Detonation Sucks
I don't know what ya have for compression, but I look at it like this:
11 to 1 or over n/a #6
11 to 1 with 100 shot #7
11 to 1 with 200 shot #8
11 to 1 with 300+ #9
once you get to 13 to 1, I'd go to a #7 n/a, and for 100 shot a #8, 200 a #9, etc.etc.
Personally, I'm on a 13.5 to 1 compression motor, and am running a 150 shot, I run a #9 plug. I'm gonna start playing with the jetting a little, therefore I got some #10 plugs for a little added safety net.
Each heat range you go down lowers the combustion chamber temps by about 100 degrees (rough rule of thumb) so that may be enough to save you from some damage. Noone ever hurt anything from running too cold a plug, ya just foul them out easier.
I gap everything at 35, gap them too tight and the car gets a little harder to start and whatnot, plus this is a way to be consistant to see what just the plug change does.
I dunno about running different plugs in different heads, I'm running ET heads and am running the AR 3933 normally, and have some AR 3932's to go to for my next outing/testing session.
What is the general feeling on a plug being colder but projected?
It would be nice to change the stickie to include a breakdown of plugs by temp and all the part numbers for projected and non projected. It appears some part numbers are missing? This is a good thread. To think I almost didnt start it. Thanks to all that participated!


Autolite my friends.... non projected and the ground strap is cut back some already on top of it. They don't melt, and are available in the cold heat ranges we need to be running.
Autolite my friends.... non projected and the ground strap is cut back some already on top of it. They don't melt, and are available in the cold heat ranges we need to be running.
are they the same part numbers that are in the stickies? if not can you post up some part numbers with the heat ranges?
Autolite AR 3933 #9 heat range
Autolite AR3932 #10 heat range.
Guys with street cars will complain that the #8 idles rough and starts a little harder then they're used to with the hotter plugs, but with these the chances of a plug turning inot a torch are alot less.
I run the 3933 with a 150 shot, but have some 3932's for the next outing
Please keep in mind I"m running alot more compression then the average guy on here with 13.5 to 1. MFG Recommendations: approx 11:1 compression
Heat Range: 8
Gap: Consult ignition mfg.
Plug: NGK R5672A-8 (stock number 7173)
I also bought a set of these as well incase the car does not like the #8's
Nitrous (two steps colder)
Heat Range: 7
Gap: Consult
Plug: NGK BR7EF (stock number 3346)
But, from the info above, it says one has a reach of .75 (3/4") and the other is .70. Does this make any difference?
Autolite my friends.... non projected and the ground strap is cut back some already on top of it. They don't melt, and are available in the cold heat ranges we need to be running.
If you melt an NGK there is a 50/50 shot you might be OK.
NGK's are much easier to read also......I run NGK's FWIW.
The Best V8 Stories One Small Block at Time
But, once you have your tuneup set the autolite plug IMO is a better way to go.
I just got a bunch of #10 plugs today, 4 sets of autolites and 2 sets of NGK's, NGK will get the first couple runs.







