Engine coolant additive,which one ??

Haven't used it myself but I heard plenty of positive reviews on Redline water wetter.

Run distilled water a bottle or two of it for a straight up racing mix, or run a mixture of coolant, distilled water and water wetter for street driving.

Stock gauge? If so, it's not accurate. Redline water wetter or the purple ice will do about the same thing and work substantially better when used with straight up distilled water. Anti freeze in the system limits water wetter type product's effectiveness. If you live where it freezes you'll have to go back to anti freeze in the winter. If you were to use 60% water and 40% anti freeze and a couple of bottles of water wetter you might see 8-10 degrees cooler on a good gauge. With straight distilled water and water wetter you could see 20-30 degrees lower temps in certain ''perfect'' conditions.

All I use is good anti-freeze and distilled water. Zerex G-05 meets the Ford spec for our cars.

What happens when a cylinder head gets hot inside is coolant flows across the head's surface and immediately boils off of the metal surface resulting in reduced surface coverage of the water. That causes more heat and less cooling. What water wetter does is change the surface tension allowing the coolant to flow across the metal surface without boiling off until higher temps are reached. The result is more coolant in contact with the metal surface and cooler temps. Water wetter works best with straight water, no anti freeze and it also contains anti corrosion and lubrication properties for seals and water pumps.