Do MSD boxes give inaccurate initial timing readings?

[dipso]

I was wondering about something. Since an MSD ignition provides spark for 20 degrees of rotation at low RPMs. Would that mean that you are registering on your timing light, for a longer period of time.

So let's say your timing is set at 0. The initial spark is hitting at 0 and the light goes on. But the light stays on for 20 degrees, and therefore doesn't go off right away.
So when you are looking at your light, it is turning off later than it would at a single spark.

I think the distributor rotates twice per crankshaft revolution, so wouldn't the reading at idle, be off by 10 degrees.
It's still firing at 0, but reading until 10.

[moneymanager]

Don't really know but you time an MSD the same way you time the stock setup. I'm going to guess that 20° of rotation on your flywheel will last about 0.00001 seconds if it does stay on.

[Bob Kontak]

Originally Posted by dipso

I think the distributor rotates twice per crankshaft revolution

Other way around.

[Eagledriver]

The light doesn't "stay on". It fires on the first spark and that is what you see on the timing light. The extra sparks May or may not show up with the light, but you will still see the primary spark lighting up the pulley.