Technical "Wrong" Tachometer

Hello everyone, i have a small "project" in mind that I'd like some tech advice on, to see if my logic is right:

I have a nonfunctional Tachometer i planned to use on my truck, everything else, housing, bezel, glass, is perfect. The plan was to replace the internals, and i can find some NOS ones that fit the housing for reasonable prices, the catch: they're for a 4 banger and I'm running a 6

Now, If I understand things correctly, it should work, but read 50% more (for example, at 2000 RPM it reads 3000). Since i plan to make a new face for it to match my other gauges, it would be simple enough to compensate for it. The tach reads all the way up to 8000, which "corrected" would be ~5333, so, about the max my stock 261 should ever run.

Is my logic right, or there's more to it than simply the pulse count of a 4 vs 6 cylinder?

 

Photos would be helpful

 

Your logic is correct.

A basic tach is simply a miliammeter. It'll read the average of the current drawn via the points, which is however many pulses per revolution.

Hook a tach for a 6 cylinder engine to a 4 and yes, it'll just under-read on the original calibrated markings by 50%. (Conversely, hook a 4 cyl tach to an 8 and it'll read 2x the engine speed, for example).

Phil

 

I agree your logic is fine, and the tach will just read 50% higher on the original scale. But if you make a new facescale "calibrated" for the 6 cyl it will work and read the actual rpm you need.

 

A lot of older tach's had a switch on the back of them to switch for different cylinders. I have not bought a new tach in years so I do not know if they still do this

 

Thanks everyone, I'll follow through with the plan then.

Aftermarket tachs still do come with the switch, but they wouldn't work in the housing i have.
Both the tach i have, and the mechanism i want to replace the old one with, are from OEM applications (4", dash mounted), so no switch there...

 

I wonder if a place like Williamson could convert it to a 6 cyl tach?

 

Don't assume that it doesn't have a switch. My OEM Olds Cutlass tach has a switch.

 

I did the exact opposite on Lotus Cortina race car . I re-printed the face on a 6000 RPM 6cyl Smiths Tacho.
This read 9000 RPM for my Lotus [whereas the 4 cyl Smiths Tachos available only went to 7000 RPM]