Inline capacitor?? Rpm gauge??


Sep 14, 2022
do I need the inline capacitor/resistor thing that goes from my distributor into my firewall for my rpm gauge? Or can I just run a straight wire without a capacitor. I have an Hei distributor


Royal Smart Person
Supporting Member
May 23, 2022
SW Arizona
If your tachometer came with one, use it.

I attached a PDF that gives simple explanation.

Think of a capacitor as either a battery (DC Electron Storage) or as a filter (AC Signal Filtering). In line (Series) connection it blocks DC but allows AC to pass through. The ground side of the HEI is what fires the spark in simple terms. There's actually a whole bunch more going on in several processes.

The signal from HEI in simple terms is alternating current. In actuality it's more of a square wave, but that has to pass through so the tach can read it. The capacitor also helps filter stray and large spikes that make the needle jump around.

Basically (Simple Terms) the capacitor is designed to help regulate (filter) the signal from distributor to tach, which thereby makes the displayed measurement more accurate.

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Royal Smart Person
Mar 9, 2016
South Florida
In car audio a capacitor is used in line between amp and speakers as a high pass filter to block out unwanted frequencies that translate to dc in the speaker(conductor). Different mF ratings for different frequency cut offs at -6db per octave iirc. People called them bass blockers. If you've got one use it, I've fried cheap tachs before on MSD setups not using them.
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