[Febo]

Soooo..... Have you maybe tried a 195* thermostat for your CRX HF, one that is 15* over OE recommendations? It won't get you the coveted 210* operating temp, but it should move that needle of yours a bit higher.

Also, where do you get the 210* optimal combustion temp?

Also, are you using fail-safe thermostats, or the ones that if they fail they fail in the closed position?

What brand are you using? Is it a quality brand? What is the ambient temperature of where you are driving? What is the ambient temperature of where you are letting your car idle? What is your radiators efficiency?

The questions can go on and on, but I assume the biggest difference here is the design of the thermostat as older ones will fail closed and blow up your car while new ones fail open and make your car cold.

If you ARE going to do this scientifically then you need to use the same car, same coolant mix, same ambient temperature, same humidity, same A/F ratio, etc.

Good luck.