Owner of Paolo66,
Spark plugs are a bit of a black art and whenever I ask at various car club meets, I invariably get many conflicting opinions - so I'll stick to the facts (as I know them).
NGK make good spark plugs (as do all current manufacturers)
Oddly, NGK code their heat ranges in reverse to most other plug manufacturers, that is, the smaller the number the hotter the plug. A BP7HS will be colder than a BP5HS whereas with Bosch plugs it is the reverse, so a W7BP is colder than a W8BP.
Heat range is important - a plug needs to be hot enough to prevent fouling and misfiring yet cool enough so as not to cause preignition.
For an air cooled engine (eg: Fiat 500, VW etc.) a hotter plug is usually indicated.
Your original BP7Hs is a fairly cool plug and you may be better off with a 6HS or even a 5HS. For Bosch plugs the equivalents are W7BP (for 6HS) and W8BP (for 5HS).
The recommended plugs for the standard Fiat 500 engine were either Marelli CW6N, Champion L87Y or Bosch W225T1. They translate to NGK B6HS or NGK BP5HS or Bosch W7BP.
As an example, I started with 6HSs in my engine but I had trouble with rough running and minor fouling so I changed them for 5HSs and cured the problem.
There is a lot of technospeak bumpf circulated by various (sometimes) knowledgeable people, often with strong opinion and in a very dogmatic fashion. Like you, within reason I use what works and currently my plugs suit my engine, the climate where I live and my driving style. If the car starts, runs well and isn't overheating then the plug is most likely correct for your pattern of use.
BTW, all of this assumes that your timing is correct, your valve clearances are set properly etc.
Hope this helps,
Chris
PS: I've attached the NGK plug guide. I have the Bosch one as well, but it's in German ...
