Multi ground spark plugs and others? First with my 2003 Mazda Protege I used a multi ground NGK BKR6EK in that noticed it did nothing for the car but make it backfire on demand. Also used non resistors but in that also made the car weak. The best for the Protege was a NGK ZFR extended projection types that I cut back the ground.
In the Mazda I used mostly a nickel electrode type plugs. That was because in the old school view was a bigger electrode gave you volume of spark. It filled the cylinder better than a little spark and copper has less resistance also. This is true in many engines and setups.
I still like to use copper but not in modern engines that run really lean at a higher rpm noted by my 2020 Nissan Versa I had. The only thing you could use was a twin-tip type of spark plug. Aka the one that I cross referenced and found a type that goes on a Nissan Skyline but a different heat range. The ground was tapered on them. It helps! The copper and the Iridium the issue was the ground kept melting away. So being that it was best to not use those types. Side gaping those plugs turned the car into something that felt like a V6 but you can't side gap the twin-tip types so finding spark plugs with a tapered ground was good enough.
Non resistors has the issue it sparks too fast not building up the charge so the combustion was weak. Like vs a capacitor type of spark plug that stores energy to amplify discharge, Pulstar spark plugs. But should note with a older car 100,000+ miles on it without knowing how the head gasket is you may not want more combustion in there you could blow your gasket over time. It depends on the issues on your type of engine.
Multi ground spark plugs in normal engines has other issues. "The additional ground electrodes can sometimes interfere with the ignition process, potentially absorbing heat energy." Also a weaker spark divided by multi grounds is bad also. And is why cars that still use them have very high voltage in their coils. https://www.youtube.com/watch?v=53yfHLdn41k
To cut back the ground or using twin-tip type of spark plugs in a car that is not noted in the car manual you would have to use higher octane in your car to slow down the combustion speed to not have the combustion too soon. You want complete combustion, whole within reason of the limits of the engine. It all depends on how things run. Also note the maker of the car also has different spark plugs, oil in their car manual like from Japan than a American standard for the car. Me I tend to go with the maker of the car as they know!
All around you just have to use what you have to or not!
