Tachometer Calibration
 

I've recently become a partner in an old Sea Ray with twin 5.7 V8 inboards. We've noticed some disparity between what the tachometers are reading and what the sync gauge is reading. I believe the tachometers and sync gauge are reading each 12V signal from the EFI box to the coil. I plan on taking some frequency readings with my DVM on the individual tachs at various RPMs and compare the Hz with the needle on the gauge.

Can someone confirm that 1000RPM = 66.667 Hz? This is based on 4 coil pulses per revolution (4px1000rev/60sec).

I'd suggest you contact Bob@Ashlocktech.com.
I'm sure he'll help you out.

Each tachometer is not reading 12v but rather dwell / on time from the Neg or trigger side of the coil(s) . They do share running ( positive) voltage and ground to operate. There may simply be resistance on one of those circuits.

I'm pretty sure that modern tachometers measure the frequency of the pulses that trigger the ignition coil and convert that frequency to a voltage that drives a coil which positions the needle.

Let us know what you find

This is of interest to me because I have an electronic tach on my Unimog 404, and it seems like it reads really high - I really doubt that I'm cruising at 7200RPM with a stock M180 and 7.1:1 compression

TACH-ADAPT

Call Bob!
Nicest guy you'll ever talk to on the phone.
He's got you covered.
Honest.
You'll thank me later.
Trust me.

(Even Pelican sells his Tach Adapter, so he can't be all that bad.)