Tach swap? G13 tach in G10 car?

[hypermiler01]

My car is a 1993 1.0L Metro. I want a tach and would rather have the oem tach in the instrument cluster than an add-on tach. But the only car in the salvage with an oem tach is a G13 4-cylinder car. Can a 4-cylinder tach be adapted to a 3-cylinder car?

[thebrad]

Pretty sure it's a wiring job.
For example, I have an Integra GSR cluster in my Honda Civic.

Any pics?

[Coyote X]

If you dig around on teamswift there is a post on there showing what resistors to swap out to get it working on a 3 cyl. It didn't look that hard to do really so it might be worth digging around over there and seeing what is involved in it.

[Christ]

The problem with using a 4 cylinder tach in a 3 cylinder is that the tach reads output from the ignitor and does math.. x/4 = RPM. Lets say x is 4000 on the 4 cylinder, your formula is

4000/4 = 1000 RPM (idle... )

Now, lets assume you put that cluster in your car, and don't change anything about it.

4000/4 = 1000RPM for a four cylinder,
but for the 3 cylinder, the actual formula should be:
4000/3 = 1333.33

So basically, your engine is revving 1330 RPM when your tach is displaying 1000 RPM. The difference won't be linear either... it's a curve.

16000/4 = 4000, 16000/3 = 5333.33.

Now, if you can change caps or resistors to make it do the math correctly, then you're set to go, but just using it without modification will definitely yield an incorrect reading.

[dcb]

Taking it apart and printing out your own label might be an option too, no electronics but a bit of artwork to sort out.

[hypermiler01]

Originally Posted by Christ

but just using it without modification will definitely yield an incorrect reading.

Yes, which is why I asked if it could be adapted (to give a correct reading).

And the difference is linear. display=actual * 3/4

Apparently 3 cylinder oem tachs were rare, but not nonexistent.

[Gary Winnick]

Just a quick addition the above post, the clear and black plastic have two long screws through them in the upper corners, but that's about it. Also, speedometer calibration is a big PITA.