NGK technical guide to plug part numbers here: http://www.ngksparkplugs.com/techinf...tnumberkey.pdf
Looks like the main difference is that the 8 is colder than the 7.
When the heat rating is too high:
The spark plug temperature remains too low and causes deposits to build up on the firing end; the deposits offer an electrical leakage path that gives rise to loss of sparks.
When the heat rating is too low:
The spark plug temperature rises too high and induces abnormal combustion (pre-ignition): this leads to melting of the spark plug electrodes as well as piston seizure and erosion.
ETA more here: http://www.ngk.de/en/technology-in-d...-the-ngk-code/