Engine Sync gauge versus rpm guage

[Dusty Bilges]

Have about 12 hours now on my new-to-me 300 Sedan and continue trying to figure out best trim/rpms/speed/ range.
When my engine sync gauge shows perfect sync, the tac's are a couple hundred rpms apart. Should I just believe my sync gauge? Is that normal, or should I consider syncing by using the tac's?

[km1125]

I can tell by sound if my engines are in sync, and the sync gauge validates that. Listen closely to the engines while you're moving one of the engines throttles to either sync with the gauge or with the tachs and see how it sounds... ONE will surely sound "synced" better than the other.

Now, I saw that but I should also admit that I don't have mufflers in my system, so it's a lot easier to hear the engines than you guys who have nice mufflers installed. I'm a bit envious there though, because my boat sounds like a Harley when cruising on the lake.

[denpooch]

Ditto km1125. I use the sound. Listen for a 'slow moving' sine wave. (https://en.wikipedia.org/wiki/Sine_wave).
Also, at times I can 'feel' the slow oscillation from the deck on my feet. The throttle and tachometer on the port side is a couple of clicks below the starboard when in sync.

[mjk1040]

I was always told to go by the sync and not the tach's. Try tapping the tach's while running and see if they come into sync closer.

[Cooler]

I always use the sound method. Tachs are not accurate for sync for a couple of reasons, primarily your props are probably not exactly the same pitch by a few millimeters or so. As soon as you hear one steady hum, you are in sync. Slightly out of sync, you will hear an up & down frequency to that hum. The sync gauge will most likely comply with that sound. er

[Viper]

Source of the post....When my engine sync gauge shows perfect sync, the tac's are a couple hundred rpms apart. Should I just believe my sync gauge? Is that normal, or should I consider syncing by using the tac's?

Depending on your engine make/model/year, your tach info can come from a couple of different sources. On older vintages, the signal is typically picked up at the coils so the discrepancy can be connections, wiring, plugs, or the tachs themselves. Unless you take a reading right at each engine with a tool, you won't know for sure if your tachs are actually off or not or by how much. The sync gauge usually gets its signals from the tachs but not totally dependent on the tach reading. If yours is an EFI application and set up properly for base timing and throttle, idle is then ECM controlled and knowing the idle spec for the engine will give you an indication of how well your tachs are reading at idle, however you're assuming they are set up properly to begin with. Checking at the engine is the best way to check actual rpm, then go from there to determine if the tachs or sync are off and take corrective measures.