Here is an article saying not to go with cooler thermostat on a 3800 Gm engine.
from (
http://engine.firebirdv6.com/) check this out scroll to near bottom of page
some more info here!!
The 3800 comes stock with a 195 degree (96-97) and a 180 degree (98+) thermostat. A common mod is to replace the thermostat with a 160 degree to increase fuel enrichment, the real facts are:
"A 160 just lowers the initial opening temperature, you still can run at 200 degrees or more, thereby creating wide temperature variations to and in the block and heads. You want your engine as HOT as you can get it without pre-ignition (which triggers the Knock (timing) Retard). Efficiency of the engine goes up the more heat the cylinder heads retain - up to the point of pre-ignition, all other things being equal. So you want it just cool enough so that the PCM does not retard the timing. A cool engine draws heat out of the combustion process - less vapor (combustion) pressure, less ability to do work (push the piston down). Not to mention the possibility of oil contamination from unburned fuel (due to lower temps). Pre-ignition can be reduced also by using higher octane gasoline. Reducing the temperature also impacts the ECT sensor, as discussed above in "PCM Operation." The reported ECT set temperature is 170 degrees, thereby forcing the PCM into Closed Loop Condition by "timing out." At which point the the desired goal of increasing the fuel enrichment goes away anyway, as the HO2 sensors will control the enrichment to maintain 14.7:1."
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