After careful consideration, I think I am going to switch to Evans NPG (100% propylene glycol) during my annual this year when I flush the cooling system. Here is how I see it:

Disadvantages:

1. I will probably run a higher cooling temp, about 20-30deg higher best I can tell,  This will put my hottest temps up around 250F.  I think the coolant o-rings should be able to withstand that.  Otherwise, the increased temps will improve heat rejection and possibly BSFC without other negative impact.  Running higher coolant temps should also partially offset the decreased specific heat of the NPG (better temperature difference in the radiator).

2. Initial cost and hassle flushing the system.

3. Higher oil temp.  - Oil temps are not a problem for me, if anything I would like to see them up into to 180 range (currently 140-160).

Advantages:

1. No risk of boil over.  I had one in-flight boil over when using 50/50 EG.  That was a real pain.  Once it gets to boiling over it is very difficult to get it to cool down, and it can be hard to find water at some airports.  Also, a small leak in an EG or water system will drop the pressure and cause boil over very quickly.

2. Antifreeze.  Running water with water wetter did decrease my temps by 20 deg, but I could not leave the plane parked overnight in the mountains for fear of possibly freezing the coolant.  So before flying to any place cold I had to drain out the water and add EG.  That was a real pain, and now my temps are back up near the boiling point of the water /EG mix.

3. Longer coolant life and better corrosion control.  Evans clams NPG+ will last the life of the engine, but I will obviously replace it every 5 years when replacing the cooling hoses.

4. Non-toxic.  I am frequently dripping EG in the hangar every time I do work on something.  Would prefer something non-toxic.

5. Low pressure cooling system that does not change pressure with altitude...  I eventually plan to get into the flight levels but that would mean that my boil over temp drops dramatically.

6. Better control of nucleate boiling.  Not sure if this is good, bad, or indifferent.  Evans claims it significantly improves the margin of safety for detonation and allows increased spark advance.  Others claim that nucleate boiling is important for cooling very hot spots.

Comments?  Anyone have real life experience with NPG they could share?

Dave Leonard