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I run a 180 thermostat with reprogrammed fans. This keeps me in the "sweet spot". 187-189 and oil temps in the 200-210 range.
According to tuner : He's found that the ls1/ls6 engines tend to make best power with h2o temps at 187 degrees.
Pointless statement by your tuner in my opinion. Depending on the ambient temperature, it would not be unusual for the coolant temp to reach 230-250 deg F and the oil temps 20-40 deg higher than that under high load track conditions. If in fact you are interested in making optimum power under high load conditions this is the range I'd be most concerned with.
Your radiator would have to be enourmous to maintain those operating temperatures under high engine loads to make "best power" as you have stated.
I'm not sure what temp is best for our C5s, but Fireball Roberts used to run his Pontiac at 240 degrees in his Nascar runs. I run a 180 with a Dewitts radiator to keep the extra ponies within a decent cooling range.
I run a 160 stat and had ECS program my PCM to work with the 160 properly. I run in the desert of So Cal when it gets above 100 degrees. In stop and go traffic my car would get above 220*. Now it runs in the 175-185 range and even in stop and go traffic it does not get above 210*. I am happy with the 160 and the way ECS reprogrammed my PCM so I guess it boils down to each his own...