Well, I searched as best I could, and was at a loss for a good explanation for this question: What is the best way, or are there certain rules to follow as far as deciding on a spark plug heat range for engines? For an example, I'm running a 350 Pontiac with a stock style (cast 16) head resluting in about 9.25:1 compression, flat top pistons, and a cam with a .408/.407 lift and 212/225 duration @ .050 lift. Any suggestions or help to answer this question would be great.

go to the NGK website, they have a thing on there where you can ask for custom specs, they came back and told me I could use UR5's (I've got similar comp ratio and cam specs), they got back to me within 24 hours

generally speaking, you should go 1 heat range colder for each "point" of compression you go up by (e.g. I went from 8.4:1 to 9.4:1)

The stock and slightly modified engines can most of the time start with the factory recommended plug. The heat range can be adjusted up or down depending upon the needs of a given engine.

Good info here: http://www.autolite.com/framer.cgi?...ucts/racing.htm

Almost without exception I have found going one heat range colder is all you will ever need for performance, if you need to go colder...you have other problems that need to be addressed. Alternate fuel engines like propane and alchohol are the exceptions of course.

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If your plug p/n is for the stock engine,look for a high perf. model of your engine in the parts catalogs and that is usually a good place to start when modifing an engine.Also get yourself a plug color chart that shows what a plug should look like and what they look like under adverse conditions and improper heat ranges so you'll be able to learn to read your plugs.They will tell you a lot about your engine,and remember a picture is worth a thousand words. :p

When I upped compression on my last350 from 8.5-9.5 I ran Delco R45TS then I switched to a R44TS and the butt dyno recorded a marked improvement.
My newest motor is only 8.6 so I decided to start at R45TS's again.