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Author Topic:   Tachometer Calibration
Binkie posted 10-11-2007 01:37 PM ET (US)   Profile for Binkie   Send Email to Binkie  
I want to install a tachometer on my raceboat powered by a 1974 three-cylinder Chrysler power head on a racing mid section and gearcase. The tach has a switch to set the poles:

switch poles
#1-- 4
#2-- 6
#3-- 8
#4-- 12

I ran the motor in a barrel at as slow an idle as I can, It is a direct drive motor, and got #1-1400-RPM, #2-900-RPM. #3-700-RPM, #4- 500-RPM, #5=1500-RPM. The motor sounds like it is running about 500-700 -RPM.

It also says on tachometer to set the switch to match the outboard engine alternator.


jimh posted 10-11-2007 08:49 PM ET (US)     Profile for jimh  Send Email to jimh     
How can we help?
Binkie posted 10-11-2007 08:58 PM ET (US)     Profile for Binkie  Send Email to Binkie     
Anyone know how many poles is used for a Chrysler 3 cylinder motor. What is a pole anyway?


Bella con23 posted 10-11-2007 09:38 PM ET (US)     Profile for Bella con23  Send Email to Bella con23     
Rich: I looked it up and your outboard has a 12 pole stator. A pole is the magnetic pickup portion of the alternator assembly. There are permanent magnets imbedded in the flywheel of the powerhead. The stator resides mounted stationary to the top of the powerhead as the flywheel rotates around it.

The magnets in the flywheel pass one of the wire-wound "poles" on the stator creating a magnetic field within the stator and thereby generating electricity.

The tachometer receives it's pulse each time the flywheel magnet passes a stator "pole". Because your outboard has 12 poles as counted by looking at the underside of the flywheel and counting the small rectangular laminated metal core pickups, the tachometer needs to be set to divide each revolution of the engine by a factor of 12.

Hope this helps.

Binkie posted 10-12-2007 06:46 AM ET (US)     Profile for Binkie  Send Email to Binkie     
Thanks for the info, and your time to look it up. Trying to find top speed for this boat is nearly impossable without a tach. The props that fit the racing lower unit are Merc speedmaster props, meant for the original 4 cylinder 140hp powerhead, so with reduced power, I have to get the props reworked to gain rpms.


sternorama posted 10-12-2007 04:24 PM ET (US)     Profile for sternorama  Send Email to sternorama     

just for fun... a handheld non-contact tachometer...

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