On our new top end( te2 heads, sp2, sv1 carb ) we upgraded our ignition to the msd grid, matching coil, msd pro billet dist, went with the larger mag cap. I have been fighting interference and when i get it the car won't rev above 4000 rpn and when I look at the data from the grid everything is smooth till that point and the my rpm look more like seismograph, just very rapid up and down till the rpm demand drops then all is smooth. It's not a setting issue in the grid, been through all that with msd. Every once in awhile it will rpm mostly smooth I'm going to add more grounds between the frame, motor, firewall, have a shield 6ft pickup wire to reroute that out and away as far as possible and hopefully that takes care or it. But I'm laying in bed, can't sleep wheels turning and I'm wondering if my non resistor plugs could be part of this too. I'm running ngk 7173/R5672A-8 I wanted the smaller 5/8 he nut for better clearance on my headers. Called ngk and they said the closest plug in a resistor was the 4644/BKR7E. TA recommend the bp6-9es plugs which are also non resistor but the larger 13/16 hex. They make bpr6-9es with a resistor. I haven't made enough passes to know what heat range I need, motor is about 11.75:1 compression. I opted to start on the cooler side to be safe. Just wondering 1st off what others are running for their plugs. 2nd I can't find what all the letters mean to make a comparison. I found it for 4644 but cant find any chart for 7173 seeing it starts with a r then all numbers. I know the 6,7,8,9 is the heat range and the larger the number the colder the plug. 3rd has anyone else gone rounds with emi/rfi and what did you have to do to correct it, just incase what I have done and am doing doesn't fix it.