Math on a tachometer

kaliningJune 30, 2008

Maybe john or someone may remember. Please, anyone chime in. On an 8 cylinder Tach. connected to a 4 cylinder engine

do you multiply by 2 or devide by 2 ? I can't remember and i can't find my paper work. Thanks in advance.


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It all depends o n where the tach is getting its trigger signal.

If the signal comes from a sparkplug lead, there may not be any change. Again, it all depends. These days on some 4 cyliner engines, only two spark colis are used; One coil fires two plugs simultaneously. One of the plugs is on a cylinder on its power stroke while its mate is on amother cylinder on its exhaust stroke.

If both cars has a distrubutor and the tach signal comes from a single coil that feeds all plugs, then the math is obvious. Again, it depends.

There is another situation: Some car engines now use individual coils mounted directly on each plug. The high tension lead of thje coil is not available, only the low tension side is available.

So, what to do? Hook up the tach to the 4 cylinder engine in the normal way. Listen to the engine idling - does it sound near normal speed? If so, look at the tach. If the tach reads about half speed, say 400 rpm and you'd expect 800 rpm, multiply by 2, or move the selector switch to register the correct speed. My very old fashioned tach has a selector switch for 4, 6, and 8 cylinders and expects to see pulses from s single coil feeding all sparkplugs. It was made before auto engines began using one coil to fire two plugs simultaneosuly.

The technique of firing 2 plugs with one coil has been used by 4 cylinder motorcycles for many years. It was in comon use in the 1970s. This eliminates the need for a distributor.

I'm afraid this has not been helpful since I need much more information to give you the correct answer. Yah, I know, it all depends!

    Bookmark   June 30, 2008 at 9:09PM
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4 cylinders on one coil with points. Single points. I think it is 4 pulses on two crank rotations means i multiply by 2 ? I get 400 on the tach and i think the engine is doing 800. awful slow 800 though. I don't hold too much faith in the tach either. It's a cheap piece of junk. Can't get a steady needle. jumps all over the place.

    Bookmark   July 1, 2008 at 2:29PM
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