Yes. The vapor pressure rating at a specific temp indicates that the fluid will condense vapor back to liquid at or above this pressure. Even at very high temps, the VP of evans npg+ is near zero absolute.

At 212 F, the VP rating of evans npg+ is almost a full vacuum. I calculated VP for 50/50 egw would be 10.7 psia (about 8" vacuum).

At 230F, the VP of evans npg+ is still near a full vacuum, while 50/50 egw is about 1 psi on a gage. This is why it is critical to keep pressure in the egw system if it is run hot, like track events in texas. The rx8, like most new cars, uses a pressurized expansion tank that functions as both an ast and an air pressure accumulator, to help keep system pressurized for all running conditions (just temp is not enough to create pressure in cooling sys).

An AST for evans will help initial filling, but is not needed to condense vapor caused by normal localized nucleate boiling. But it would likely help if air or exh gas enters the system for some reason. A 7 psi cap would help assure no vacuum/collapse at the lower rad hose, esp in cold weather.

As i noted in other rolled thread, npg+ is inferior to egw as a basic heat transfer fluid .... false impression of quicker engine warm-up, etc. But the high BP and low VP make it better if your system is running very hot for some reason and/or system pressure is not holding properly .... example: return valve leak in cap.

 

Just curious, where did you get the vapor pressure data for these compounds?

 

been a while, but I got component values from web tables. Then used mole fraction ratios to get the partial pressures and sum, fir the egw mix.

 

Thanks KevinK2, you have saved me from extensive burns in the kitchen

As I said, I'll post Evans' reply when it comes in. But I will probably rely on KevinK2's (I'm guessing you're the same Kevin) "mini-AST" concept, and run without the standalone AST when I get around to converting to NPG+.

 

As always, draw your own conclusions.

 

FWIW, I have five gallons of NPG+ sitting on the shelf waiting for my car to be finished, and I am still contemplating using an aluminum AST, despite the fact that I eliminated the AST when my car was still rotary.

My reasoning is a little different, and has to do with the difficulty of burping the cooling system of my new engine properly. The AST simply makes it a lot harder to leave air trapped into the system, but I agree with Evans... once burped, it's probably just taking up space.

 

I may run the AST so I can use a 7lb cap. Stant also makes a 4lb cap, but I can never find it at the local auto part stores.