in a hemi we never usually index the plug,, just put them in,,, and its always the same hp,, with the same type of plugs,, if indexing would matter it would make power change some up or down cause the "new" plugs wouldnt be indexed unless you got lucky

in bb chevy motors i do,,, i have to index the plugs,, or fords,,, or else the piston would hit it

the biggest gain i ever seen in indexing plugs was about 200 or so, made a high 600 number, piston hit just enough to close the gap off,, so just swapped the 8 plugs around in the motor till they tightened in the right spot,, i looked at the numbers as the dyno guy was laughing, along with his dad,, after the 1rst pull,,,, he didnt see what i did,, then i walked back in and said pull it again, 950hp, he asked me about 10 times.... what did you do??? the dyno operator was charlie westcott,,,, he has a dyno in his barn,, this was 6 or so years ago