Will a four stroke tach work on a two stroke?

1978Galaxie

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So I have this automotive tachometer that I want to install on my boat, and on the back, there is a switch where you can either hook it up to a four, six, or eight cylinder engine. You hook it up to the ignition coil I believe, and I was wondering if you just switch it to the six cylinder setting, if it would read accurately on a three cylinder two stroke engine.
 

GA_Boater

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Need some details;

What motor and what tach?

The answer is probably no, it won't work because of how the ignition signals are converted to RPM inside the tach.
 

dennis461

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Interesting,
Check my math please and TY.

For every two revolutions on a 4-stroke, there is one fire pulse(per cylinder).
So the tach has to double the number and then divide by number of cylinders.
So for two revolutions per minute of a six cylinder, the number displayed must be 1*2*6/6=2.
A six cylinder 2-stroke will have twice the number of firing pulses, the number displayed will be 2*2*6/6=4
A three cylinder 2-stroke would have 2*2*3/6=2.

It actually might work.
Does your engine engine have a suitable connection for the tach?
 

GA_Boater

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Interesting,
Check my math please and TY.

For every two revolutions on a 4-stroke, there is one fire pulse(per cylinder).
So the tach has to double the number and then divide by number of cylinders.
So for two revolutions per minute of a six cylinder, the number displayed must be 1*2*6/6=2.
A six cylinder 2-stroke will have twice the number of firing pulses, the number displayed will be 2*2*6/6=4
A three cylinder 2-stroke would have 2*2*3/6=2.

It actually might work.
Does your engine engine have a suitable connection for the tach?

What if the tach is driven by the stator? A 12 pole stator gives 6 pulses per revolution, 10 pole is 5, etc. Gotta know what motor.
 

1978Galaxie

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It has the 75hp 757h8d Chrysler Outboard. The tach runs off the negative wire on the ignition coil. I got it from O?Reillys.
 

dennis461

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An automobile tach cannot work connected to the stator (like many aftermarket tachs intended for outboard engines with no tach signal).
So # of poles on the stator does not apply to the OP plan.

If your outboard coil has two wires, connect it up and see it it works. The outboards with CD ignition probably will not work.
Your engine model seems to have a tachometer connection on the WHITE wire of the CDI.

http://www.maxrules.com/graphics/force/wiring/75-135_cdMagDelta.jpg

Just remember, you may be risking some electrical damage by experimenting!
 
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GA_Boater

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So # of poles on the stator does not apply to the OP plan.

Trying to supply all possibilities before the OP told us what he had.

Just remember, you may be risking some electrical damage by experimenting!

All the more reason to return the auto tach and get the right one. I would, but that's just me because I don't like burning money.
 

sam am I

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It'll work..............On your single ig. coil 3 cylinder 2 stroke, hook the new automotive tach sig wire to the boats "-" negative coil wire.(think it's the white wire on your CD unit or at ig sw.) Your 3 cylinder 2 stroke boat coil has 3 pulse/rev (3 cylinder, 2 stroke = 3 pulses/rev).

Once hooked up, be sure to set (as you mentioned) your new 4 stroke automotive tach to "6" cylinder (4 stroke, 6 cylinder also = 3 pulses/rev), you're good to go and seemed to have already knew your answer....:laugh:

FWIW, and as you probably already knew, if that type of motor (single ig. coil) was a 4 cylinder (4 cylinder, 2 stroke = 4 pulses/rev), just set your tach to "8" cylinder (8 cylinder 4 stroke = 4 pulses/rev)............



BTW, analog tach's (front end) such as the one you bought, for the generality of a pulsed signal derived from inductive sources, manu's/designers will internally de-coulple the input signal through an ac coupling cap (block DC) and/or also limit current typically into 100K ohms or greater to ground. It would be next to impossible to hurt any ignition system/coil, mag's output or alternator's tach output (low voltage sides) connecting up these type tachs, pulse matched or not.
 
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