First of all, I did do a search, and I must not be good at it, cause I could not find how to do this:
How do you wire up a 4-cylinder tach on a rotary? I have heard that it can be done, but no ones knows exactly how. My tach just has an input and ground, so it shouldn't be that hard. I want to use my original British guages on the car. Also, does anyone have a source for aftermarket custom speedo cables? Thanks.

 

Use the leading coil to trigger the tach (input)

 

You want the ground side of the trailing coil don't you?? Because that is where you tach signal is anyway?? I may be wrong, I've never wired a diff tach, but that would make sense to me.

 

Actually its nice to have the factory tach on the trailing, and an aftermarket on the leading. This way you can tell immediately if you lose an ignitor, and which ignitor it is. Both coils fire the same amount of times per revolution, so the tach signal doesn't differ between them.

 

Nice that would make sense too. Then you can technically do it on both coils.

 

Seems to me that if you only counted the number of pulses from one of the coils then a 4-cylinder tach would show half of the actual rpm of the rotary engine. I am certainly not a wiring expert though, so correct me if I am wrong and explain.

 

Set your tach to 4 cyl mode. Also hook it to the trailing coil because it is the only indication of a bad trailing circuit. If trailing went out, you wouldn't feel the difference, so you need a visual aid.

 

Yea I was thinking about it some more.. It wouldn't matter if you hooked it up to the leading to find out if it went bad. YOU WOULD FEEL IT. Trailing you really don't feel that much, so if both tachs go out then you know the trailing is bad. That is a better indication. I would always wire up to the negative on the trailing.

 

When you feel a big loss of power it isn't necessarily the ignition. This way you can be sure because one of the tachs will go out. There is no reason not to do it.

Xavier, it will show full rpm because the rotary engine sparks once per revolution per rotor housing. Whereas a four cyl sparks once EVERY OTHER revolution per cylinder. So the same number of sparks per rev, got it?

 

I guess it would just be personal preference. IT doesn't seem to matter.

 

I tried to hook my aftermarket tach to the trailing on a stock ignition and it would not work so I have it on the leading. But before this I had it on the same coil(Trailing) but with the direct fire set up(as per jeff20b) and it was fine? Why is that?

Thanks, Ed