I did an experiment today while winterizing my vdrive. I recovered and measured the volume of coolant/water in the vdrive, intake hoses,block,heater,exhaust manifold and muffler. I recovered 20 litres or 5 gallons of water. The muffler holds a fair bit of water. So for all those who think sucking up 5 gal of plumbing antifreeze is good you are mistaken. You need 30% prop glycol for protection to -40. If you are using 100% prop glycol then you would want 4 gallons of it. If your using -50 plumbing antifreeze you would need 40 gallons of it.
So unless you have 100% prop glycol you need to drain your system to get any freeze protection IMO