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Dead Parrot · 05-05-2017, 11:17 AM

At idle, a single noisy plug should have a frequency of about 6 pops per second.
Math: Assume 700rpm idle. Divide by 2 since plugs fire every other rev. (350) Divide by 60 since RPM is minutes and sound freq is seconds.(5.8 round to 6).
8 noisy plugs will be about 50/sec. Very similar to the 60hz power hum you sometimes get from stereo equipment when connecting stuff while it is powered on.

A noisy alternator will likely be around 3~6khz at idle. There is an internal cap in the alt that is for noise reduction. Brushes near EOL might be arcing more then normal.

Could also be bad or non-rf suppression wires. Think the solid copper 'racing full power' wires.

05-05-2017, 11:17 AM	#3
Dead Parrot Registered User Join Date: Aug 2012 Location: Oklahoma City, OK Posts: 2,675	Re: RPM related AM radio interference? At idle, a single noisy plug should have a frequency of about 6 pops per second. Math: Assume 700rpm idle. Divide by 2 since plugs fire every other rev. (350) Divide by 60 since RPM is minutes and sound freq is seconds.(5.8 round to 6). 8 noisy plugs will be about 50/sec. Very similar to the 60hz power hum you sometimes get from stereo equipment when connecting stuff while it is powered on. A noisy alternator will likely be around 3~6khz at idle. There is an internal cap in the alt that is for noise reduction. Brushes near EOL might be arcing more then normal. Could also be bad or non-rf suppression wires. Think the solid copper 'racing full power' wires.