Thanks guys! Here's some follow up.
What sort of motorcycle is it? Perhaps this is an X -> Y problem and you could much more easily get your signal directly from the CDI or tachometer.
2017 Harley-Davidson Sportster. Clamps like this are just a convenient way of getting to the timing without having to dig in to other wiring or components. Since they're a very common tool for this very job that's why I chose one. However, if I eventually install a coil-on-plug system then I'd need an alternate solution. I've thought about getting a signal prior to the coil, but I'd want a preexisting connection I can tap in to, or I'll be back in to measuring current flow with a clamp-on sensor I'd guess.
@DaveAK What is wrong with the circuit that defragster highlighted for you?
I don't think there's anything particularly wrong with the circuit, although I don't fully understand it, which isn't helped by having 3 components marked with red asterisks. Also I don't have the thyristor mentioned and don't want to pay $5 shipping for a 50 cent part. I want to understand what I'm doing better before I start ordering parts. Another concern is that one post mentions a clamp with 200ohm resistance suggesting many secondary turns, and mine isn't like that. Having said that the circuit is from a Sport Devices SP1 Dyno DAQ unit which I am currently using. It has a very simple clip on clamp with no turns and maybe is capacitance driven? Again just a very uneducated guess. If I can find where the image is referenced from maybe I could get more info. The link shows it's hosted on the Sport Devices server. The SP1 clip on clamp has a connector that isn't easy for me to test with otherwise I'd see what I could do with it.
connect teensy to CAN of bike, and read the RPM over the network?
I have already done extensive CAN work on this bike and isolated the RPM messages amongst others. Unfortunately the RPM is only output every 200ms and I feel like I need something more responsive for my dyno application. This was going to be my preferred method until I embarked down my current path. Just as an FYI because I know you're a CAN guy I do now have the ability to read and write the entire ECM map over CAN and I've decoded about 80% of it.
You can use a lower value load resistor to reduce voltage. The big issue is that spark current is highly variable - hence the limiter circuit.
If you want to learn/understand, I recommend putting the circuit into LTSpice.
So am I right in heading down the V=IR path? Only knowing that I'm working in the low mA range until until I get a corresponding voltage reading on my scope. Then depending on resistor value chosen I should be able to work backwards to calculate current to make sure I'm heading in the right direction.
And yes, the spark is highly variable and will need some kind of limiter circuit, maybe exactly like the one defragster originally linked. What I'm hoping to achieve is to first get a signal I can reliably measure and understand with my scope. Then try some circuitry with components that I do have on hand to see what improvements I can make. Once I have a comfort level with that I can move on to making a circuit that's Teensy friendly.
I've tried LTSpice before and didn't have much success. I can't remember what I was trying to achieve though. Maybe it's time to try it again now I have something specific in mind. I can model each of my steps and compare with actual observations. Is there anything comparable to LTSpice that'll work on Linux to save me breaking out my laptop?