Picking a coolant for some can put you on that proverbial fence line and you’re just not sure which way to fall. You need your cooling fluid to prevent high pressure and air or vapor within the system. You need it to transfer heat away from the engine,but it also helps distribute temps more evenly too. Your choices for the most part are a waterless coolant or a mix like 50/50. The waterless coolant most often talked about is Evans NPG (NPG =non-aqueous propylene glycol) waterless coolant (Rotax recommended). Evans NPG is basically non toxic,non-corrosive and can operate at zero pressure. Its boiling point at zero pressure is 375F and the freeze point is -40F and was originally developed with the race car in mind.

Now the next choice is an ethylene or propylene glycol diluted mix. Most store bought brands are 50/50, but some people buy the full strength or non-diluted coolant and then add distilled water to make a mix of 50/50 or a 60/40 mix. The 50/50 mix is good up to approximately 270F with the Rotax 1.2 bar radiator cap. If you have the old .9 bar (13 psi) cap you should replace it with the newer 1.2 bar (18 psi) cap (SB-914-029 & SB-912-043). The freeze point is approximately -34F. Making the coolant with too much anti-freeze(80/20) or too little (20/80 gives you all the wrong characteristics that you want in an engine coolant and can be detrimental to your engine. Having a mix too heavy with anti-freeze can cause loss of cooling and the freeze point will actually start back up. Having too much water and not enough anti-freeze will allow the coolant to heat too high and cause vapor areas within the engine which will also aid in detonation and metal fatigue and not protect well against freezing outside air temps.

So now you think Evans waterless coolant is the best thing since sliced bread. Don’t jump on that horse and race to buy Evans yet. Let’s take a look at the Rotax 912ULS (100 HP) engine as an example. The max oil temp is 266F, the CHT is 275F and the coolant exit temp is 248F. Your coolant temps are usually close to the CHT temps. Rotax has stated in SB-914-029 and SB-912-043 that if you use Evans NPG you can go up to the max temp of 266F, but if you use a 50/50 mix then your max must be lowered to 248F. They did this because the 50/50 mix boiling point of 270F is too close to the 266F max temp. You may get vapor spots within your cooling system and if that were to happen you will lose the cooling in that area and metal fatigue will set in and loss of cooling. If you use Evans to stop a boil over problem while idling then you may have another issue duringcruise. Even though Evans has that great boiling point it does have a drawback.Evans does not absorb and distribute heat as well as water. So your temperatures across the board (i.e. oil, CHT’s and coolant) will all be higher by about 25F-30F over the 50/50 mix. If you were already too high in temps then Evans will make it higher. If you have an open air engine that is exposed to the air and not cowed then Evans may work very well for you. If you have a tightly cowed engine then Evans may drive your temps up to red line and negate any benefits you might gain from the boil over protection at idle. For example I tried Evans NPG in my 2006 Flight Design CTSW because it was getting up to 250F+ in the summer. The problem then became that all my high temp alarms were going off on the oil and CHT’s because Evans raised those temps another 30F which put me over the 912ULS max allowable. I drained the Evans and went back to the 50/50 mix and then just unloaded my prop pitch to a little less course and the temps were all fine. If you have an engine heat issue then don’t throw coolant types or dilution strengths at the problem, but fix the cause (i.e. air flow, a lean fuel issue or reduce the prop pitch).

If you use Evans then you must drain and purge the system of all water with Evans Prep fluid which helps remove water from the system. This isn't hard and is quite easy. Now the other thing to consider with Evans is that if at any time your coolant level is low then you can never add water to the system and must add only Evans NPG and if you are not at your home port that could be a problem.

If you use a 50/50 mix then you can top off with justdistilled water. Do not use tap water in your coolant system.

One last commenton coolants, if you use a pure Dex-Cool coolant do not mix any other coolants with it unless it states that itis Dex-Cool compatible and then I would still think twice. Mixing some of these coolants with Dex-Cool can cause thickening of the fluid which will cause several problems. Do yourself a favor and just don’t go there, so pay attention to what you put in your cooling system or what you add to it later.

I hope you now will look at your Rotax cooling system and have a little more information to make that decision on which coolant type is right for you.

Enough with all this one paw typing, time to curl up on the couch.