The question came up at work:  Does the timing light flash as the current passes through the inductive pick up or does it flash as the current jumps to ground via the spark plug?  We set timing at WOT at 6000 rpm fully loaded on the dyno.  We set individual cylinders to read the same as cylinder #1.  We adjust this timing by bending the reluctor inside the distributor for any cylinders that are differant.  The question arises because as the cylinder pressure increases the resistance across the spark plug increases, thus slowing the actual ignition timing and perhaps delaying the signal to the timing light?  Any thoughts?

We have checked multiple timing lights and found that it is not uncommon to see 1 degree variation.  Let me add another twist:  We had a customers engine that he had assembled.  After he had adjusted the valve/ball vavle lash, he left an adjuster loose.  We began to perform a timing loop to determine the best timing, while doing this we checked individual cylinder timing and found that cylinder 4 was 3 degrees retarded.  We shut the engine off and rechecked valve lash, at this point finding the valve lash on cyl 4 had loosened up by about .030 due to the loose adjuster.  After repairing this problem the cylinder 4 was rechecked and did NOT need the reluctor modified...the only change was a variation in cylinder pressure.  

There is a time constant for sufficient voltage to build, required to 'fire' the plug. The constant is determinent on inductance/resistance, and includes the entire circuit. Rather than a timing light, you could use a current transformer and read the voltage and current on a dual trace scope. As mentioned, you are looking for consistency across cylinders, noting the voltage levels required to commence current flow. Variations in voltage will translate via the time constant, into degree variation. A variation over the speed range could imply gap resistance, whereas a consistent, but different voltage level compared cyl to cyl could imply a circuit problem.

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