hey guys,
i want to put new plugs in my 509 and was wondering what to use,or if i should just but the same ngks i have now,not sue if the number,it is 620hp, aluminum heads,big cam etc.

Ngk Tr 55

If it ain't broke don't try and fix it. If the current plugs work fine, then keep using them.

If not post what they are and what the issues are. The numbers are on the plugs.

ac mr42t

Put in what the head manufacturer recommends for starting point and then check plugs for location of the heat marks and change heat range, if you need to, accordingly.

Note: when doing this it's better to start with a colder plug than it should end up with than with a hotter plug. A colder plug will just foul faster whereas a 'too hot' plug can seriously wound your motor. Most head manufacturer's recomended starting point recomendation is on the cold side.