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what revs is the rev counter counting

  • Thread starter Thread starter rlevenson
  • Start date Start date
R

rlevenson

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The tach on my '80 550 L works real good, or seems to, like the rest of the bike. The tach on these bikes runs off a wire that sockets into a spiral gear-like spinner located in cylinder head. When I'm on the bike and the tach says, for example, 6k, what am I learning exactly: that the exhaust side valve camshaft is going at 6k revs per minute ? and does this translate one-to-one to countershaft sprocket r's ?

I am also seeing that some electrical tester equipment has a tach that uses alligator clips that go to spark plug wire. I am not following the arithmetic on that device's taking a pulse off one of four plug wires and translating it to the r's we need to know.

What r's do we need to know, and what is the best device to read those while going fast on the motorcycle ?

Thanks.

--Richard
 
Your tach measures crankshaft revolutions. Cams turn at half that speed I believe. So at 6k your cams are turning at 3k.
 
Last edited:
Each spark plug fires every other revolution, 6k = 3k at each plug the meter just doubles it.
 
Tachs measure and/or calculate

Tachs measure and/or calculate

Mechanical tachs on engines are attached to any part that is directly driven by the crankshaft. The tach ALWAYS reports the RPM's of the crankshaft. If it's attached to the camshaft it doubles the RPM's as cams turn once for every two revolutions of the crank.

Electric tachs are normally attached to a coil wire. The tester unit you mention is probably also intended to be attached to the coil. They have a sensor that counts the number of electric charges sent to the coil and using a timer calculates the RPM's. If the alligator clip is attached to a wire that is about the same size as the ones in the wiring harness then it goes to the coil. If you attach it to the plug the voltage will FRY the tester. If its a tester built for autos it will have a selector for 4, 6 and 8 cylinders. It needs to know how many times the coil fires per revolution. A 4 cylinder fires twice with each revolution, a 6 fires three times and an 8 four.

I've never seen one but a tach can be built to attach to the spark plug wire or use an impulse sensor like some timing lights. An impulse type would have a large clip with a space big enough to fit it around the spark plug wire when its opened. If it has a coil of wire in the clip and a large wire attached to it (the size of a spark plug wire) then pull the spark plug boot, put the open end of the coil on the plug and the small end into the boot.

Hope this helps
 
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