This is interesting SC...I don't have experience with it, but you did peak my interest.
A couple of things I gleaned from reading about the NPG cooling system:
- It uses Propylene Glycol...a close relative of Ethylene Glycol you find in standard coolant
- It's designed for use in a non-pressurized system and may require the Evans radiator, coolant pump
In a pressurized system like on the Supra, I'm not entirely sure this would work well. Pretty much everything I've read says the max ratio of coolant to water is 60/40 to 65/35. I suppose you could convert the Supra coolant system to non-pressurized, but it just may be more trouble than it's worth.
JetJock did a good write-up on coolant at MK III Tech:
http://www.mkiiitech.com/forums/viewtopic.php?t=859
I do know that you would have to thoroughly flush the system to change over to the NPG. I also know Propylene Glycol is an organic hygroscopic compound with a definite affinity for water. It is non-toxic (in contrast to Ethylene Glycol) and is actually used in some cosmetics, food additives and "personal lubricants"
It looks like it gets pretty viscous as it cools, making it harder to push through your radiator...probably ok for warmer climates and the reason Evans has an "NPG friendly" radiator and coolant pumps in stock. It doesn't look like NPG will flow well enough on a street car at low speeds in a cold climate...a race car is a different story.
From what I've read, it's a poorer heat transfer medium than the water based coolants...that would make your temp gauge read higher even though there's no danger of boil over (NPG boils at 370 deg F). Another reason for a bigger radiator with very good air flow. The stock Mk III ECU might not like it either...when the coolant sensor detects temps over ~100 deg C, ignition timing is retarded (I can't tell the exact temp and how much from the book I've got) to prevent knock when above idle (IDL off). Also, the A/C will cut-off under high temp conditions. There's no way I know of to "tell" the stock ECU that the motor is not over heating. Of course, if you're using a stand alone, these points are not an issue.
Hope this helps