My mazda loves the copper plugs. I'm running to much boost on a way to big turbo but with a colder copper plug it seems to do ok.

With propane you want to run at least a stage colder. I have a buddy that runs propane in a big 454 with high comp and he runs 2 or more stages colder. There are a lot of benifits to propane in a gas truck. Like $2.25 galon and over 100 octane.