inside within a couple of days of extreme overclocking but you hear that about every version of Bitmains miners, there are always a few that have immediate or near immediate failures.
I guess it could be argued how much more profitable the miner could be at 15-16 vs 12-13k vs power usage. at 575Mhz mine are running under 66C with fan set on auto and between 12.5-12.8K, haven't pulled power usage at this speed though.
The net result for ~3k additional hashrate is running the chips at 10-15C hotter and who knows what power usage... I'm wondering if its worth it if the miner dies in the short term (under 90 days).