Mostly stock 340 engine what Spark plugs

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My 1970 340 with X-heads. Original bore,280 mega cam,600 Holley and stock cast mani's dual exhaust.. Who would like to take a crack at which plugs for this setup. Is a 4 speed car with 3.23 posi. Have had the car one summer and have done some improvements. Just a fresh tune-up left. Any thoughts much appreciated.
 
I usually run a little cooler plug like RN9YC. Have for a long time. Was originally to prevent pre ignition in the 10.5-11:1 small blocks at the strip probably no need to run them in a 9.5:1 engine, but I bought a surplus of them so Im using them. They do foul easy.
 
I used to use Champion N9Y's but can only get them in resistor form. Switched to accel 114's some time ago and they seem to work okay. The 340 runs about 10.5:1 and the plugs have the correct coloration. Before the popular availability of unleaded premium gas plugs only lasted about 1500 miles before the engine started missing at high rpm runs.
 
i run champions in both my cars (340 in the demon and 440 in the charger ) both are slightly modified - I put 3 to 4 thousand miles each a year and have done so for years with no issues
 
RN9YC was the recommended stock plug for 340 '68-69 and I presume '70 and '71.
That's a slightly colder plug than the RN12YC. Without testing, its hard to guess which one is best for your motor today. If you go by the the theory "run the coldest plug that doesn't foul" then the slightly colder plug might be the one you end up with.

If you don't like Champions, just find a cross reference chart from the manufacturer you prefer. IMO Autolites are harder to read.
 
Autolite 64 have served me well. Champions have always fouled too easily. Also like NGK.
 
How does the spark plug know what engine it is going into...as in I only use such and such brand in my whatever brand?

I have used Champions since 1973 in EVERYTHING. In fact, if you pay me to tune your car, bike snowmobile...whatever, the very FIRST thing I do (after considering all the variables) is install a set of Champions. Which leads to the real issue...which is...most guys can't read plugs. When I see a black shell plug in a racing engine I know for a fact the tuner can't read plugs.

Also, never use a cross refernce chart. Use the chart provided by whatever plug manufacturer you pick.

To the OP RC12YC Champions, gapped at .030-.035 and learn to read plugs. even if it's a daily driver.
 
A nice easy way to find what plugs are compatible you can do a search on one of the auto parts store websites like I did here:

http://www.oreillyauto.com/site/c/search/Spark+Plug/03435/C0334.oap?model=Duster&vi=5144039&year=1970&make=Plymouth

They list the model number which you can use to look up the heat range of the plug and also all the other specs for size, threads, etc.

I have only tried NGK and Autolites, I'd assume NGK are higher quality because A) they're more expensive and B) they're Japanese lol. Regardless I have "only" tried these brands because I've only ever had to replace spark plugs on my own cars once since I've owned them ('93 Jeep Cherokee, my Duster w/ 360 Mag, and '94 Mustang GT 5.0) and those two brands last a loong time.
 
I ran NGK's in my old 340. It was a high mileage engine that burned a lot of oil out the exhaust. When I pulled the engine, I checked the plugs thinking they would look horrible. To my surprise they were a light tan color and could easily have been put back in for another ten thousand miles or so!
 
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