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Author Topic:   Tachometer Dial Face Maximum RPM
Spray posted 07-08-2007 12:37 PM ET (US)   Profile for Spray   Send Email to Spray  
[Is there an] electrical tachometer with a top scale to 4,000 or 5,000 RPM? My motor tops out at less then 4,000-RPM thus using less than half of my existing tach's 8,000-RPM [scale].
jimh posted 07-09-2007 09:17 AM ET (US)     Profile for jimh  Send Email to jimh     
Outboard motor tachometers can usually be obtained with a maximum dial calibration of 6,000, 7,000, or 8,000 RPM. I do not recall one which went only to 4,000-RPM.

You could adapt your tachometer with 8,000-RPM calibration to be a 4,000-RPM device by making an adjustment to the calibration selector switch. Usually these switches offer a choice of the number of poles in the motor alternator, and, if you were fortunate, you might have a motor with say twice as many poles as the lowest calibration offered. By setting the tachometer to the wrong calibration mark, you could change the reading to match your motor by halving the scale calibration. For example, a motor with [ten] poles could be used with a tachometer set for [five] poles, thus halving the scale calibration--or at least that is my theory.

Spray posted 07-09-2007 09:44 PM ET (US)     Profile for Spray  Send Email to Spray     

Thanks for the resopnse. My alternator pole count is 12 according to the Tach manufacturer. My options are 5 or 6 on the back of the Tach. The Tach is broken and stops at 1K for some reason so my usabke scale is almost nill. I was hoping to buy a new Tach with a more reasonable - usable scale. The only options I have found thus far are 0 - 6000.


jimh posted 07-10-2007 12:16 AM ET (US)     Profile for jimh  Send Email to jimh     
If you have a 12-pole alternator and use the 6-pole calibration you should get double the reading, that is, 8,000 indicated on the dial will be actually 4,000-RPM from the motor. Maybe you can try this and report the results.
jimh posted 07-10-2007 12:18 AM ET (US)     Profile for jimh  Send Email to jimh     
Do not confuse the behavior of the dial pointer when the tachometer is OFF. Most do not return to the zero but read upscale around 1,000-RPM indicated. This is just about a universal artifact of how they are made.

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