Pontiac Problems? Ask a Mechanic for Answers ASAP
Nothing wrong with the brands you have tried.
Although I favour NGK.
Heat range is compression ratio depend.
Can you give some more info about your engine?
Ok, you'll be limited if you need the short plugs.
Best to stay with that style to stop your leads burning up.
Depending on compression ratio, cam profile, fuel octane, driving style as to whether one range colder would be necessary.
How hard do you rev the engine, and how often?
Street car only?