I have been thinking about the Evans tonight & googling it.
It does seem a bit "snake" oil, so possibly fitting for a cobra...but at the end of the day a 50/50 glycol water mix under a 7psi rad cap pressure will boil at 130 degc,not 100degc.
What is important(along with mass flowrate) is the specific heat capacity of the coolant fluid, and water is a fluid with a high specific heat capacity. I don't think Evans state the engines run cooler but just say they don't boil. If you have an inefficient flow system, or poor /clogged radiatorthen even with the evans the temperatures can get high,so whilst you may not have any pressure in the cooling system, it may well be very hot.
I picked this up from the Toyota supra forum and it makes sense:
The amount of temperature rise (what we call "delta T across the engine") is given by the formula:
delta T = P / (m x Cp)
Where:
P = the heat power rejected to the coolant (kW).
m = coolant mass flow (kg/s)
Cp = Specific heat capacity of the coolant (J/kg.K)
The thing to take away from this is that "m" and "P" are pre determined by the engine, so effectively delta T (the temp rise across the engine) is inversely proportional to Cp (the specific heat capacity of the coolant). Therefore if you switch coolant to something with a lower Cp value, the delta T will increase. For water the delta T is usually 6 to 8 deg C, for 100% glycol it would be about 12 deg C.
Whats not given for the Evans is the Cp,and given a water glycol 50/50 mix boils at 130degc with a small pressure rad cap (7psi) if you cant keep your engine cool then theres an issue with your coolant flow rate or radiator efficiency.
I am not saying the Evans is a bad idea, as many may like the "fit & forget" and non expansion aspects,but then again its effectively the same as 100% glycol, which is a lot cheaper and to which you can add low cost water wetter.
I also thought it interesting that some fluids used in solar energy heat transfer are supplied from the same address as Evans:
Evans Cooling Systems UK
Unit 5,
Europa Way,
Swansea West Business Park,
Fforestfach,
Swansea
SA5 4AJ
and Hydratech:
http://www.hydratech.co.uk/contactus.html at same address
They at least state they sell etheyne glycol /propylene glycol....
Graham