@XRTEngineered
Thanks for the comments. I'm definitely not advocating using Evans and to keep on riding even when you are overheating. It's common sense (I hope) that one should avoid overheating your engine at all much less intentionally run it long periods of time in an overheated state. I do watch my temps and pull over, or otherwise cool it off, when temps overheated. It's only that if I happen to not catch it, Evan Waterless doesn't boil off. You are correct that a higher pressure cap and/or catch are also viable solutions if using standard coolants when one happens to not notice temps overheating.
I agree it's meant to be used pure, however, you can top off with standard coolant/water/snow perfectly safely in a situation where you have no other choice. You'll just have to drain, prep, and refill with pure Evans when you get back home. An inconvenience and also a minor expense, but if you are in a situation where you need to use other coolants/water, you can. Nothing bad will happen as far as compatibility and you can even continue to use it mixes. It will just behave like standard coolant and vaporize/pressurize if there is more than about 3% water in your system. I'd argue that you typically won't have standard coolant available on the mountain either and, if you add snow, water from your water jug, etc. you are going to be in the same situation when you get home; you'll have to drain and re-fill to get your water/coolant ratio correct again and to get out any contaminants, hard (non-distilled) water that you sourced on the mountain.
Your points about Evan Waterless thermal properties are noted and I was aware of those, but didn't see it as much of a con for snowbike use. We are using thermostats and temp gauges and are typically running in a cold environment rather than hot summertime trail/motocross use. If a particular engine's cooling system was always unable to keep that engine from overheating, or always right on the verge of overheating, during it's typical use, I agree that Evans thermal properties may be a con in that situation. In our case, we have a temp gauge keeping us up to date on engine temp and a thermostat that is opening and closing to control temps. Part of the reason I switched to a tunnel cooler was to give even more cooling overhead and so the thermostat could open and close as needed. Agreed there could be localized hot spots in the head, but we have that when standard coolant is vapor rather than liquid on those same hot spots since vapor has poor heat exchange compared to liquid.
@everytime5.9
Agreed that poorer thermal properties in a hot environment very well could require a larger radiator/heat exchanger, higher flow rate over the heat exchanger, etc. However, snowbikes are ridden in a much cooler environment than dirtbikes and so we have an environmental temperature advantage compared to riding in 100+ degree heat. I have no idea the calculations, but I think it's fair to say that -10F air hitting a radiator will pull much more heat out of a cooling system than 100F air hitting the same radiator. There is more to it than radiator size; air temps, air speed over a radiator (fan or riding speed) and rpm (which increases waterpump and coolant circulation speeds) play as much a part. And, since this particular thread has us swapping to tunnel coolers, we effectively are enlarging our heat exchanger capability greatly over stock radiators. Then, are actively using a device to reduce that heat exchange (a thermostat). In the big picture of cooling an engine, the only time reduced heat exchange becomes an issue is if one's cooling system is flowing at 100% capacity (no thermostat or thermostat open) in your normal operating environment but your engine is in a constant state of overheating (single track riding in high temps is a good example). In that case, heat exchange would need to be increased which there are many ways to do: add a fan to a radiator to increase airflow, increase radiator size or better design, or, like you said, ensure you are using a coolant with better thermal properties.
In summary, I see Evans Waterless and standard coolant as two ways to skin the same cat. Standard coolant can be used with higher pressure caps/systems and coolant overflow bottles. Evans boils at a higher temp. Neither should intentionally or regularly be run overheated. Evans has poorer thermal properties so if the engine being cooled is constantly overheating, one will need to make a change to exchange more heat (enlarge heat exchangers, make existing heat exchangers more effective (exposure to more or cooler air/water/snow), or switch to a coolant with better thermal properties).
In my particular case, I'm running perfect temps in the powder because the thermostat opens and closes as needed. My heat exchanger and it's environment (in this case, tunnel cooler blasted with snow) offers more than enough heat exchange. I do have to solve my hotter-than-desired temps on the trail, but I knew I'd have to do that when I did a tunnel cooler and radiator delete. If I'd retained on radiator, I'd be fine on the trail. I'd intended to a ski scratcher, but ran into a snag with ski shape and ski-scratcher compatibility so I'll be resolving with rail mounted scratchers. Once scratcher solution is implemented, the needed heat exchange will be more than adequate on the trails as well and thermostat will open and close to control temps.
So based on the real world engine temp data for my particular build, there are no cons to Evans. I fully agree there are plenty of engine, environments and use cases where using Evans could be challenging, but not seeing any for a situation where a thermostat is regularly closing because the heat exchange is adequate to cool the engine in question.
This is a great conversation! Fun stuff to think through.