[301 Madman]

I was wondering about other Pontiac peoples thoughts on spark plug indexing with the stock heads. My thoughts are that the pontiac's stock sub-par plug location in the cylinder warrant indexing the plug for flame propigation to be optimum. My experiences with my 301 turbo was that the motor ran better with less detonation. I also Know that my 7.5 cr has a low flame front speed and it woul probally make more of a difference with low cr motors. Any thoughts?

[301 Madman]

I was wondering about other Pontiac peoples thoughts on spark plug indexing with the stock heads. My thoughts are that the pontiac's stock sub-par plug location in the cylinder warrant indexing the plug for flame propigation to be optimum. My experiences with my 301 turbo was that the motor ran better with less detonation. I also Know that my 7.5 cr has a low flame front speed and it woul probally make more of a difference with low cr motors. Any thoughts?

[Half-Inch Stud]

Yes Madman, Here is a performance tweak that escapes indexing: In my judgment, the ground lugs can glow orange at peak-torque [highway cruise or harder, or leaner]. Evidence of this can be gleened from studying the light rust and/or white ash that may appear along the ground lug of used plugs. If the lug glows, and I believe it can, then it would be the hottest spot in the combustion chamber & can be tripping off some detonation/pinging.

My practice has been to shorten the ground lug to half-length, by using a medium file. Run the file edge along the groung lug, where it bends over to reach the center conductor. File a cut into the ground lug, prep the remaining end to a blunt end.
Whack the remaining ground lug toward the center conductor for .045" gap. I use HEI with .045 gap so that the spark jumps reliably with high compression. Full timing advance of 34 degrees around 2600 ought to give you a foundation.

This mod is the closest thing to an aircraft plug yet the cost of Autolight 85.

[301 Madman]

Yes, I suppose filing part of the ground lug off shortens the distance heat has to travel back to the head, making the plug "colder" somewhat. It probably helps flame propegation as well by getting the ground lug partially out of the way of the initial flame kernel. I found that 18-20 degrees of initial advance worked well in my turbo, with the mechanical advance locked out. 9psi changes the cr to something like 11 to 1 under boost, with like 250deg F intake charge temp so I couldn't run much advance anyway.

[Half-Inch Stud]

You got it right on the thermal path to the plug body/cyl head. Sounds to me like your turbo car will benefit [you will feel and hear] a performance improvement with the shortened ground lug. Maybe able to try more total advance for those predominantly light-boost driving situations?