Distilled water for sure, and about 75/25 mix. Like mentioned, you want antifreeze to up your boiling point, water is 212, way too many GTO's hit that on this board so stay far away from straight water if you have a bunch of stock cooling parts still.
RMI 25 is the most referenced additive for cooling, helps with core build up and reduces temps better than any other. I know a few guys who had to flush the system after a month of use due to it cleaning up the core ( older core though ).
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69 GTO Convertible, 4000lbs
462ci, 606tq/569hp - 93 oct @ 34 deg (207psi)
11.7:1, KRE H Ports, Lunati HR 282/290 w Johnson Lifters & 1.65 Scorp, E30, EFI, Holley HP + Dual Sync, 12-1 Crank Trig, 120lb Inj & 1000cfm TB, Torker II EFI Int & Rails, PTC 10", Rock Valley SS Tank, Dual 450s, Aerom EFI Reg, Aero Front & Wilwood Rear Disc Brakes, Dougs 1 7/8" & Borla Pro XS 3", Alum Rad & Dual Fans, 12:1 Box, UMI Arms & Viking Berz Fr + Rear CO Shocks, Hella UP5.0 Vac Pump, 12 Bolt, 3.73, 33 Spline
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