Subject: Re: Suitable Spark Plugs for 16/80?
Rich, I tend to only think in terms of "hard" and "soft" so cant be exactly certain about cold and hot. Someone else will put us straight I am sure!
A hard plug ie a 7, 8 or 9 in NGK terms would be used in my experience on a higher performance engine. For instance when we ran a 2.8 Hart engine on about 12:1 compression ratio we used NGK 8s.
If your engine is fouling either from too rich mixture or oil at lower engine speeds then I would tend to go to a softer plug - more like 5 or even 4. I use NGK 6 on my 4.5 engine and have been thinking of trying 5s with this horrible modern fuel.
In my experience the Green Spark Plug Co are not only reasonably priced but also extremely helpful and knowledgeable.