Tach and dwell meter on an old outboard.

[BuzzStPoint]

On my old outboard.
Chrysler 55hp. 1970 magneto engine

I've hooked it up on my tach terminal and you can see it moves
This one has 6 & 8 cylinder.

Knowing this is made of a 4 cycle.
I'm told I can't use this for setting a good RPM.

Since everything spins on timing and cycles.
Shouldn't there be a formula to convert this?

[BuzzStPoint]

I did find this, a snippit of an page using a tach and dwell on a single cylinder 2 cycle engine.

The arithmetic is not too difficult. If the gauge "thinks" it is monitoring a 4-cylinder 4-stroke engine it counts the number of discharges per minute, divides by two, and displays the answer as the RPM. If, in fact, it is monitoring a single-cylinder 2-stroke engine, then it ought not to have divided by two. So, when a tachometer set to "4-cylinder 4-stroke" is fitted to a single-cylinder 2-stroke engine, the reading displayed is half the true RPM, e.g. 600 RPM displayed on the gauge corresponds to 1200 RPM on the engine.

How would I translate that to 6 or 8 cylinder setting



So if I have my math right, ( I am by no mean good at math)
8 cylinder setting
RPM on scale X 2 = RPM X 2 = RPM of outboard.

[BuzzStPoint]

Just an update.

Posted on an automotive board and got this answer.

if the outboard has one coil and set of ignition points, then for a 2-cylinder 2-stoke you would multiply the 8-cylinder rpm reading by two

So, with the tach/dwell on one set of points, multiply the 8-cyl rpm by four. Each 2-stroke cylinder has one "ignition event" per revolution; an old-time 4-stroke V-8 would have four events per revolution.

Just incase someone else was wondering if they can use an old tach and dwell on the old outboards.

[Rick.]

I was wondering about the same thing a week ago cause I was looking to buy a timing light. Don't let the number of cylinders mess you up. It doesn't matter how many it only matters if it's a four cycle or two cycle. As far as I'm concerned your math is correct. The one I bought has a switch that goes four cycle or two cycle. Good of you to share your knowledge. Best of luck. Rick.

[wademckissick]

You need to know how many pulses/rev. A 4stroke V8 has 4 pulses/rev which is the same as my 4 cyclinder 2 stroke.

[Silvertip]

Unfortunately, outboard tachometers are NOT dependent on the number of cylinders the engine has. The tach signal is generated by the stator in the charging system so it doesn't matter how many cylinders the engine has. Every revolution of the engine produces a specific number of pulses. Most outboards produced five or six pulses per revolution but some Chryslers were oddballs in this area so you need to get to the Chrysler forum to find that number. Once you have that the conversion is quite simple -- provided the tach is accurately interpreting the signal it is seeing. So a six cylinder four stroke engine produces three pulses each revolution and a V8 produces 4. So therefore a two stroke engine that produces six pulses per rev (as most Johnson/Evinrude/Mercury engines do) then your tach on the six cylinder setting would be reading double the actual engine rpm because it is getting twice the number of pulses that the six cylinder engine would be producing. I think your Chrysler produces 10 pulses/rev.