been using accel u groove 416s on my 383 for the first 3000miles. unfortunatly the ceramic part of the plugs has hair cracks all over them. so im replacing them... ive decided to get away from accel ( had there plugs/wires/control module and i didnt like any of them) so im going with NGKs which i used on my stock 350TPI and loved them.
heres the accels im using now, car runs fine. http://www.summitracing.com/parts/ACC-8189/
problem is that the manufacture heat range is at 6... and the NGKs i got are at heat range 4. http://www.summitracing.com/parts/NGK-5155/
the car gets beat on and sees some track duty, ignition upgrades are MSD blaster coil, kinda new mallory wires (making new MSD wires when i go the NGKs plugs), DUI performance control module, DUI cap and rotor. and ive open the plug gaps from stock .035 to .045". engine is a aluminum head 383 with 9.8:1 compression and will see a peak RPM of 5500-6000 at the track.
just trying to figure out why the accels are heat ranged at 6 and the NGKs are set at 4. i kno how they make the plugs a different heat range but idk what the actual NUMBER means and how it relates to the type of engine.