In an effort to clarify in my own head the setting of spark plug gaps, I've gone and done a wee bit of webcrawling.
I have absolutely nothing against Ron's method of pulling and re-gapping and reinstalling until the engine runs rough and/or misfires and/or mileage goes down, but if you've gotten to know me through my posts on here, I appreciate a somewhat more thoughtful, if not outright more scientific approach... Especially since I now drive a vehicle with a transverse mounted engine that requires special procedures to access half of the sparkplugs!!
What we're looking to accomplish is to optimize efficiency by improving ignition. Basically, this gets down to determining the Breakdown Voltage, the minimum voltage required to create a spark that will ignite the air/fuel mixture under compression in a cylinder.
Here's a neato little calculator I found. (it requires javascript to work correctly):
Megasquirt Sequencer Coils
Apologies for it being Metric, but that's how most of the world rolls outside of the US
I suggest working backwards with it, if you will. Start with the factory gap to deteremine the breakdown voltage, and then make the gap larger until it starts to approach or exceed the design capabilities of your coil. Don't worry about the MAP pressure or the temperature for now (our groove makes for lower manifold pressures and temperatures, which means lower voltage required => bigger gap!), and you can probably find a compression ratio spec for your engine online to really get close to the bullseye, which is the voltage.
Once you've found the voltage, creep the gap up and the pressure down to keep the voltage within a reasonable tolerence; 3-5% is probably pretty close. a 5% voltage tolerence means +/- 2.5 percent, and over 25kV, that means +/- 625V. I would personally err on the side of caution, meaning fudge the numbers to stay under the calculated voltage to make sure I'll always have spark. Also keep in mind that the pressure can only go so low, especially if you live and drive above 3000' above sea level or so. I wouldn't go to 98 kPa, for instance, and that's pretty low.
Hopefully now you can ballpark a bit more accurately, so you don't have to keep pulling and gapping your plugs and rolling your engine like me. You might be surprised at how wide you might be able to go, as Ron has always said...especially if you have a scantool that measures some of the numbers you can plug into that calculator.
