Tach hook up

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Sep 27, 2013 | 01:06 PM

264WSM

Thread Starter

Tuned

Joined: Dec 2011

Posts: 313

Likes: 8

From: Wyotana

Tach hook up

Not exactly a Ford question but I need help with my thought process.

I have an old Case Back-Hoe (that I pull w/ my Ford) that I want to put a Tach in. It has a 4 cyl gasoline engine that spins to 2300rpm. The tach I have goes to 8000, so trying to look at 1500-2200 will be difficult. However, the tach has a switch on the back for 4-6-8 cyl engines. If I install this and put the switch on 8, will it read 4600 when I am actually doing 2300rpm??
I want my max rpms to read around 60% or so of the face. I can "recalibrate" the face with a Sparpie marker.

Sep 27, 2013 | 04:43 PM

fmc400

MSEE

Joined: Apr 2004

Posts: 10,386

Likes: 35

From: Austin, TX

RPM is proportional to the frequency of the switching waveform at the coil, call that FREQ, measured in cycles per second (Hz). For a 4-stroke engine, RPM = 120 * FREQ / CYL where CYL is the number of cylinders (4, 6 or 8). For a given frequency, RPM decreases as the number of cylinders increases. Therefore, 2300 would appear as 1150 if you set the switch to 8. The 4-cylinder setting is the highest you'll get.

You could just do like I did, and build your own homemade digital tachometer