Spark Plugs

[hooD]

In 1984 I installed Champion N9YC spark plugs into my cuda. 
I have pulled them out a couple of times during the years to clean them. 
There is only 2,000 miles on them. 
But I am going to replace them very, very soon.

So here is my question:  What would be an excellent plug to use?
The engine is completely stock except for headers. 

[Chryco Psycho]

RN12YC     IMO

[hooD]

RN12YC     IMO

Is the 12YC a cooler or hotter plug?  And what's the advantages of it versus the 9YC? 

[Chryco Psycho]

not sure hwere to get the 9s anymore but you can use them also but you said you have to clean the 9s so the hotter plug should stay cleaner

[vinb]

Champions numbers : bigger number the hotter the plug.
I run 9's in the race engines & 11's or 12's on the street.
The factory called for 12's..
You are talking your '73 340 , correct??

[hooD]

Champions numbers : bigger number the hotter the plug.
I run 9's in the race engines & 11's or 12's on the street.
The factory called for 12's..
You are talking your '73 340 , correct??

Yes, this is for my '73 340 smog engine. 
The reason behind cleaning the plugs was not because they were fouled or dirty, but just as a maintenance thing I do every so often, like once every 5 years!  Considering that these plugs are 25 years old, I'm wanting to replace them with some fresh ones as I am also replacing the ignition wires after 35 years. 

So let me ask this question again in a performance aspect.  Would 9's or 12's work better in my 340?  I have to loosen the header on the drivers side to get at cylinders 5 & 7, so I want to put the best plug in for the bang.

Next my TQ needs a rebuild and then I'll be set for another 35 years or so if I live that long.

[Aussie Challenger]

  9's were more for highway/spirited driving, 11 or 12's for street cruising as 9's can foul causing low speed missing. On hard highway you may even want to go to 7's, on my old Aussie Hemi 265 I had to use them, but then change them back to 9's for the street. That was also 30 years ago when good fuel and high highway speeds were allowed.      

[vinb]

HooD ,
I was running RN-9-Y's and a 160' thermo-stat last year. I have the same as you '73 340.
This year I'm running RN-11-Y's and a 180' stat. The Cuda runs much better with heat in the engine. Temp wise 0n a 90' day the engine runs about 185'.....

I also run 34 degrees of timing, but the engine is not totally stock either....

Hope the info helps...

[hooD]

HooD ,
I was running RN-9-Y's and a 160' thermo-stat last year. I have the same as you '73 340.
This year I'm running RN-11-Y's and a 180' stat. The Cuda runs much better with heat in the engine. Temp wise 0n a 90' day the engine runs about 185'.....

I also run 34 degrees of timing, but the engine is not totally stock either....

Hope the info helps...

I went to autozone and they said that Champion RN14YC is the OEM for 1973 340's.  I bought them but haven't put them in yet, because I'm wanting some feedback on this. 

Champion      Traditional spark plug      405      RN14YC      .035"     
(Original Equipment Manufacturer and/or Original Equipment Equivalent)

[Chryco Psycho]

I haven`t run a small block for a Long time but I would try the 12s first , 14 could trigger detonation

[Aussie Challenger]

  14's way tooo hot for any performance motor   

[vinb]

I would return the 14's and get 11's or 12's.....

[hooD]

Well, I am returning the 14's later today and will be putting new RN9YC plugs back in.  These are resistor plugs this time around. 
The last time I cleaned the plugs was 1998.  They looked pretty good for 11 years.  And really good for being 25 years old.
I figure if my engine liked them that well, I might as well use the same ones again. 
I have never had a pre-ignition or detonation issue, ever.  And the 14's might have caused some troubles.

[hooD]

The new RN9YC champions are in.  The new wires are on, and the new air cleaner & air filter is on.  New master cylinder and brake booster and new front shocks on also.