Engine RPM graph “pulses”

What is most likely cause of RPM fluctuations or “pulses” on the data logger graph? This is in relation to engine RPM. Graph line is smooth until reaching just over 7k rpm all the way up to 9k it has fluctuations as if the timing is jumping around. I have always heard this was a mechanical issue and not an electrical issue. I run a crank trigger so one would think the timing should be stable no matter what rpm range it’s in. Rpm bounces around +/- 100 to 200 pretty rapidly, and can really be seen when taking the graph filter down to 1.


Points box issue? Trigger issue? Something else?

Thoughts?

 

Post up a picture of a graph with issues. How does it pick up rpm? A “tach” signal or an optic pickup?

 

You can see on the pic where it just looks like noise but smooths out during the shift? Picks up RPM signal through an MSD 8132. Some runs look worse than others. Mag, coil and points box tested ok at Leahy shortly before these runs.

 

Some good information and practices stated here:

https://computech.com/wp-content/uploads/2016/09/Engine-RPM-Sensor-Instructions.pdf

 

Do you have a cross over switch. If so is it mounted in a spot that gets some vibration starting at 7000 rpm and bouncing the contacts??

 

I do run a crossover switch. Not sure how the contacts could be bouncing though.

 

Good info. Thanks guys.

My crossover switch is mounted with zip ties to the mag support bracket for that reason. But could still be too much vibration. I have thought about taking the switch off completely since getting a stronger battery, it seems to start just fine on the trigger. All grounds are good, I check them more often than I should. I don't run a grid so I cant see any timing signals.

Could trigger pickup gap be a contributor?

Also, interesting article from rb0804 in regards to mounting 8132 as close as possible to the mag. Right now, mine is mounted in front of the foot box so it would make sense as to why rpm signals aren't stable. This could be why I have been having issues with shift activation through an rpm activated switch. I have gotten used to shifting the car manually because it just doesn't seem to want to work like it should. Testing it on the stand with a 3k chip it works great, but down the track trying to shift at 8600-8800 it doesn't take it.

I will try relocating tach signal converter to somewhere by the coil and run wires up from there.