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The stock GM thermostat is set to be fully open at around 195 - 203 deg F.
If you plan to take it out and test it, then keep in mind that the rubber seal (GM #10226107) is a 'one use' part and attempts to reuse it may be a waste of time and coolant, so have a new one on hand from your local GM dealer.
The only acceptable alternative Thermostat is the Stant (part #14068) which is fully open around 180 - 185 deg F. The seal that comes with it is not as good as the GM seal which is tapered on one side. You may also wish to drill a bleed hole in it, between 1/16 or 3/32" should be sufficient, and when you install it, remember that the bleed hole should be in the 12 o'clock position.
In general, lower temperature thermostats on electronically controlled engines will keep keep temperatures lower resulting in richer mixtures, more advanced spark, and some increase in power at the expense of fuel economy. A small but usually negligible impact on emissions will also occur. This assumes the engine temperature regulates around the thermostat opening point.
Removal of the Thermostat:
- Raise the car on the passenger side.
- After fiddling with it, .... install a new seal and clean the two mounting surfaces of the thermostat housing.
Install the thermostat housing and hose back onto the radiator. Pry the housing back and install the thermostat (and seal), being careful to position the seal properly (taper side of seal facing out). You can try using a thin bladed screw driver to hold it in place. Then gently position the housing in place, install the 2 screws, lower screw first, then the upper screw and the bracket, etc.
I considered trying to use crazy glue to hold the seal in place, but didn't. The alternative, is the procedure in the GM service manual which entails removing BOTH haves of the thermostat housing and 5 hoses. It appears to me that this is extremely difficult to do without the radiator removed.
If you desire an all Aluminum radiator, Fluidyne (www.fluidyne.com) makes a drop in replacement for the ZR-1, list price $620, and a dealer offered it to me for $520. I got a new GM radiator for my 93 for $300, hopefully it will last. Other radiators, such as the Lingenfelter are NOT drop in replacements.
Another fun task thanks to the good job the GM engineers did of DFM (Design for Maintenance). There are two problems that one will face when removing the upper radiator shroud for the first time. One is that the 7mm sheet metal screws on the lower sides of the shroud are difficult to reach and require a 7mm U-joint socket to get to them.
Another trick suggested by Jim Milstead is to reverse the direction of the 7mm sheet metal screws (held in place by Tinnerman nuts) by filing some material from the lower shroud and moving the nuts to the upper shroud. The distance of the hole from the edge of the shroud is 3/8". You will have to file 1/8" from the lower hole and 1/16" from the middle hole. Use a coarse file or a hacksaw blade (and holder) to do it.
If you wish to remove the thermostat, do it with the radiator removed, in the case that you have to remove both halves of the thermostat housing to get the seal to seat.
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ZR-1
Net Registry
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In
either case, after removing all the bolts (including the 5 holding
the fans to the shroud), you lift the shroud up and back on the
drivers side, and then on the passenger side, clearing the oil
cooler and AC lines. The rest of the procedure is straight forward
and doesn't need any explanation. 
