Ideally 195 degrees. Any cooler and incomplete fuel combustion. However as high as 210 is ok. This assumes a 50/50 mix of antifreeze and distilled water. Yes distilled water. You want to screw up a $10.00 gal of antifreeze because you were too cheap to buy a $1.00 gal of distilled water. Advantage, no minerals or other chemicals in the water... Also recommend a 9 lb. Closed system radiator cap.