Use the same plugs, just different gaps. The plugs don't care what the distributor is.
I keep pointing out a no load test because, it takes very little throttle or ignition to run an engine this way. When loaded, the engine is working and the cylinder pressures are way different, this changes the way plugs fire, and this is where the differences are going to show up if there are any.
The throttle is the key to the test, not the rpm. As you open or close the throttle, you add or delete fuel, so did the distributor change the fuel milage, or was the carb adjustment ? You changed to variables at once.
Lets say you go from HEI to points. All you do is drop in the distributor and set the timing. The carb is exactly where it was with both units. The rpm is lower so you open the throttle to raise the rpm back to the HEI setting. You just increased the fuel usage, test over, HEI is more fuel efficient! You absolutely cannot change the carb setting from one to the other if you are checking fuel usage.
The only way you will convince me of a true fuel advantage is taking two 100 mile test trips, one with each distributor, and each on the same route in similar weather.
Read this is if thing pressure in the cylinder doesn't matter.
http://www.camaroz28.com/forums/advanced-tech-38/cylinder-pressure-spark-gap-260024/Joe
