If ignition timing is being intermittently retarded, say for example from an advance problem on a mechanical or vacuum distributor, or from abnormal noise in a knock sensor wiring harness on an electronic ignition vehicle (and the retardation is also evident w/a timing light at the flywheel), will shortened burn times be evident on the scope?

If a Dynamic or Running Compression Test is abnormal - restriction in air flow, a flat camshaft lobe, collapsed lifter, etc. - will that be corroborated by an abnormal secondary (or primary) pattern on the scope? What would you look for, in particular?

Thank you,
Mark

If the ignition timing is being retarded, that would force cylinder pressures up (plug is being fired closer to BTDC) so the firing line will be higher and the spark line will be proportionally shorter.  A coil puts out a fixed amount of energy.  Anything that would change the cylinder pressures so that they are lower would result in the opposite, firing lines will be lower and spark lines longer.  A primary/secondary scope trace is our "window" into the combustion chamber, it will show many, many problems.  For me, it gives direction, further tests have to be conducted to pinpoint the cause.