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EITHER buy some Pentosin antifreeze, or go by a BMW dealer and get some BMW antifreeze. Mix 30/70 per John's instructions. Alternative is Zerex Z05 available at O'Reilly's.

Very few brands are truly phosphate free. I have never had issues with any of the above. All the anti freeze with phosphate will cause that white chalky build up.

JMHO.
 

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1969 Camaro Restomod
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Adding water would raise the temp rating of the mixture and creating a weaker mixture that cant handle real cold days. So instead of -12F, it could be at or above 0F depending on amount of water added. Being in New England, you should prepare for power loss to the buildings for a day or two and no heat in garage for a prolonged time. I would add more straight coolant until you got enough to cover temps expected in the area for the winter plus 10 degrees. That is what my dad did for his 79 years of living in Maine. My 50 years of auto ownership, I have never needed any coolant rating value less than -15F and that was in North Georgia. Don't risk a block or heads thinking heat failure is a very remote possibility.
According to the chart printed on the Prestone bottle, the straight 50/50 mix I've been using for years is supposed to be good down to -37 Fahrenheit, and a 40/60 mix good to -12 degrees. Should be fine here in my area... I haven't seen temps inside my garage much lower than 25 degrees. My car has never been parked outside overnight since I've owned it. I've got a Koehler whole house generator so I'm not too concerned about power outages, and if I need it, I have heat in the garage. If I go with a 40/60 mix I could easily add more coolant for the winter months if I thought it was needed. I ordered a refractometer that should be here by Thursday so I can double check my little Prestone floater tool. I think adding straight coolant to a 50/50 mix would hurt my cooling efficiency severely. Might be fine for someone up in northern Maine but not necessary for this car and it's limited use.
 

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My math skills are rusty so I tried using the Google machine to see if they could give me a conversion chart LOL. No such luck. So typically, we have about 23-24 quarts of coolant in our BBC cars, and typically, we buy 50/50 premix stuff up here in New England. How much of my 50/50 coolant would I have to drain and replace with ionized/distilled water to get a 30/70 or a 40/60 mix? Starting with straight, undiluted coolant would make this simpler, but I'm not dumping 6 gallons of fresh coolant to experiment LOL. So calculating using the 50/50 that's in the car... what's the formula?

I came across this page "QUEST FOR THE IDEAL COOLANT" and the chart there suggests that a 70/30 mix would be OK down to 7° Fahrenheit and a 60/40 mix would be ok down to -10° Fahrenheit. Strangely, they suggest that the boiling point in either case would be 220° Fahrenheit. Based on the chart and article, here in Western New England they'd recommend the 40/60 mix or maybe a 35/65 mix if your car is safe in a garage that never hits 0°?

View attachment 281209

Check out the chart that Prestone prints on the bottle. A 40/60 Coolant/Water mix says its good to -12° Fahrenheit and the boil over protection is 260° Fahrenheit.
If you have 24 qts in the system and it's 50/50, there are 12 qts of coolant in it now. If you want a 30/70 mix you need to end up with 0.3 x 24= 7.2 qts of coolant. You'll need to replace 9.6 qts of 50/50 (12 - 7.2 = 4.8 qts of coolant times 2) with 100% water. For a 40/60 mix you'll need to replace 4.8 qts of 50/50 with 100% water.
 
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