There are a couple of methods you could use, I'm still thinking about what my approach will be for my side oiler.
On some blocks there may be a water passage with a screw in plug that could be removed. I will face the engine downhill, or even jack it up, to facilitate a better drain angle. You could use Evan's Flush, fill, run, drain and it takes dilutes and removes the bulk of the remaining water. Thats an expensive method as the Flush isn't cheap and you use it and then throw it away.
Another method is to simply increase the engine temp to about 280 degrees or so by blocking the radiator so no heat exchange can take place. Do that after you have drained as much as you can and refilled with the NPG. Any remaining water will "flash boil" and vent through your overflow tube or possibly the surge tank with a loose fitting or zero pressure cap in place.
280 engine temp (which your gauge may not be able to read that high, by the way) will not hurt the engine at all.
The reason we "worry" about engine temps exceeding 220-230 or whatever is for a couple of reasons:
1.
Water boils at 212, so were already close to the boiling point of our cooling agent to begin with, not a lot of wiggle room for temp's above that. So we "pressurize" the system in hopes of containing a boil over. Remember, once the water DOES boil, your screwed, you've lost all hope of retaining any heat transfer ability and the temp's from that point on can get extremely high. Especially inside the engine near the combustion chamber, where the water IS NOW steam. Local extreme temp hot spots will result, THAT is what warps your cylinder heads and blows head gaskets and things get real nasty. Otherwise, temps in the 280 range IF the coolant remains in a liquid state are not a problem, good heat transfer still takes place. NPG boils at 350 or so degrees with zero pressure, it WILL be liquid.
2.
The other concern is the
oil temp. You don't want your
oil temp over 250 in any case.
Oil temp is generally lower than water temp so this won't be a problem using the "hot purge" method.
Smoky Yunick, back in the day, did some excellent research utilizing very high engine temps and retainment of engine heat. His results were increased horse power and better mpg. Smoky is a legend for his various contributions to the automotive and racing world.
I've not decided on my approach, or combination of things approach as yet to purge all the water from the block. I MAY choose to run a very low pressure cap instead of zero pressure. My concern with zero pressure is any coolant that DOES leave the system and enters my coolant recovery tank MAY not be drawn back into the motor with a zero pressure cap???? Thoughts on that anybody??