Need a little tach education please. (Mercury)

[WRX03]

On a 2011 Mercury 40hp 4-stroke EFI 3 cyl, I'm using an automotive tachometer (Revolution Gauges). On this tach you can adjust from 1/2 spark per revolution up to 6 sparks. To get the readings to be correct I have the tach set to 6 sparks per revolutions. Can someone give me the down-low on why a 3 cylinder motor needs to be set to 6 spark/rev? I contacted Mercury online help, asked the same question, and got "Your motor does not have a waste-spark ignition system so it would only fire once for each cylinder." Most it's likely something simple-right. :facepalm:

 

[Silvertip]

Re: Need a little tach education please. (Mercury)

1) what do you have the sender wire connected to on the engine?
2) A two stroke three cylinder engine does not use the ignitiion system to trigger the tach. It is triggered by pulses from the charging system. An engine with a 12 pole alternator produces 6 pulses per engine revolution. Since the charging system is used as the trigger, it doesn't matter how many cylinders the engine has which means marine tachs are pretty generic.
3) If you are using the four-stroke ignition system (one of the coils for example) to trigger the tach, then the coil will deliver one pulse for every SECOND engine revolution. Therefore an auto tach cannot possibly read correctly regardless of the number of cylinders you have selected. A 4-cylinder four stroke engine using a single coil ignition system for example (which you don't have) provides 2 pulses per engine rev. a 6 cylinder does 3 pulses per rev and a V8 does 4. An engine with a separate coil for each cylinder there is the same as reading a ONE-CYLINDER engine.
4) If you connected that tach to the stator output of charging system you are back to the 12 pole alternator producing 6 pulses per engine rev.

 

[WRX03]

Re: Need a little tach education please. (Mercury)

Thank you for the reply. The connection is the magical grey wire of the Mercury wire harness. By your answers it looks like the reason is a 12 pole alternator producing 6 pulses per revolution. So this is why the 6 sparks per revolution setting of the gauge is working-makes sense. By the gauge manufacture's instructions for a 3 cyl motor I'm suppose to set the tach at 1.5 pulse, but the engine idling at 3100 rpm just didn't look right.
Thanks again!

 

[Silvertip]

Re: Need a little tach education please. (Mercury)

When dealing with outboard tachs, ignore ANY reference to the number of cylinders an engine has. It doesn't matter if the engine has oneor a dozen cylinders. It is simply the number of pulses per rev from the the electrical system stator. An example is: Johnson/Evinrude 2 cyl, 3 cyl, V4, and V6 engines all have 12 pole alternators. One tach and one setting (6P) works for all of them. Number of cylinders is a non-factor.