Right off the NGK site.....

6097 REPLACED BY 4644


Plug heat range should be chosed based on the plug condition.....not because someone says so. Read the heat signs on the plugs and decide. You want the porcelain around the center cathod to be a slight tint of tan. if its black then the plug is too cold, if its white or bright white, its too hot of a heat range. Also look at the electrode, where the electrode changes color, you will see a line between the colors, you want this line to be closer to the bend if not right in the middle of it. The electrode will give you kind of a Fine tuning heat range where the cathode porcelain will give you a broader range or too hot or too cold ect. Those two things will help you choose the heat range you want. If your interested in learnig how to tune by your spark plugs....do a search or send me a PM. I will hook you up with a few links. Remember, wideband tuning was not used years ago.

I ran stock heat range at 10psi on mine and it was proper cylinder temps. By the time I got to 22psi I had to drop 2 ranges colder.