TEJ

Apologies for the delay as I have been on lunch.

Various literature around the globe will specify differing heat ranges depending on the climate. 7 or 6 heat range is used in most engines including your Alfa. You are correct to question this factor as running a 'hotter' plug than specified sometimes causes problems for owners. Usually moving 1 digit hotter does not cause any engine damage but on a long run it is not ideal. If you are looking to change the plugs then I would suggest fitting the 7 heat range as Mr. Miles has fitted in his vehicle. The only thing that can go wrong is that you will experience very slight misfiring due to the plug tip fouling up with carbon deposits as it is not retaining heat to burn them off. This will then tell you that you need to use 6 heat range plugs as your particular vehicle runs better on them. It could be the reason that they were fitted in your engine in the past.

The projected plugs are generally better as they push the spark position closer to the piston which improves throttle reponse. In the '60s manufacturers started using projected plugs as they had not been around for long so this is why some vehicles with the same engines list different plugs as standard equipment. The Bosch super 4 items that Mr. Miles has fitted are actually more projected that B7ES but not as much as BP7ES. The difference is approx 2mm which actually will make no difference. I would personally stick with the projected type if it has been fitted currently but would not fit plugs with more ground electrodes such as the super 4. It is ideal that the initial ignition of the inlet charge happen with minimum material getting in the way of the expanding 'explosion' so more force is transferred to the piston face. The more ground electrodes that are present absorb more of this energy, so they are not the best plugs to fit IMHO. The only benefit of them is that they will last longer than a single ground electrode plug, that's it. The best plugs are Platinum or Iridium types as they have very sharp electrodes which means even less material in the way than a standard plug.

So basically, try the 7s if they foul up, you know to go back to 6s. The projection issue is something I would stick with but it's your choice at the end of the day.

Hope this helps.

Regards,
Tej