Selecting spark plugs with the correct heat rating

[DartVadar]

Currently I've been running NGK spark plugs with a heat rating of 10 (11 being the coldest plug they make) I took a look at the plugs and from the looks of it they are too cold a plug, a bit sooty and black, no oil though. My motor is a 360 with aluminum heads and 10.6:1 compression ratio, making about 375-400hp. It's a street engine running 94 octane fuel. I think the stock heat rating is about a 4, but because mine has been modified would a 7 or 8 be a better choice?

I need new ones even if you guys think it's a good heat rating, well one at least, I don't know what I did but somehow the white part of the plug that is on the outside of the head by the plug wire got cracked, so I want to replace it before I run it again.

 

[clinteg]

I'm running NGK 7's on 11:1 iron head on 91 octane pokin around town. Will probably go with 8's at the track. Running 7's because that's what the local race shop recommended for that motor and the previous owner said to run 8's at the track (that was when the motor was 13.9:1 CR). I would say that 10's are way too cold for your setup. You'd probably be alright with a 6 or 7 plug.

 

[DartVadar]

I'm running NGK 7's on 11:1 iron head on 91 octane pokin around town. Will probably go with 8's at the track. Running 7's because that's what the local race shop recommended for that motor and the previous owner said to run 8's at the track (that was when the motor was 13.9:1 CR). I would say that 10's are way too cold for your setup. You'd probably be alright with a 6 or 7 plug.

I went and got some KGK 7 plugs, and the car is running alot better than it did with the other plugs, put some miles on it and the plugs look perfect!

 

[toolmanmike]

Warm for the street, one step colder for strip and another step or two colder for high compression. Not your engine combo but when I built my 273 (which went from 10.5 to 9.6/1 compression, the engine didn't run clean. It didn't run bad it just ran rich and blubberly especially at idle. I messed around switching carbs and ignition systems and never changed anything. I found out that the N9Y Champion converted to a N8Y which is colder than a 10.5/1 engine needs and way too cold for my 9.6/1 mill. If you can follow me, my compression was in the middle of the 10.5 and the 8.5 so I settled for a N12Y. As soon as I changed the plugs out and adjusted the timing and carb, it ran much better and idled smoother. Bottom line is....warmer plug for the street. It will run like poop with too cold a plug in there. tmm