Hello
I have this fans
Silverstone Tek 140mm x 38mm Fan for CPU Cooler and Computer Cases Cooling FHP-141
but Antminer on 350 mhz have the 75-78C temeperature
More overclocking is impossible
but the sound is much less
but the room quickly heated as the air coming out is much hotter
140mm just means you're sending more air around the heatsink, not through it.
If your thermistor reads 75-78°C I can promise you that the chips at the back of your miner are at 95°C or hotter.