Calibrating the tach with not so simple math?

[Guest tony78_280z]

Not a math wiz, but...

 

If I hook up my GM Dizzy strait to the Nissan tach I will get the wrong reading as the GM has 8 sparks for 1 RPM and the Nissan has 6 sparks for 1 RPM. 6 to 8 or 3 to 4.

 

So if I set my idle for a steady 1000 RPMs as displayed on the Nissan tach, then how many RPMs am I really at? 750?

 

So with the idle still steady, if I then adjust the displayed RPMs on the tach to 750, am I then adjusted properly to display the RPMs correctly throughout the rev range?

 

If my math is some how off (which I feel it is), correct me please.

I'm just tryin to figure out how to calibrate the tach without hooking up a second tach for comparison purposes.

 

THANKs

[blueovalz]

I do not believe you will have any acuracy thoughout the entire RPM range by simply adjusting the tach speed to actual engine speed at idle. There has been some discussion on the board about this before and I cannot remember if a resistor (shunt) was used to calibrate the tach or not.

[Z-8]

I believe the JTR manual discusses this. Something about a 1/4" or 1/2" turn. Don't really remember off the top of my head, my books at home. If nobody responds by the time I get home, I'll read it, and post again.

[Z-8]

Alright here it is, word for word:

 

"To calibrate the tach, it must be removed from the instrument panel. The wires do not have to be disconnected from the tach, but the ignition must be turned off. On the back side of the tach case, there will be a slot about an inch below the hole. With a very small screwdriver, turn the slot about 45 deg clockwise. The tach is now calibrated for a V8. This adjustment should be checked with a tach known to be accurate. Again, be sure the ignition is turned off when turning the adjusting slot."

 

From the picture in the book, the hole for the slot is 3/4 of the way up the tach, and a little to the left(looking from the back). I hope you haven't tried this yet. Let me know what kind of Z you have, there are a couple of different ways to hook it up after that.

[JMortensen]

The stock tach is more of a suggestion anyway, if you really want a tach, buy a tach and put it in. The hole in the dash is just the right size for a 5" gauge so installation couldn't be easier.