Another spark plug question

[coupe32]

I had another 340 engine built by a machine shop. I just got it back from the shop. I have edelbrock aluminum heads on this engine. The machinist said it will be slightly over 10.5 to 1 compression. I have always used champion
N9Y sparkplugs on my mostly stock 340 witht the X heads. Should I use a hotter plug with the aluminum heads? I will have also have to use the smaller type that takes the 5/8" socket to fit into the heads. Any ideas on which sparkplug to use? I'm also using MSD ignition with a Mopar performance distributor if that matters.

[ehostler]

Did you read the documentation that came with the heads? I ask as it clearly states the preferred plug is the Champion RC12YC.

[coupe32]

Nope I bought the heads slightly used so I did not see that. The machine shop said the heads were like new. The RC12YC would be a much hotter plug than the stock 340 with iron heads used. Thanks for the info.

[340_GTS]

I've been running Autolite XS3924DP, which are iridium plugs, in my '69 340 with Eddy heads, 10.5 compression, Mopar ignition. No problems at all.

[Dennis Jokela]

I use Champiom RC9YC on my race only car. You might need a hotter plug for street use.

Dennis Jokela

[ehostler]

The RC12YC is the recommended plug. It doesn't specify compression or cam. So, the RC12YC is a good starting point. Then read the plugs and see if you need to go hotter or colder.

[cudabob496]

NGK sells them based on compression ratio. A lot of people like
NGK.

[madscientist]

If you are actually 10.5:1 (a little low IMHO even for pump gas) you can prolly use a C63Y. Fine wire, extended tip, 5/8 hex.

That is close to a 9 but a better plug. If you try the 12 you'll fight to get the timing up where it should be. So if running 32* total sounds cool to you try the 12. If not start with the 63, about 35-36* total and jet from there.

[Dart 360]

In the Champion heat range N12YC, RC12YC are usually the standard heat range and for harder driving you should use N9YC or RC9YC depending upon style of plug, lower the number the cooler the plug and higher the number the hotter the plug with Champion Spark Plugs. For any competition start with N7YC or RC7YC which is regarded as the hottest to start with or go with the N63YC.

[coupe32]

Thanks for all the info. coupe