Had this in the 'puter pretty helpful...its from MSD tech. assist.
Spark Plugs
Choosing the correct spark plug design and heat range is important when trying to get all the performance possible. Since there are so many engine combinations and manufacturers, MSD cannot recommend which plug or what size gap is exactly right for your car. It is recommended to follow the engine builder’s or manufacturer’s recommendations for plug heat range and gap. Once a proper plug is selected, you can experiment with the gap to get the best performance. By using these recommendations as a starting point, you can experiment by opening the gap in 0.005" increments then
test. When the performance falls off, the gap is too large. Carbureted or fuel injection engines can use larger gaps, but turbo or supercharged engines should stick with
the smaller gap. If no significant gains are achieved, go with the smaller gap. Remember, the larger gap taxes the plug, wires, cap and rotor. After changing the gap, reading the spark plugs is a helpful tool in getting the right tune up. The air/fuel ratio and how it’s burning can be read on the ceramic portion of the spark plug. A ring can be seen where
the effective heat transfer takes place. The closer this ring is to the tip, the richer
the mixture. Closer to the base of the plug means the engine is running more lean.
Rule of Thumb Plug Gap
COMPRESSION GAP
UP TO 10.5:1 .040 - .045
10.5:1 - 13:1 .032 - .040
OVER 13:1 .025 - .032
Note: These are recommended for normally aspirated engines only. Reduce the gap for blower, turbo or nitrous applications.
Thanks,
msdtech2