Question, I'm going to change the plugs on my dad's car and I was just wondering what plug is everyone using with the stage 6 aluminum heads? The motor is a stroked 440 from muscle motors that is 496ci, 10.5 to 1 comp, with a 850 double pumper demon carb, and backed by a 727. I think he has Champions in there now, not sure if they are the RN12YC's or not.

Should be a 3/4 reach plug with a gasket but if the car is new to you then you need to double check. Sometimes things are what you expect. I use NGK plugs for anything I work on. That engine probably wants a 6 heat range so I'd use a NGK 6987.

Autolite 3924
Maybe a Champion RC9YC
Your way better off running a colder plug than the motors needs versus to hot
If the car sits a lot between uses it may have a fuel quality issue when the gas in the fuel tank gets older than 90 days
if it does sit a lot drain it and use that gas in some other newer vehicle with knock sensors and put new fresh gas in it to help not hurt itself

what they said^^^^. I ran both NGK and the Autolites in my 472 with Stage 6 heads. It went 175 mph in the quarter so that should get you in the neighbor hood. Birdtracker

Ive been using Autolite AR3933X plugs on my Stage 6's for the past 7 years. Never had an issue with them. 12:1 pumpgas.

The 3920 series plugs are resistor plugs and legal here in the U.S. on the street, the 3930 series are race plugs with no internal resistor and they are illegal on streetcar here in the U.S.
If a smart cop hears you go buy making RFI interference and static on his squad car radio, he can make our lives miserable

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Wow thats really anal.

The advantage to running resistor plugs and resistor wires is that makes the coil go up on voltage to fire the plugs
Same as opening the sparkplug gap
The MSD 7 series race boxes don't like non resistor plug wires, resistor plugs work fine though

So I talked to my dad and he changed the plugs over winter to a RJ18YC Champions. The car was running super rich with the 850 demon and I messed with the MSD distributor at first timing was 18 degrees at idle 36 advanced with lite springs and pulling 9-10 inches in vacuum. Changed out the spacer for the advance so now timing is 28 degrees idle and 36 degrees advanced and the car runs way better now with an idle around 850-900rpm pulling about 12 inches of vacuum, car drives like a new car, no kidding. Now my question is should I run the colder plug and my buddy says since my dad is running a digital 7 box we could lock out the distributor for 36 degrees at idle and retard the timing on the msd box. Would running a colder plug with the timing locked out would that make a big difference running and performance wise? We also mix a combo of non reformulated 91 fuel with leaded 110 race fuel. Also this car doesn't have any vaccuum advance or power brakes.