Auto Tach on V4 2 stroke OB?

V153

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Apr 16, 2011
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Since I've had little luck finding a period correct tach. A buddy gave me this Sun Pro to try. It read way high on the 4 cyl setting but on 8 cyl it appears to be about right. Can this be?

Gonna run the boat tomorrow so I guess I'll find out. Just wondering if anyone had any experience with this?
 

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Feb 17, 2012
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depending on the engine and where the tach is connected it may be write or wrong. If the engine is old and has a coil/points then I can see it being correct as 2 strokes have one spark per rpm where 4 strokes have one spark every 2 rpm. On a modern engine I wouldn't have though the tach would read anything.
 

gm280

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If any tach can pick up the correct pulse count for the given RPM, why wouldn't it work? Tachs are merely counting devices that display their counting via digital displays or on an analog display. So I can see that working... The pulses are turned into a voltage proportional to the number of pulses received over time and operates a meter...
 

Silvertip

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Outboards typically derive a tach signal from the charging system, not the ignition system. Over the years outboard charging systems provided 6 pulses per engine rev. It therefore made no difference whether the engine had one cylinder or a dozen. The only thing that needed to be set on the tach was the number of pulses per rev.. Tachometers for cars typically have a 4-6-8 cylinder selector switch on them because those tachs were looking at ignition system pulses. Since a car engine is a four stroke design, there is one pulse every other engine revolution per cylinder. Therefore a four cylinder provides two pulses per engine rev, 6 cylinders provide 3 and eight cylinders provide 4. So the answer to the question has two parts. 1) yes -- the tach may register. 2) Some math will be necessary to determine a correct reading because you need to determine a) how many pulses from the charging system on your engine (probably 6/rev) b) what the # of cylinders setting is on the tach (use "6") and d) divide the indicated reading by 2 to obtain the correct rpm. To review: The engine is producing 6 pulses/rev. The tach is looking four 3 since you have it set for 6 cylinders. Therefore the tach is seeing twice as many pulses as it needs so you divide by 2.
 

V153

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It doesn't work right. But it looks really cool ... Dang! Anybody got a 80's JohnnyRude Tach?
 

Silvertip

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A V12 setting would make it read correctly -- provided the modern V12 didn't use a wasted spark ignition system, but then one could set the tach for 6P and you would be in business. Ahhhh -- how fun it is to work with numbers.
 

V153

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Gee thanks guys. Now I gotta go steal a tach outa a Lamborghini ...
 
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