measure rpm with voltmeter

[Jaydela180]

Trying to measure acurate 1200 rpm idle

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with no tach....

Read a little bout measuring rpm with voltmeter in hertz/ frequency mode.

I understand that reading will show per second and times 60 will get you the rpm, but will that be double actual rpm with our wasted spark?

And can I just measure from coil terminal? If not, where would be ideal to measure from?

Thanks in advance.

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[TwoManyXS1Bs]

Oh, this'll be fun.

Probing on the coil's grounding side will give you a nominal voltage swing of about 12v-14v, but will be higher due to inductive kickback. Depending on your meter's input features (Hi-Z?), may need to put a 1-2 megohm resistor between your probe and the coil's grounding side.

With a wasted spark, you get one pulse per revolution. So, 20Hz would work out to 1200 rpm...