These have to perform some sort of internal RPM calculation to determine how much time to delay the light flash, correct?

Doug

 

The Actron light I have also provides RPM so yes, the RPM may factor into the calculations...

 

From a previous post of mine:

Beware that the adjustable advance lights are not necessarily accurate. I have an Actron CP7528 (Inductive pickup with the advance dial). I compared it's readings with a distributor on my Sun Machine, to check it's accuracy (firing a separate ignition coil to fire the Actron strobe/circuit).

The Actron readings showed a consistent 2 degrees of error at all positions above zero, as tested @ both 500 & 1000 dist. RPM on the Sun (1000 & 2000 crank RPM). In other words, 2 separate sets of tests.

When the Actron's advance dial was set on "0", Sun agreed "0".
Actron set on 10, Sun read 4 dist. (8 crank).
Actron set on 20, Sun read 9 dist. (18 crank).
Actron set on 30, Sun read 14 dist. (28 crank).
Actron set on 40, Sun read 19 dist. (38 crank).
Actron set on 50, Sun read 24 dist. (48 crank).
Actron set on 60, Sun read 29 dist. (58 crank).

In each case, if I added 2 degrees to the Actron setting as a correction factor, it would fire at the correct point.

Examples:
Actron set at 12 deg. would actually be 10 crank deg.
Actron set at 32 deg. would actually be 30 crank deg.
Actron set at 62 deg. would actually be 60 crank deg.

While I agree that 2 deg. of error is not a lot, it still can matter.

And your results may vary......

I also tested my other 2 timing lights at the same time (both non "advance" type). An older Craftsman Inductive pickup, and a cheap Equus Inductive pickup unit. Both fired at zero, and worked fine at all RPMs.

The Actron was the brightest, and the Equus the dimmest.

Plasticman

 

We have to have our timing lights at work calibrated. The digital ones always pass. The old analog Snap-on dial back lights (like what I have at home) are 3 degrees off. All of them are like that. Mine is 38 years old, and 3 degrees off doesn't bother me a bit....I can compensate by turning the distributor. The dial back lights are very useful to those of us who do NOT have a distributor machine in calibration of our advance curves.

 

Likely uses a "phase locked loop" triggered by the #1 pulse.
It can then divide the cycle into 360 slices (EDIT: actually 720 because spark only happens every 2 revolutions), each being 1 degree.
The dial-back **** selects how many 'slices' to delay the light.

So the locked loop, in effect, is tracking the RPM of the engine in real time.

I have the cheepo version from Harbor Freight. I've often wondered how accurate it really is.
Although, dialing it back 10 degrees does match the timing plate... so it's at least 'pretty close'.

 

I believe this is how it's done. If I were designing a dial back light, I'd certainly employ a PLL.

 

Makes me wonder how accurate my "Sears Craftsman" (IIRC)" is.

 

I've had my Snap On for 35 years as well. But it stays at a friends engine shop for dyno tuning. He's bought new high end timing lights but they don't work with all of the different ignition types. Mine seems to work great with any kind so he just keeps there. He builds mostly sprint car and drag race engines.

 

I don't use a "dial back light". I have a timing tape on my harmonic balancer. It's always accurate---NEVER VARIES--NEVER OUT OF CALIBRATION----And...….You can use any old timing light with it !!!